Amandine Pinget
email: amandine.pinget@univ-smb.fr

Rachel Bocquet
email: rachel.bocquet@univ-smb.fr

Caroline Mothe
email: caroline.mothe@univ-smb.fr

Barriers to Environmental Innovation in SMEs: Empirical Evidence from French Firms 1

Keywords: Antecedents, Barriers, Environmental innovation, Multiple treatment model, CEO perceptions, SME

Recent literature explores the determinants of environmental innovations (EI) but rarely addresses obstacles to these innovations. To our knowledge, no previous study accounts for antecedents of EI to explore the various perceived barriers to EI for small and medium-sized enterprises (SMEs). Noting the importance of SMEs in European economies, this article identifies the extent to which SMEs perceive barriers to environmental innovations, considering their type, number, and intensity. With a merged data set of 435 French SMEs, we investigate different perceptions of environmentally innovative SMEs, compared with those of technologically innovative SMEs and non-innovative ones, using a multiple treatment model that integrates the antecedents. We thereby analyze SME CEO's perceptions of barriers to EI. The barriers are not only more numerous but also more important for SMEs that engage in environmental innovation activity compared with those that have introduced only technological innovation or those that do not undertake any innovation activity.

INTRODUCTION

In the past decade, concerns about firms' wrongdoing, especially in relation to the environment, have expanded. In response to pressures for a cleaner environment, firms might pursue environmental innovation (EI), which differs from "traditional" innovations in its externalities and drivers. Because regulations for adopting EIs may exist, institutional pressures trigger such innovations, especially among polluting firms (e.g., [START_REF] Berrone | Necessity as the Mother of 'Green' Inventions: Institutional Pressures and Environmental Innovations[END_REF][START_REF] Jaffe | A Tale of Two Market Failures: Technology and Environmental Policy[END_REF][START_REF] Porter | Toward a New Conception of the Environment-Competitiveness Relationship[END_REF][START_REF] Rennings | Redefining Innovation-Eco-Innovation Research and the Contribution from Ecological Economics[END_REF]. Most literature explores the determinants of EI adoption, so we know little about the elements that hinder EI. In particular, we find limited research into their barriers, which suggests the need for further empirical research on this topic [START_REF] Del Río González | The Empirical Analysis of the Determinants for Environmental Technological Change: A Research Agenda[END_REF].

To contribute to current debates on EI, we study EI determinants and barriers, both theoretically and empirically. Noting that the barriers to EI remain largely unexplored, we investigate the possibility of transferring or adapting existing theories and conceptual frameworks to environmental innovations [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF][START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF][START_REF] Rennings | Redefining Innovation-Eco-Innovation Research and the Contribution from Ecological Economics[END_REF], which tend to be more complex than other technological innovations [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF]. Theoretical and empirical research started to investigate the environmental benefits associated with innovations about a decade ago, including their drivers and determinants, but few studies (cf. De Marchi, 2012;[START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF] compare environmental and non-environmental innovations. Even fewer investigations address these issues in relation to small and medium-sized enterprises (SMEs), even though the 20 million SMEs in the European Union represent 99% of all European firms. Due to their resource constraints, SMEs tend to focus less on environmental questions than their larger counterparts, even though they account for approximately 64% of all industrial pollution [START_REF] Calogirou | SMEs and the Environment in the European Union[END_REF]. In this sense, SMEs have a major role in global sustainable development issues and represent an important target for public policies aimed at developing a sustainable society, yet they also face unique challenges, because even if they want to reduce their environmental impacts, they are limited by a relative lack of resources. This article therefore seeks to identify the extent to which SMEs that innovate in environmental contexts perceive barriers to their innovation, compared with SMEs that introduce technological innovations only or SMEs that do not innovate. As Mairesse and Mohnen (2010: 8) indicate, "most of the data collected in innovation surveys are qualitative, subjective and censored." Many variables, whether qualitative or quantitative, are subjective in nature, based largely on the personal appreciation and judgment of the respondents. Such perceptions are important in relation to public policy issues, because perceived obstacles to innovation constitute reflections of failed innovation policies, in that "If an obstacle is perceived to be high by a respondent, it means that somewhere there is a deficiency in innovation policy" (Mairesse & Mohnen, 2010: 22). This subjective approach requires an understanding of subjective visions of business opportunities and the mobilization of resources and capabilities to transform knowledge into business reality [START_REF] Shepherd | Prior Knowledge, Potential Financial Rewards, and Opportunity Identification[END_REF]. 2 We follow subjectivist entrepreneurial theory [START_REF] Penrose | The Theory of the Growth of the Firm[END_REF], which acknowledges the economic importance of an entrepreneur's personal knowledge [START_REF] Polanyi | Personal Knowledge: Towards a Post-Critical Philosophy[END_REF], which is subjective per se [START_REF] Kor | Resources, Capabilities and Entrepreneurial Perceptions[END_REF].

Adopting Kor et al.'s (2007) use of [START_REF] Penrose | The Theory of the Growth of the Firm[END_REF] to elaborate how entrepreneurs' perceptions and personal knowledge shape a firm's subjective productive opportunity set (including innovative activities), we assert the importance of understanding perceptions of the factors that hinder or make it difficult for SMEs to innovate environmentally. Such understanding has major implications in terms of macroeconomic policies to support EI.

Prior literature on such barriers to EI is very scarce, though two recent working papers testify to a growing interest. First, [START_REF] Marin | SMEs and Barriers to Eco-Innovation in EU: A Diverse Palette of Greens[END_REF] propose a taxonomy of European SMEs in terms of barriers to eco-innovation. Second, [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF] reveal the importance of financial barriers for SMEs' environmental innovation. With this study, we extend this line of research by accounting for the type, number, and intensity of such barriers and thereby answering two main research questions: What are the main barriers to EI, as perceived by SMEs? And are these perceived barriers more numerous or intense for environmentally innovative SMEs compared with technologically innovative or non-innovative SMEs? To test our predictions, we use a novel multiple treatment model and a merged sample of 435 SMEs in the French Rhône-Alpes region.

We find that barriers to EI are not only more numerous but also more important for SMEs that engage in environmental innovation activity, compared with those that do not undertake any innovation activity. This predominance also holds for the comparison with SMEs that have introduced technological innovation only, though to a lesser extent and mainly as a matter of intensity rather than of number.

With this approach, we contribute to prior literature in several ways. First, in line with [START_REF] Klewitz | Sustainability-Oriented Innovation of SMEs: A Systematic Review[END_REF], we seek to develop of a more integrated theoretical framework of EI in SMEs that encompasses, for the first time, both antecedents and barriers to EI. Second, we identify specific EI determinants for SMEs and compare perceptions of barriers to EI across three SME categories (environmentally innovative, technologically innovative, and non-innovative), using an original methodology based on a multinomial logit model with treatment effects. Our findings show that EI has a more binding character than more classical technological innovations, and the barriers to EI appear more numerous and more intense. Third, our novel data set of French SMEs enables us to address conventional questions about innovation while also considering SMEs' specific antecedents and barriers to innovation.

In the next section, we present our theoretical framework and draw hypotheses about barriers to EI. We then present the data and methodology, followed by the main results of our econometric models. Finally, with our discussion and conclusion, we note some limitations of this research and avenues for further research.

ANTECEDENTS OF ENVIRONMENTAL INNOVATION

Environmental innovation has been defined in various ways, to include different types of innovation (i.e., technological or non-technological), depending on the researchers' objectives and questions. For example, Kemp (2010: 2) defines EI as the "production, assimilation or exploitation of a product, production process, service or management or business method that is novel to the organization (developing or adopting it) and which results, throughout its life cycle, in a reduction of environmental risk, pollution and other negative impacts of resources use (including energy use) compared to relevant alternatives." In contrast, Rennings (2000: 322) views EI as "measures of relevant actors (firms …) which: (i) develop new ideas, behavior, products and processes, (ii) apply or introduce them, and; (iii) contribute to a reduction of environmental burdens or to ecologically specified sustainability targets." The various definitions sometimes refer to EI as green or eco-innovation, which are broader terms that encompass unintended environmental innovations too [START_REF] Arundel | Measuring Eco-Innovation[END_REF]. We prefer the term "environmental innovation," because it aligns with our research perspective, in which EI is a result of the firm's strategy. Moreover, it is the term most often used in innovation literature [START_REF] Schiederig | Green Innovation in Technology and Innovation Management-an Exploratory Literature Review[END_REF].

Accordingly, we focus on technological EI, which we regard as new or modified processes, products, or services that reduce environmental harms [START_REF] Beise | Lead Markets and Regulation: a Framework for Analyzing the International Diffusion of Environmental Innovations[END_REF][START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF]. This definition includes changes to products and production processes that generate environmental benefits, whether those benefits accrue to final customers (i.e., products and services) or the firm itself (i.e., processes). Note that this definition "is based on the effect of the innovation activities independent of the initial intent and includes both incremen-tal and radical improvements" (De Marchi, 2012: 615). Because EI holds increasing interest for both firms and scholars, a question arises: Does it require specific theory and public policy? This question is particularly pertinent for SMEs [START_REF] Cuerva | Drivers of Green and Non-Green Innovation: Empirical Evidence in Low-Tech SMEs[END_REF][START_REF] Del Río González | The Empirical Analysis of the Determinants for Environmental Technological Change: A Research Agenda[END_REF], for which the frontier between the determinants and barriers of EI remains tenuous.

Prior EI literature discusses whether EI is triggered by supply-push or demand-pull factors, or both [START_REF] Costantini | Demand-Pull and Technology-Push Public Support for Eco-Innovation: The Case of the Biofuels Sector[END_REF][START_REF] Di Stefano | Technology Push and Demand Pull Perspectives in Innovation Studies: Current Findings and Future Research Directions[END_REF][START_REF] Peters | The Impact of Technology-Push and Demand-Pull Policies on Technical Change-Does the Locus of Policies Matter?[END_REF]. Beyond such technologypush and market-pull factors, regulation is another important driving force (e.g., [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF][START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF][START_REF] Horbach | Determinants of Eco-Innovations by Type of Environmental Impact-The Role of Regulatory Push/Pull, Technology Push and Market Pull[END_REF][START_REF] Rennings | Increasing Energy and Resource Efficiency through Innovation -An Explorative Analysis Using Innovation Survey Data[END_REF]. Some authors also introduce firm-specific factors [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF][START_REF] Horbach | Determinants of Eco-Innovations by Type of Environmental Impact-The Role of Regulatory Push/Pull, Technology Push and Market Pull[END_REF].

We adopt a classification3 of four types of antecedents, in line with [START_REF] Horbach | Determinants of Eco-Innovations by Type of Environmental Impact-The Role of Regulatory Push/Pull, Technology Push and Market Pull[END_REF] and [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF]: regulation, technology push factors, demand pull factors, and firms' characteristics.

Regulation

Similar to most studies of firms' EI antecedents, we do not adopt a standard "policy-oriented" approach. We instead prefer the Porter hypothesis, which stresses that regulation can drive innovation in certain circumstances. This hypothesis has been formulated twice [START_REF] Porter | Towards a Dynamic Theory of Strategy[END_REF][START_REF] Porter | Toward a New Conception of the Environment-Competitiveness Relationship[END_REF] and indicates that well-designed regulations can enhance firms' innovation and competitiveness. In contrast, a standard view considers environmental regulation solely an additional cost for firms.

With this reliance on the Porter hypothesis, we consider environmental policy a main potential driver of EI [START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF]. For example, environmental regulations incentivize innovation through environmental taxes or certificates [START_REF] Wagner | The Porter Hypothesis Revisited: A Literature Review of Theoretical Models and Empirical Tests[END_REF]. Two key points differentiate EI from other innovations: externalities and drivers, which [START_REF] Rennings | Redefining Innovation-Eco-Innovation Research and the Contribution from Ecological Economics[END_REF] refers to as the "double externality problem" and the "regulatory push/pull effect," respectively. That is, just as innovation and R&D activities induce positive externalities, green innovators can produce positive environmental externalities [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF]. Part of this created value gets appropriated by society-in the form of reduced environmental damage-so there are some disincentives for firms to invest in products or processes that reduce their environmental impacts [START_REF] Jaffe | A Tale of Two Market Failures: Technology and Environmental Policy[END_REF][START_REF] Rennings | Redefining Innovation-Eco-Innovation Research and the Contribution from Ecological Economics[END_REF]. This additional externality may prompt a lack of investment or interest among firms, because direct returns from investments in EI are difficult to reap. The potential for market failure also induces a greater need for policy intervention to drive EI [START_REF] Rennings | Redefining Innovation-Eco-Innovation Research and the Contribution from Ecological Economics[END_REF].

In this vein, recent studies indicate a positive correlation between regulation and environmental innovations [START_REF] Horbach | Determinants and Specificities of Eco-Innovations Compared to Other Innovations-an Econometric Analysis for the French and German Industry Based on the Community Innovation Survey[END_REF]; environmental regulation offers the initial incentive for firms to develop environmental innovation processes [START_REF] Del Río González | The Empirical Analysis of the Determinants for Environmental Technological Change: A Research Agenda[END_REF]. [START_REF] Antonioli | Is Environmental Innovation Embedded within High-Performance Organisational Changes? The Role of Human Resource Management and Complementarity in Green Business Strategies[END_REF], in their comparative analysis, find that polluting sector firms tend to innovate more environmentally than firms outside a polluting sector. This effect of more stringent environmental regulation exists for innovation in general [START_REF] Ford | How Environmental Regulations Affect Innovation in the Australian Oil and Gas Industry: Going Beyond the Porter Hypothesis[END_REF], such that some firms even overcomply to gain competitive advantages and an improved social image, in which case the costs associated with reduced pollution might be balanced by realized gains [START_REF] Ambec | Does it Pay to Be Green? A Systematic Overview[END_REF].

For SMEs, though regulation is a powerful driver of EI, meeting environmental regulations is arduous [START_REF] Brammer | Environmental Management in SMEs in the UK: Practices, Pressures and Perceived Benefits[END_REF], especially when the regulatory system is complex (certifications, policies, institutions). With a sample of Chinese SMEs, [START_REF] Zhu | Institution-Based Barriers to Innovation in SMEs in China[END_REF] find that unclear laws or regulations, together with excessive taxation, have hampering effects on small firms. Thus, well-designed regulation must be adequate and appropriate to support SMEs' EI processes.

Technology push factors

Literature on the determinants of EI that adopts a technology push (supply-side) view generally suggests that improving organizational, strategic, and technological capabilities triggers EI [START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF]. Using a novel data set of 1566 U.K. firms, [START_REF] Kesidou | On the Drivers of Eco-Innovations: Empirical Evidence from the UK[END_REF] emphasize the importance of allocating organizational capabilities and resources to EI. A strong positive relationship emerges between technological capabilities and EI [START_REF] Cuerva | Drivers of Green and Non-Green Innovation: Empirical Evidence in Low-Tech SMEs[END_REF]; in addition, environmental management systems (EMS; e.g., ISO norms) favor the EI process [START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF][START_REF] Kammerer | The Effects of Customer Benefit and Regulation on Environmental Product Innovation: Empirical Evidence from Appliance Manufacturers in Germany[END_REF][START_REF] Wagner | On the Relationship between Environmental Management, Environmental Innovation and Patenting: Evidence from German Manufacturing Firms[END_REF]. The ISO 14001 norm has a positive influence on R&D activities, and a more mature EMS increases environmental R&D investments [START_REF] Inoue | A New Insight into Environmental Innovation: Does the Maturity of Environmental Management Systems Matter?[END_REF]. [START_REF] Kesidou | On the Drivers of Eco-Innovations: Empirical Evidence from the UK[END_REF] also find that cost savings, especially on material and energy, are important incentives for EI. [START_REF] Horbach | Determinants and Specificities of Eco-Innovations Compared to Other Innovations-an Econometric Analysis for the French and German Industry Based on the Community Innovation Survey[END_REF] confirm this result among French and German firms and show that savings on energy and material enhance EI. In addition, SMEs with an external acquisition strategy likely innovate less, because both acquisition and innovation strategies incur important costs [START_REF] Hitt | The Market for Corporate Control and Firm Innovation[END_REF]. Instead, an internally focused strategy should enhance a firm's propensity to innovate environmentally [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF].

Cooperation in R&D also appears to drive EI by enabling economies of scale, especially for firms in the same sector [START_REF] Cainelli | Environmental Innovations, Complementarity and Local/Global Cooperation: Evidence from North-East Italian Industry[END_REF]. As an important characteristic of EI, it may require knowledge and competences that do not belong to the firms' core competences [START_REF] Horbach | Determinants of Eco-Innovations by Type of Environmental Impact-The Role of Regulatory Push/Pull, Technology Push and Market Pull[END_REF][START_REF] Rennings | Increasing Energy and Resource Efficiency through Innovation -An Explorative Analysis Using Innovation Survey Data[END_REF]. In this sense, [START_REF] Cainelli | Environmental Innovations, Local Networks and Internationalization[END_REF] not only show that interfirm network relationships are the most important EI driver for firms located in a local production system but also that EI is stimulated by firms' interactions with "qualified partners" (e.g., universities and suppliers, but not customers or competing firms). Other authors demonstrate that cluster policies could leverage EI, in both clean-tech sectors and other industries [START_REF] Barsoumian | Eco-Innovation and National Cluster Policies in Europe. A Qualitative Review[END_REF]. [START_REF] Wagner | On the Relationship between Environmental Management, Environmental Innovation and Patenting: Evidence from German Manufacturing Firms[END_REF] emphasizes the need to collaborate with environmentally concerned stakeholders, especially for SMEs. In one of the rare studies dedicated to SMEs, Del Río González (2009) asserts that other actors, such as industrial associations or public and private entities, can engage in cooperative processes to support innovation. Research into EI determinants also highlights the crucial importance of interactions between firms and between SMEs and various actors [START_REF] Marin | SMEs and Barriers to Eco-Innovation in EU: A Diverse Palette of Greens[END_REF], which implies a link to open innovation considerations. Open innovation is key for EI [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF], especially for SMEs [START_REF] Klewitz | Sustainability-Oriented Innovation of SMEs: A Systematic Review[END_REF][START_REF] Worthington | Strategic Intent in the Management of the Green Environment within SMEs[END_REF], and in their systematic review of the sustainability-oriented innovation of SMEs, [START_REF] Klewitz | Sustainability-Oriented Innovation of SMEs: A Systematic Review[END_REF] argue that interactions with external actors (e.g., authorities, research institutes) ultimately can increase the innovative capacity of SMEs for such environmental innovations. [START_REF] Triguero | Drivers of Different Types of Eco-Innovation in European SMEs[END_REF] suggest, on the basis of their analysis of the drivers of different types of EI in European SMEs, supply-side factors thus appear more important for environmentally oriented innovations than for more traditional product innovations.

Demand pull factors

Firms have strong incentives to engage in EI that are congruent with customer benefits [START_REF] Kammerer | The Effects of Customer Benefit and Regulation on Environmental Product Innovation: Empirical Evidence from Appliance Manufacturers in Germany[END_REF]. [START_REF] Kesidou | On the Drivers of Eco-Innovations: Empirical Evidence from the UK[END_REF] argue that firms initiate EI to satisfy minimum customer and societal requirements. Environmental consciousness thus is a relevant parameter for innovative firms [START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF], especially in environmentally sensitive industries. In the pulp paper industry for example, public pressure is the strongest determinant of EI, even more so than regulation [START_REF] Popp | Environmental Policy vs. Public Pressure: Innovation and Diffusion of Alternative Bleaching Technologies in the Pulp Industry[END_REF]. Although some SMEs likely are reluctant to implement EI, out of a concern that most of their customers are not willing to pay more for green products or services [START_REF] Bianchi | Greening" SMEs' Competitiveness[END_REF], their investment in EI can represent a means to develop their markets [START_REF] Brammer | Environmental Management in SMEs in the UK: Practices, Pressures and Perceived Benefits[END_REF]. Moreover, close proximity between a firm and its customers can help it implement an EI strategy [START_REF] Madrid-Guijarro | Barriers to Innovation among Spanish Manufacturing SMEs[END_REF]. As [START_REF] Triguero | Drivers of Different Types of Eco-Innovation in European SMEs[END_REF] show, European SMEs that collaborate with various actors (including consumers) increase market demand for green products, and market share in turn has a significant positive influence on EI.

Firm characteristics

Firm size has a positive effect on EI, such that larger SMEs, which enjoy greater access to financial and human resources [START_REF] Rehfeld | Integrated Product Policy and Environmental Product Innovations: an Empirical Analysis[END_REF], are more likely to engage in EI processes [START_REF] Cuerva | Drivers of Green and Non-Green Innovation: Empirical Evidence in Low-Tech SMEs[END_REF][START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF][START_REF] Galliano | Les Déterminants de l'Adoption de l'Eco-Innovation selon le Profil Stratégique de la Firme: le Cas des Firmes Industrielles Françaises[END_REF]. Older firms have acquired more competencies, knowledge, and resources to support an EI strategy, whereas younger ones tend to seek an understanding of their market first, then search for venture capital funding [START_REF] Mazzarol | The Influence of Size, Age and Growth on Innovation Management in Small Firms[END_REF]. In addition, international firms are more conscious of environmental pressures [START_REF] Del Río González | The Empirical Analysis of the Determinants for Environmental Technological Change: A Research Agenda[END_REF] Overall, SMEs appear generally less likely to introduce EI than large firms, because of their lack of resources. Of the various explanatory factors for SMEs' EI, those related to the demand side seem to have the smallest impact. Examining barriers to EI offers another path of interest, which may provide a better understanding of SMEs' decisions related to EI.

BARRIERS TO ENVIRONMENTAL INNOVATION

Because of the scarcity of studies on the drivers or barriers of environmental innovation, we rely on literature on barriers to technological innovation. This analogy between EI and other types of technological innovation requires consideration of two main elements: First, some studies based on large samples demonstrate that environmental innovations are more complex and costly, such that they require knowledge and competences that are not necessarily among the firm's core competences [START_REF] Horbach | Determinants of Eco-Innovations by Type of Environmental Impact-The Role of Regulatory Push/Pull, Technology Push and Market Pull[END_REF][START_REF] Rennings | Increasing Energy and Resource Efficiency through Innovation -An Explorative Analysis Using Innovation Survey Data[END_REF]. Second, Ghisetti et al. (2015) show that EI is not always a question of financial resources but rather reflects managers' perceptions and the organization; as such, EI often requires more transverse process activities than do traditional, "dirty" technological innovations (which mainly emerge from R&D departments).

Previous studies of barriers to (technological) innovation do not address EI specifically but rather seek to explicate the impact of such barriers on firms' attitudes toward R&D activities [START_REF] Blanchard | Where There Is a Will, There is a Way? Assessing the Impact of Obstacles to Innovation[END_REF][START_REF] Hyytinen | Do Financial Constraints Hold Back Innovation and Growth? Evidence on the Role of Public Policy[END_REF][START_REF] Mohnen | Financial Constraints and Other Obstacles: Are They a Threat to Innovation Activity[END_REF][START_REF] Mohnen | Complementarities in Innovation Policy[END_REF][START_REF] Savignac | Impact of Financial Constraints on Innovation: What Can Be Learned from a Direct Measure?[END_REF][START_REF] Segarra-Blasco | Barriers to Innovation and Public Policy in Catalonia[END_REF]. 4 For example, research carried out for the Commission of European Communities, featuring contributions from researchers across eight European countries, reveals that major barriers relate to the education system, skilled labor, venture capital and bank financing, norms, legislation, and public bureaucracy (European Commission, 2004). In Canada, [START_REF] Baldwin | Impediments to Advanced Technology Adoption for Canadian Manufacturers[END_REF] study barriers to advanced technology adoption by manufacturing firms and identify five classes: cost, institution, labor, organization, and information. [START_REF] Galia | Complementarities between Obstacles to Innovation: Evidence from France[END_REF], investigating the complementarities among barriers to innovation for French manufacturing firms, show that firms that postpone projects are more prone to economic risk, lack of skilled personnel, innovation costs, lack of customer responsiveness, lack of information about technologies, and organizational rigidities. In contrast, firms that abandon projects tend to face economic barriers rather than technological or organizational ones. [START_REF] Mohnen | Complementarities in Innovation Policy[END_REF] also assess complementarities among barriers to innovation in a sample of firms from Ireland, Denmark, Germany, and Italy. They cite four groups of barriers-risk and finance, knowledge, knowledge skills outside the enterprise, and regulation-and assert that a lack of internal human capital complements all other barriers in almost all industries. In the Netherlands, [START_REF] Mohnen | Financial Constraints and Other Obstacles: Are They a Threat to Innovation Activity[END_REF] show that financial barriers significantly affect firms' decision to abandon, prematurely stop, slow down, or not start an innovative project; in addition, these financial constraints depend on firms' size and economic situation.

As these studies show, many firms are constrained by financial barriers. However, when barriers to innovation serve as an explanatory variable for R&D activity or innovation output, a non-significant or even significantly positive coefficient often results [START_REF] Baldwin | Impediments to Advanced Technology Adoption for Canadian Manufacturers[END_REF][START_REF] D'este | What Hampers Innovation? Revealed Barriers versus Deterring Barriers[END_REF][START_REF] Galia | Complementarities between Obstacles to Innovation: Evidence from France[END_REF][START_REF] Hölzl | High-Growth Firms, Innovation and the Distance to the Frontier[END_REF][START_REF] Hölzl | Distance to the Frontier and the Perception of Innovation Barriers across European Countries[END_REF][START_REF] Iammarino | The Perception of Obstacles to Innovation. Foreign Multinationals and Domestic Firms in Italy[END_REF][START_REF] Mohnen | Financial Constraints and Other Obstacles: Are They a Threat to Innovation Activity[END_REF][START_REF] Mohnen | Complementarities in Innovation Policy[END_REF][START_REF] Mohnen | Barriers to Innovation in Service Industries in Canada[END_REF], such that firms facing stronger barriers appear more likely to innovate, all else being equal. The positive correlation of innovation with perceived obstacles is, at first sight, a typically counterintuitive result. However, most studies of innovation obstacles, especially those using Community Innovation Surveys (CIS) that are based on perceptions (e.g., [START_REF] Galia | Complementarities between Obstacles to Innovation: Evidence from France[END_REF][START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF][START_REF] Mohnen | Complementarities in Innovation Policy[END_REF][START_REF] Rennings | Increasing Energy and Resource Efficiency through Innovation -An Explorative Analysis Using Innovation Survey Data[END_REF], indicate that barriers to innovation are perceived as stronger by those firms that actually are innovating. Therefore, closer inspection suggests "that innovating firms are more likely than non-innovating firms to perceive the various obstacles that stand in their way" (Mohnen et al., 2008, p. 208). In other words, the perception of hampering factors is itself endogenous and co-determined by some of the same factors that condition innovation. [START_REF] Clausen | Search Pathways to Innovation[END_REF] provides an original, additional explanation: The key variable is not actual barriers but their perception by managers. In that sense, those who wish to innovate are more inclined to perceive barriers, and this perception relates positively to the will to innovate. Obstacles to innovation should be interpreted as a measure of how firms overcome them, rather than as preventers of innovation [START_REF] Baldwin | Impediments to Advanced Technology Adoption for Canadian Manufacturers[END_REF][START_REF] Tourigny | Impediments to Innovation Faced by Canadian Manufacturing Firms[END_REF]. The definitions of barriers to innovation applied in innovation surveys in turn might indicate how successfully a firm has overcome those barriers. D' Este et al. (2012) similarly propose a distinction between "deterring" and "revealed" barriers: The former prevent firms from engaging in innovation activities, whereas the latter invoke a positive effect, such that firms can overcome barriers and innovate. [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF] offers support for the deterrent barrier hypothesis for financial constraints, which deter innovative strategies.

In line with these previous studies, we analyze three major sets of perceived barriers to EI: cost, knowledge, and the market. First, cost barriers reflect the firm's difficulties in financing its innovation projects. During the innovation process, available financial resources might not be sufficient to cover the investments required, so high costs and a lack of financial resources (internal and external sources) constitute important barriers to innovation. Second, knowledge barriers pertain to limited access to information about technology and skilled labor. Managers and employees who can incorporate and support innovation as a business strategy thus attain a competitive advantage. That is, EI require specific information and knowledge, so qualified personnel and associated skills are important for exploring new environmental technologies. Third, an ability to connect a technical opportunity to a market opportunity encourages successful innovations, but technology push-and demand pull-related barriers may constrain innovative activity. The technology and markets linked to EI tend to be complex and evolve rapidly, so firms that pursue EI must address these two issues even more intensively than firms that innovate in other realms. The market barriers thus reflect market structures and pull technology derived from demand.

However, these barriers might not be identical or perceived in the same way in relation to environmental innovation, and especially for SMEs. The European Commission's Environmental Technologies Action Plan (European Commission, 2004) cites several barriers to environmental innovation: economics, inappropriate regulations or standards, insufficient or weak research systems, and lack of market demand. [START_REF] Ashford | Understanding Technological Responses of Industrial Firms to Environmental Problems: Implications for Government Policy[END_REF] also provides a detailed list of barriers to pollution prevention: technological, financial, labor force, regulatory, consumer-related, supplier-related, and managerial. Empirical studies indicate that EI is often costly, because it requires specific procedures to measure, manage, and adapt benefits for the environment, which could hinder an innovative firm's performance [START_REF] Konar | Does the Market Value Environmental Performance? Review of[END_REF].

Market uncertainty also tends to be greater for green products, because of their relative newness and volatile consumer markets. Similarly, access to both knowledge about markets and technologies and skilled personnel is more difficult for goods outside the mainstream.

These barriers get reinforced for SMEs, which lack various resources and are more constrained in their day-to-day operations. At a regional level, [START_REF] Freel | Barriers to Product Innovation in Small Manufacturing Firms[END_REF] observes barriers to product innovation among a sample of small manufacturing firms and breaks the resource constraints down into four sets: finance, management and marketing, skilled labor, and information. [START_REF] Madrid-Guijarro | Barriers to Innovation among Spanish Manufacturing SMEs[END_REF] consider the lack of financial resources, poor human resources, weak financial position, and high cost and risk as internal barriers, as well as turbulence, lack of external partners, lack of information, and lack of government support as external barriers. The cost of innovation affects Spanish SMEs more, and barriers' impacts depend on the type of innovation. According to [START_REF] Madrid-Guijarro | Barriers to Innovation among Spanish Manufacturing SMEs[END_REF], costs represent the most significant barriers to innovation, with a disproportionately greater impact on small firms, probably because SMEs suffer more limited financial resources than large firms. In this sense, SMEs are especially subject to barriers linked to the lack of financial resources and costs [START_REF] Iammarino | The Perception of Obstacles to Innovation. Foreign Multinationals and Domestic Firms in Italy[END_REF][START_REF] Savignac | Impact of Financial Constraints on Innovation: What Can Be Learned from a Direct Measure?[END_REF]. [START_REF] Alessandrini | Bank Size or Distance: What Hampers Innovation Adoption by SMEs[END_REF], considering Italian small firms during 1995-2003, show that their different time patterns cause process and product innovations to be associated with different financial constraints.

Del Río [START_REF] Del Río González | The Empirical Analysis of the Determinants for Environmental Technological Change: A Research Agenda[END_REF], in a review of empirical studies, indicates that barriers to environmental technological innovation for SMEs are not the same as those encountered by large firms but provides no further details. He only indicates that small firms lack sufficient human, technical, and financial resources, which bars their EI, and he calls for more research.

The two (known) empirical studies on barriers to EI for SMEs (beyond the systematic review by [START_REF] Klewitz | Sustainability-Oriented Innovation of SMEs: A Systematic Review[END_REF] highlight that SMEs have very different profiles in terms of their perceptions of such barriers [START_REF] Marin | SMEs and Barriers to Eco-Innovation in EU: A Diverse Palette of Greens[END_REF] and that perceived financial barriers deter their environmental innovative activities; that is, they prevent SMEs from adopting environmental innovations [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF]. Two other studies that include both large and small firms and rely on CIS data related to obstacles to innovation conclude that legislation and bureaucratic processes [START_REF] Rennings | Increasing Energy and Resource Efficiency through Innovation -An Explorative Analysis Using Innovation Survey Data[END_REF] as well as a lack of knowledge [START_REF] Horbach | Determinants of Eco-Innovations by Type of Environmental Impact-The Role of Regulatory Push/Pull, Technology Push and Market Pull[END_REF] are perceived barriers that hamper EI.

Because SMEs face relatively more, and more intense, barriers to innovation than large firms, due to their inadequate or insufficient internal resources, we explicitly investigate variance in the number and intensity of barriers for SMEs, according to the type of innovation (i.e., environmental versus technological versus no innovation). We hypothesize:

Dependent variables

Environmental innovation is generally measured with input, intermediate output, direct output, and indirect impact measures [START_REF] Arundel | Measuring Eco-Innovation[END_REF]. For our sample of SMEs, "objective" measures, such as patents and R&D (i.e., input and intermediate output) are less rele-vant. Therefore, in line with our theoretical subjectivist perspective, we use perceptual measures and adopt a direct output measure, following the Oslo Manual (OECD, 2005) and the CIS 2008. Several authors (e.g., [START_REF] Madrid-Guijarro | Barriers to Innovation among Spanish Manufacturing SMEs[END_REF] assert that subjective measures (e.g., managers' perceptions) are superior to objective measures [START_REF] Hughes | Innovation and Business Performance: Small Entrepreneurial Firms in the UK and the EU[END_REF], as well as highly correlated with objective measures [START_REF] Frishammar | Managing External Information in Manufacturing Firms: The Impact on Innovation Performance[END_REF][START_REF] Zahra | Business Strategy, Technology Policy and Firm Performance[END_REF].

We asked the SME CEOs whether, between 2009 and 2011, their firm had introduced significant novelties or improvements in its manufacturing processes or production of goods or services. A subsidiary question asked if those innovations provided any environmental benefits (e.g., reduced energy consumption, lowered CO2 emissions, waste recycling) for the firm and/or for customers. We combined these two questions-about technological innovations (yes/no, binary variable) and their environmental benefits (yes/no, binary variable)-to determine if the firm was environmentally innovative.

To investigate differences in perceptions of barriers to EI across SMEs, we used a multiple treatment model [START_REF] Cattaneo | Efficient Semiparametric Estimation of Multi-Valued Treatment Effects under Ignorability[END_REF], which enabled us to compare differentiated perceptions of barriers, according to the SMEs' innovation status. That is, we compared perceived barriers by environmentally innovative SMEs to those perceived by technologically innovative and non-innovative SMEs. Other econometric modeling-based measures (e.g., [START_REF] Böhringer | Clean and Productive? Empirical Evidence from the German Manufacturing Industry[END_REF] instead tend to consider only EI firms and therefore measure a global effect among EI firms. Some other studies on technological innovation [START_REF] Baldwin | Impediments to Advanced Technology Adoption for Canadian Manufacturers[END_REF][START_REF] Galia | Complementarities between Obstacles to Innovation: Evidence from France[END_REF][START_REF] Hölzl | High-Growth Firms, Innovation and the Distance to the Frontier[END_REF][START_REF] Iammarino | The Perception of Obstacles to Innovation. Foreign Multinationals and Domestic Firms in Italy[END_REF][START_REF] Mohnen | Complementarities in Innovation Policy[END_REF][START_REF] Mohnen | Barriers to Innovation in Service Industries in Canada[END_REF] and the very few studies on EI [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF] also focus on perceptions of barriers among innovative firms or treat non-innovative firms as an undifferentiated group. To extend these approaches, we investigate differences among EI firms, technologically innovative (TI) firms, and non-innovative (NI) firms, which provides a better adjusted measure. With these comparisons, we highlight potential specificities of barriers perceived by environmentally innovative SMEs.

Independent variables

We introduced a series of variables that prior empirical literature lists as determinants of product and process EI. Public policies and regulation are powerful levers for inciting firms to adopt EI. Because institutional pressures trigger EI even more among high polluting firms [START_REF] Berrone | Necessity as the Mother of 'Green' Inventions: Institutional Pressures and Environmental Innovations[END_REF], we used this proxy to measure the impact of regulation, with the prediction that SMEs operating in polluting sectors are more prone to adopt EI [START_REF] Antonioli | Is Environmental Innovation Embedded within High-Performance Organisational Changes? The Role of Human Resource Management and Complementarity in Green Business Strategies[END_REF]. 6 With regard to SMEs' strategy, we anticipated that SMEs engaged in external growth strategies should be more likely to allocate resources to this strategic priority, to the detriment of other activities, such as those linked to innovation [START_REF] Hitt | The Market for Corporate Control and Firm Innovation[END_REF]. In contrast, SMEs engaged in R&D cooperation might be able to compensate for their lack of resources to innovate [START_REF] Triguero | Drivers of Different Types of Eco-Innovation in European SMEs[END_REF]. If SMEs belong to a cluster,7 they also should be more likely to introduce EI, because they can benefit from interfirm network relationships and agglomeration economies [START_REF] Porter | Location, Competition, and Economic Development: Local Clusters in a Global Economy[END_REF], though the role of agglomeration economies is not clear-cut [START_REF] Cainelli | Environmental Innovations, Local Networks and Internationalization[END_REF]. These effects are important in relation to social proximity and processes of collective learning [START_REF] Mirata | Industrial Symbiosis Networks and the Contribution to Environmental Innovation: The Case of the Landskrona Industrial Symbiosis Programme[END_REF]. [START_REF] Cainelli | Environmental Innovations, Local Networks and Internationalization[END_REF] also confirm the effects of agglomeration economies on EI in areas that have historically rooted specialization patterns in "EI-friendly" sectors, that is, those that have developed a typical social capital. A firm's implementation of a pollution reduction strategy also could significantly influence its decision to adopt EI, in the form of products or processes (i.e., environmental monitoring), in line with [START_REF] Wagner | On the Relationship between Environmental Management, Environmental Innovation and Patenting: Evidence from German Manufacturing Firms[END_REF] assertion that implementing an EMS increases the probability that firms pursue innovation in general and EI in particular. [START_REF] Wagner | On the Relationship between Environmental Management, Environmental Innovation and Patenting: Evidence from German Manufacturing Firms[END_REF] also notes a positive relationship between cooperation with predominantly environmentally concerned stakeholders and environmental product innovation.

Finally, we introduced variables to control for firms' characteristics. We measured firm age by its logarithm. Because efficient firms are more likely to survive and grow [START_REF] Bartelsman | Understanding Productivity: Lessons from Longitudinal Microdata[END_REF], firm age should have a positive impact on EI. We included firm size, measured by the logarithm of the total number of employees. Medium-sized SMEs seemingly innovate more than the smallest ones [START_REF] Laforet | Size, Strategic, and Market Orientation Affects on Innovation[END_REF], so we expect firm size to have a positive impact on the decision to adopt EI, in that larger SMEs have more resources to innovate.

When a SME is part of a group, it can benefit from additional resources necessary to innovate.

In contrast, SMEs that face financial constraints (measured for 2010, using the debt-to-equity ratio computed from the Orbis data; [START_REF] Cassar | Capital Structure and Financing of SMEs: Australian Evidence[END_REF][START_REF] Michaelas | Financial Policy and Capital Structure Choice in UK SMEs: Empirical Evidence from Company Panel Data[END_REF]) should be less likely to adopt EI [START_REF] Madrid-Guijarro | Barriers to Innovation among Spanish Manufacturing SMEs[END_REF]. The relationship between innovation and exports has been well demonstrated, so we included a dummy to indicate if the firm engaged in export activity [START_REF] Basile | Export Behaviour of Italian Manufacturing Firms over the Nineties: the Role of Innovation[END_REF][START_REF] Roper | Innovation and Export Performance: Evidence from the UK and German Manufacturing Plants[END_REF]. Finally, we defined two sector variables (manufacturing versus service). Table 1 provides definitions for the variables in our multinomial logit model.

Sample

The descriptive statistics refer to the balanced sample, namely, the 435 SMEs in Appendix 2.

When we compare the environmentally innovative and non-innovative SME groups (see Table 2), several differences emerge for variables related to the polluting sector, R&D cooperation, belonging to a cluster, having environmental monitoring, exports, and being from the services sector. We find fewer differences between environmentally and technologically innovative SME groups, related to external growth, environmental monitoring, and age of the firm. We also find several significant differences between the technologically innovative and non-innovative groups, which are quite similar to those between the environmentally innovative and non-innovative SME groups. That is, there is a more important gap between environmentally innovative firms and non-innovative ones, and between technologically innovative firms and non-innovative ones, than between environmentally and technologically innovative SMEs, which exhibit few differences. Appendix 2 provides the descriptive statistics.

Table 2. Sample composition

Methods

To analyze possible differences among firms in terms of innovation input, innovation strategies, innovation output, and firm performance, two main methods serve evaluation purposes. First, a propensity score matching approach establishes two groups of firms that are similar in variables that determine a certain feature-here, the introduction of EI-such that one group exhibits this feature (i.e., EI or "treated" firms) and the other does not (i.e., noninnovative or "untreated" firms). Second, multiple treatment effects models 8 compare not just two groups but several (e.g., EI versus TI versus NI firms). Because this multiple treatment effects approach provides more accurate and differentiated estimates across groups than propensity score matching, we chose it to assess differences in the perceived barriers of environmentally innovative SMEs, compared with those of technologically innovative SMEs and non-innovative ones. Such multiple treatments also are of particular interest and relevance when differential impacts within and across treatments are likely [START_REF] Cattaneo | Efficient Semiparametric Estimation of Multi-Valued Treatment Effects under Ignorability[END_REF]. Here, we expect different perceptions of barriers among SMEs, according to their innovation type.

Generally, estimations of innovation decisions are likely subject to endogeneity biases, because firms perceive innovation obstacles particularly when they are engaged in innovation, so that variables are not independent from the phenomenon itself. In cross-sectional data, variables related to R&D and innovation outcomes are generally endogenous [START_REF] Mairesse | Using Innovations Surveys for Econometric Analysis[END_REF], especially barriers [START_REF] Mohnen | Financial Constraints and Other Obstacles: Are They a Threat to Innovation Activity[END_REF]. In contrast, a multiple treatment 8 We thank an anonymous reviewer who suggested the multiple treatment effects methodology, which allowed us to extend the results that we obtained initially with propensity score matching. [START_REF] Morgan | Matching Estimators of Causal Effects Prospects and Pitfalls in Theory and Practice[END_REF][START_REF] Wooldridge | Inverse Probability Weighted M-Estimators for Sample Selection, Attrition, and Stratification[END_REF]. The independent variables used to match untreated with treated firms are exogenous and not affected by the treatment, in line with the requirements for matching and related techniques [START_REF] Imbens | Nonparametric Estimation of Average Treatment Effects under Exogeneity: A Review[END_REF].

Frequency Percent

Thus, to estimate the treatment effect, we apply a doubly robust model. 9

First, the multinomial logit estimates allow us to distinguish the effects of EI antecedents for each group of SMEs (EI, TI, and NI). It contains all dependent and independent variables previously mentioned, as indicated in the following equation: 10

Second, the results of this regression estimate the population average of the treatment effect on perceived barriers (ATET). This estimator allows us to measure the difference in the perceptions of environmentally innovative SMEs compared with those of technologically innovative and those of non-innovative SMEs. More formally, ATET estimates the causal effect of a treatment (having introduced EI) on an outcome (number and intensity of perceived barriers), and thus, it assesses the difference in perceived barriers by comparing treated SMEs (EI) with the control groups (TI or NI). We introduce barriers (outcome) variables that reflect the respondents' answers to a series of questions we designed to identify barriers to innova- 9 We use an inverse probability-weighted regression adjustment (ipwra), because the estimation method models both the impact variable (EI) and the treatment effect (barriers). This model actually combines two models: a regression adjustment model (ra) and an inverse probability-weighted model (ipw). These estimations have a double robust property, in that if either the outcome model (for estimating barriers' effect) or the treatment model (for EI) is correctly specified, the impacts are consistently estimated. Moreover, inverse probability weighting is a robust method that leads to efficient estimators [START_REF] Hirano | Efficient Estimation of Average Treatment Effects Using the Estimated Propensity Score[END_REF], and weighting by the inverse of the estimation is more efficient than using population probabilities of the treatment to estimate the average treatment effect [START_REF] Hirano | Efficient Estimation of Average Treatment Effects Using the Estimated Propensity Score[END_REF][START_REF] Rotnitzky | Semiparametric Regression Estimation in the Presence of Dependent Censoring[END_REF][START_REF] Wooldridge | Inverse Probability Weighted M-Estimators for Sample Selection, Attrition, and Stratification[END_REF]. 10 The equation is the same for technologically innovative SMEs, except that we substitute "EI" with "TI" in the previous formula.

tion, as perceived by SMEs' top managers. Each respondent indicated his or her perception of nine barriers to innovation, related to (1) excessive costs of innovation, (2) lack of external financial sourcing, (3) lack of internal financial sourcing, (4) domination of markets by insiders, (5) demand uncertainty, (6) lack of skilled employees, (7) lack of information about technologies, (8) lack of information and visibility on markets, and (9) difficulties in finding partners with which to innovate. Each barrier measure used a five-point scale, from 0 (very low perception) to 5 (very high perception). The perceptual measures involved two assessments, that is, the perceived intensity of the barrier (barrier's intensity), equal to the sum of all barrier scores by a respondent, and the number of perceived barriers (barriers' number), or the sum of high and very high barriers indicated by the respondent (see Table 3). Finally, we grouped the nine barriers to innovation into three theoretically coherent categories (financial, market, knowledge) and computed a measure of the intensity of each perceived barrier by category. We also summed the perceived barriers, to calculate the number of perceived barriers in each category. The variable definitions are in Table 3.

Table 3. Definitions of barriers (outcomes)

Outcome Definition Barriers' intensity Sum of all barriers' scores by a respondent, ranging from 0 to 45, because for each of the 9 barriers, potential intensity scores range from 1 (very low) to 5 (very high)

Barriers' number Sum of high or very high barriers perceived by the firm, from 0 to 9 Knowledge barriers' intensity Sum of knowledge barrier scores by a respondent, ranging from 0 to 20, because for each barrier (lack of skilled employees, lack of information on technologies, lack of information and visibility on markets, difficulties in finding partners), potential intensity scores go from 1 (very low) to 5 (very high)

Knowledge barriers' number Sum of high or very high knowledge barriers perceived by the firm, from 0 to 4

Financial barriers' intensity

Sum of financial barrier scores by a respondent, ranging from 0 to 15, because for each financial barrier (innovation costs too high, lack of external financial sourcing, lack of internal financial sourcing), potential intensity scores range from 1 (very low) to 5 (very high)

Financial barriers' number

Sum of high or very high financial barriers perceived by the firm, from 0 to 3

Market-related barriers' intensity

Sum of market-related barrier scores by a respondent, ranging from 0 to 10, because for each market-related barrier (market dominated by insiders, demand uncertainty), potential scores range from 1 (very low) to 5 (very high)

Market-related barriers' number Sum of high or very high market-related barriers perceived by the firm, from 0 to 2

FINDINGS

The results of the multinomial logit are in Table 4. As expected, environmental regulation has a positive effect on the probability of adopting EI, such that SMEs from polluting sectors are more likely to innovate environmentally [START_REF] Antonioli | Is Environmental Innovation Embedded within High-Performance Organisational Changes? The Role of Human Resource Management and Complementarity in Green Business Strategies[END_REF]. This effect is not significant for TI. Regulation has a binding effect on SMEs' EI decisions. The influence of environmental monitoring indicates that firms that are conscious of their environmental impact are more proactive in their EI. Quite logically, this variable has no effect on TI. Strategic behaviors also influence the likelihood of EI adoption [START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF]. Not surprisingly, an external growth strategy through merger and acquisitions affects environmental innovation negatively, because environmental innovation and the acquisition strategy both have important costs that make them exclusive (Hitt et al., 11 The correlation matrix is available on demand. It has not been included here, because there are no significant or important correlations that may disrupt our models.

1996). In contrast, this strategy neither impedes nor fosters TI. The results also confirm the importance of networks and open innovation for SMEs, especially when EI is concerned. As expected, joining a cluster and R&D cooperation strongly increase the probability that an SME introduces EI [START_REF] Cainelli | Environmental Innovations, Local Networks and Internationalization[END_REF]; merely belonging to a cluster favors TI [START_REF] Baptista | Do Firms in Clusters Innovate More?[END_REF].

Among the control variables, firm size has a significant positive impact on both EI and TI, but with a larger effect on EI compared to TI. Large SMEs therefore are more likely to innovate in the environmental field than are small ones, regardless of their age [START_REF] Cuerva | Drivers of Green and Non-Green Innovation: Empirical Evidence in Low-Tech SMEs[END_REF][START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF].Younger SMEs are more likely to adopt TI, whereas this effect is nonsignificant for EI SMEs. Finally, unlike environmentally innovative SMEs, export is significant for TI and appears to have a positive effect on TI [START_REF] Roper | Innovation and Export Performance: Evidence from the UK and German Manufacturing Plants[END_REF].

In addition to defining these EI and TI determinants, we compare barrier perceptions among the different groups of SMEs. Table 5 provides the descriptive statistics of the barriers; the perceived barrier estimation is given in Table 6. Environmentally innovative, technologically innovative, and non-innovative SMEs thus have different perceptions of barriers. Specifically, SMEs that have introduced EI perceive more barriers, with a stronger intensity, than the two other groups of SMEs (technologically innovative and non-innovative firms). Although their effect is significant, the lesser perceived barriers by EI SMEs are those related to the market. This result holds when we investigate perceived barriers in number and intensity too.

In particular, EI SMEs perceive more barriers, more intensively, than do NI ones.

When comparing EI SMEs with TI ones, differences in perceptions remain quite similar even though they are less pronounced. The only exceptions are the perception intensity of financial and market-related barriers, whose effects are not significant. With this original methodological approach, we effectively assess perceptions of barriers by EI and their specificities.

DISCUSSION AND CONCLUSION

This study is the first, to the best of our knowledge, to test for perceptions of barriers to environmental innovation among SMEs, in line with [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF] recent analysis of deterrent financial barriers for SMEs' environmental innovation activities. It provides several important results and contributions.

Theoretical implications

First, with regard to perceived barriers, SMEs engaged in EI believe that they face more barriers than other SMEs (those that pursue "dirty" technological innovations and noninnovators). They also perceive those barriers as more intense than do the other two groups of SMEs, except for financial and market-related barriers' intensity, which do not differ between EI and TI SMEs. These results indicate a key distinction of EI SMEs: Because of the complexity of EI, they must deal with many more dimensions than TI. Our research is in line with previous results that show that size affects eco-innovation propensity, emphasizing the difficulties small and medium enterprises face with regard to the complexity of environmental innovations and investments needed to switch to greener technologies [START_REF] Hemmelskamp | The Influence of Environmental Policy on Innovative Behaviour: an Econometric Study[END_REF].

Among these constraints, the number of financial barriers is critical for SMEs. In line with recent findings, we offer two explanations. On the one hand, without a consistent, pre-dictable policy framework, uncertainties in eco-investment profitability might increase, with new financial risks [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF]. On the other hand, systems failures [START_REF] Foxon | Overcoming Barriers to Innovation and Diffusion of Cleaner Technologies: some Features of a Sustainable Innovation Policy Regime[END_REF], such as in infrastructure provision and investment, technological transition, lockin, or restriction of financial credit for SMEs' EI, may contribute to affect EI SMEs' perceptions of financial burdens.

We also note that market barriers are less perceived than any other kind, perhaps because environmental innovation for SMEs is less market driven than are other innovations [START_REF] Horbach | Determinants of Environmental Innovation-New Evidence from German Panel Data Sources[END_REF]. However, they remain influential in number for EI SMEs, compared with TI SMEs, which suggests a context of demand uncertainty. Despite predictions about "green" market growth, demand often remains uncertain, because customers are not willing to pay more for environmentally friendly products or services [START_REF] Bianchi | Greening" SMEs' Competitiveness[END_REF][START_REF] Gabler | The Environmental Belief-Behaviour Gap: Exploring Barriers to Green Consumerism[END_REF][START_REF] Rennings | Redefining Innovation-Eco-Innovation Research and the Contribution from Ecological Economics[END_REF]. Environmental features also are often not easily detectable by end users [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF]. Moreover, market characteristics may facilitate or hinder the diffusion of environmental innovation [START_REF] Calleja | Promoting Environmental Technologies: Sectoral Analysis, Barriers and Measures[END_REF]. Due to the complexity and "systemicness" of environmental innovations, market and technological uncertainties that characterize many environmental technologies may be perceived as important, because there are no widely accepted standards, in terms of either specific technological solutions or measures, to evaluate the environmental performance of products and processes [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF]. These aspects also explain our results linked to perceptions of knowledge-related barriers.

Second, EI SMEs perceive knowledge barriers as more intense and more numerous than TI SMEs, possibly due to the higher level of complexity and novelty of the knowledge required to innovate [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF][START_REF] Petruzzelli | Organizational Factors and Technological Features in the Development of Green Innovations: Evidence from Patent Analysis[END_REF] but also because EI is more knowledge and information intensive [START_REF] Horbach | Determinants and Specificities of Eco-Innovations Compared to Other Innovations-an Econometric Analysis for the French and German Industry Based on the Community Innovation Survey[END_REF]. Environmental innovation often relies on knowledge and competences that are not core to the firms [START_REF] De Marchi | Environmental Innovation and R&D Cooperation: Empirical Evidence from Spanish Manufacturing Firms[END_REF][START_REF] Marin | SMEs and Barriers to Eco-Innovation in EU: A Diverse Palette of Greens[END_REF]. Environmental features also may require sophis-ticated technical knowledge, such that EI represents a technological frontier at which firms continue to lack experience (De Marchi, 2012)-especially SMEs. In such small firms, CEOs may lack knowledge and expertise about subjects related to environmental innovation, which is strengthened by the lack of suitable information [START_REF] Walker | Small and Medium Enterprises and the Environment: Barriers, Drivers, Innovation and Best Practice: A Review of the Literature[END_REF]. Our results also align with Horbach et al.'s (2013) assertion that EI relies more on important external sources of knowledge than do other innovations. Firms engaged in collective actions (e.g., R&D cooperation, cluster membership) are more likely to introduce EI, possibly because they enjoy information and knowledge diffusion about the benefits of EI, as well as advice and assistance from partners or other cluster members.

Third, regarding the antecedents of EI, we confirm the effect of regulation, in that firms in polluting sectors tend to introduce more environmental innovations. Beyond these regulatory aspects, firms that have the highest probability to introduce environmental innovations are those that are the most mature in their environmental strategy. Three major antecedents relate to firms' strategy: belonging to a cluster, R&D cooperation, and environmental monitoring. Although the logit estimation shows that environmental innovations are driven by firms' strategic behavior, defensive motives (e.g., decreasing costs and risks, complying with regulation) emerge as important motives (e.g., stimulating growth). For example, practices and tools designed to reduce environmental costs favor EI. Similarly, SMEs operating in polluting sectors are more likely to introduce EI. These two results in turn suggest that coercive and mimetic pressures [START_REF] Dimaggio | The Iron Cage Revisited: Institutional Isomorphism and Collective Rationality in Organizational Fields[END_REF]) are crucial levers of SMEs' environmental adoption. Regulations represent a significant coercive pressure, and regulatory efforts are effective for guiding green behaviors. These findings also support [START_REF] Porter | Location, Competition, and Economic Development: Local Clusters in a Global Economy[END_REF] hypothesis, initially developed for large firms, both theoretically and in successive empirical studies with SMEs. Suitable regulation even favors SMEs' EI and may compensate for the related costs [START_REF] Porter | Toward a New Conception of the Environment-Competitiveness Relationship[END_REF], through enhanced innovation activities that accord with the firm's strategy. Our results thus reaffirm Porter's dynamic vision of the link among public policy, strategic behavior, and innovation.

As in any study, our findings are subject to several caveats. We did not separate product and process environmental innovations, so further research should delineate whether barriers differ with the type of environmental innovation (process/product) or its beneficiary (firm/client). Nor did we distinguish incremental from radical innovations; incremental innovation is much less resource and competency demanding than is radical innovation, which destroys previous products and skills.12 It would be interesting to compare our results with findings obtained from a sample of large firms. Market barriers, for example, could have more substantial influences on large firms. Further research could consider the impact of the managers' profiles too, which tend to determine SMEs' strategies. Finally, interactional effects among different categories of barriers could be studied to determine if barriers are interrelated [START_REF] Ashford | Understanding Technological Responses of Industrial Firms to Environmental Problems: Implications for Government Policy[END_REF]. Such extensions are critical, because of the importance of EI for the sustainable growth of both economies and societies. In the meantime, our study provides interesting insights that may help managers and policy makers.

Managerial implications

All SMEs should have strategic goals to facilitate their adoption of EI. Our study reveals that perceptions are as important as objective barriers. Because these perceptions are linked to the experience of the manager, managers must acknowledge the importance of networks and open innovation for SMEs [START_REF] Chesbrough | Open Innovation: The New Imperative for Creating and Profiting from Technology[END_REF][START_REF] Horbach | Determinants and Specificities of Eco-Innovations Compared to Other Innovations-an Econometric Analysis for the French and German Industry Based on the Community Innovation Survey[END_REF][START_REF] Lichtenthaler | A Capability-Based Framework for Open Innovation: Complementing Absorptive Capacity[END_REF]. To decrease their perceptions of the number and intensity of barriers to this type of innovation, SMEs should engage in collective actions. By joining a cluster, SMEs enter an innovative environment, which may favor EI [START_REF] Cainelli | Environmental Innovations, Local Networks and Internationalization[END_REF]. Small firms already face informational and knowledge asymmetries, leaving them underinformed about public subsidies and environmental innovation strategies. Public subsidies are also unequally distributed, such that they tend to benefit firms with either very minimal or very intense innovation activities [START_REF] Blanes | Who Participates in R&D Subsidy Programs? The Case of Spanish Manufacturing Firms[END_REF]. [START_REF] Horbach | Determinants and Specificities of Eco-Innovations Compared to Other Innovations-an Econometric Analysis for the French and German Industry Based on the Community Innovation Survey[END_REF] demonstrate that eco-innovation activities require more information and knowledge than non-environmental innovation. Public bodies should take initiatives to inform non-innovative firms about the opportunities and subsidies associated with EI, to encourage greater adoption.

Public policy implications

In gathering SMEs' perceptions, we reveal that managers must perceive viable strategic opportunities before they will engage in EI, because overall, SMEs lack knowledge about environmental and sustainability issues and specific practices they could implement. They also lack understanding of environmental problems and risks and the potential benefits of environmental improvements [START_REF] Walker | Small and Medium Enterprises and the Environment: Barriers, Drivers, Innovation and Best Practice: A Review of the Literature[END_REF]. They do not have expertise or knowledge about environmental issues, as confirmed by our results pertaining to overall perceptions of more numerous and more intense barriers, especially in terms of knowledge. Finally, they see environmental responsibility as too costly [START_REF] Walker | Small and Medium Enterprises and the Environment: Barriers, Drivers, Innovation and Best Practice: A Review of the Literature[END_REF], so financial barriers are deterrents to EI [START_REF] Ghisetti | Do Financial Constraints Make the Environment Worse Off? Understanding the Effects of Financial Barriers on Environmental Innovations[END_REF]. Thus, public policies have important roles to play: They must raise firms' awareness, provide necessary information [START_REF] Porter | Toward a New Conception of the Environment-Competitiveness Relationship[END_REF], and reduce uncertainties. Such policies should be oriented toward changing perceptions instead of just providing incentives, tools, or instruments. Efforts to support EI might involve information diffusion, technology transfer, or public-private partnerships, because such undertakings can reduce barriers' representation.

At a more local level, the crucial role of collective engagement, which enables firms to benefit from knowledge externalities, suggests that French competitiveness clusters can effectively support EI. Clusters are not only privileged sites for information diffusion but also places to organize and manage knowledge [START_REF] Bocquet | The Role of Cluster Governance in External Knowledge Integration: The Case of Small Firms[END_REF]. The traits that define port to engage in EI, especially because the market incentives are not as powerful for them.

By supporting the cluster policy, our research demonstrates that public policies should focus less on financial help, subsidies, or concrete and objective measures and more on shaping and modeling SME CEOs' perceptions of their environment and the potential benefits of environmental innovations. In clusters, they might rely on experience sharing; entrepreneurial perceptions partly originate from entrepreneurs' experiences in specific business settings, as defined by the industry [START_REF] Kor | Resources, Capabilities and Entrepreneurial Perceptions[END_REF]. Because of SMEs' importance in the European industry, such an approach could foster macroeconomic sustainable development by emphasizing ecological and environmentally friendly innovations.

  and more likely to elaborate and adopt a proactive environmental strategy (Aguilera-Caracuel, Hurtado-Torres, & Aragón-Correa, 2012). Finally, SMEs have more flexibility than large firms, due to their structure and size, which increases their reactivity (Aragón-Correa, Hurtado-Torres, Sharma, & García-Morales, 2008); Madrid-Guijarro et al. (2009) also note that less bureaucracy has a positive effect on EI.

H1:

  Barriers to EI are perceived as more numerous by environmentally innovative SMEs than by (a) technologically innovative SMEs and (b) non-innovating SMEs. H2: Barriers to EI are perceived as more intense by environmentally innovative SMEs than by (a) technologically innovative and (b) non-innovating SMEs.DATA AND METHODSDataWe used data from two main sources, both pertaining to French SMEs located in the Rhône-Alpes region. This region exhibits important research and innovation activity; it ranks second in the nation in terms of research potential (after the Paris region). With a specially designed survey, 5 conducted in 2012, we asked SMEs' top managers for information about different types of innovation activity (technological, non-technological, with environmental benefits for the firm or end users). The questions paralleled those included in the 2008 CIS. Moreover, the survey provided detailed information about SMEs' sources of innovation and perceptions of barriers. The focal period was 2009-2011, though questions related to general firm information specified that answers should reflect 2011 values. We obtained 671 completed questionnaires. In addition, we referred to the Orbis database, which gathered balance sheet information for all SMEs located in the Rhône-Alpes region; we used those from 2009-2011. As recommended by[START_REF] Arundel | Measuring Eco-Innovation[END_REF], we linked our unique Rhône-Alpes data set to these official data that included financial information, to ensure higher reliability. After merging the two databases, we obtained a final balanced sample of 435 French SMEs, each of which employed between 10 and 249 people. The final data set is representative of SMEs located in the Rhône-Alpes region, across firm size and sector affiliation, though manufacturing SMEs are slightly overrepresented (see Appendix 1).

Table 1 .

 1 Variables in the multinomial logit model

	Variables	Definition
	Innovation	=0 if the firm did not innovate in the last three years;
		=1 if the firm has introduced technological innovation in the last
		three years;
		=2 if the firm has introduced environmental in the last three
		years.
	Polluting sector	= 1 if the firm is part of a polluting sector, 0 otherwise
	Environmental	= 1 if the firm measures its environmental impact (e.g., environ-
	monitoring	mental audits, ISO 14001), 0 otherwise
	External growth	= 1 if the firm engages in an external growth strategy (mergers
		and acquisitions), 0 otherwise
	Cluster	= 1 if the firm belongs to a cluster, 0 otherwise
	R&D cooperation	= 1 if the firm cooperates in R&D with other firms, 0 otherwise
	Firm Size	Logarithm of firm size (number of employees) in 2011
	Firm Age	Logarithm of firm age in 2012
	Export	= 1 if the firm exports, 0 otherwise
	Group	=1 if the firm belongs to a group, 0 otherwise
	Debt ratio	= sum of long-term debts and loans, divided by shareholders'
		funds and provisions in 2010
	Services	= 1 if the firm is from the services sector, 0 otherwise
	Manufacturing	=1 if the firm is from the manufacturing sector, 0 otherwise

Table 4 .

 4 Results of the multinomial logit model 11

		EI -Robust	TI -Robust	Differences
	Variable	Coefficient	Standard Error	Coefficient	Standard Error	between EI and TI
	Polluting sector	0.674	0.384*	0.645	0.398ns	ns
	Environmental monitoring	0.735	0.356**	-0.108	0.398ns	significant
	External growth	-0.574	0.326*	0.364	0.297ns	significant
	Cluster	1.750	0.567***	1.352	0.565**	significant
	R&D cooperation	0.479	0.280*	0.268	0.276ns	significant
	Firm size	0.471	0.183**	0.455	0.179**	ns
	Firm age	0.002	0.151ns	-0.311	0.144**	significant
	Export	0.460	0.280ns	0.699	0.278**	significant
	Group	-1.046	0.643ns	-0.619	0.561ns	significant
	Debt ratio	-0.031	0.131ns	-0.112	0.127ns	significant
	Services	-0.088	0.273ns	0.204	0.271ns	significant
	_cons	-2.060	0.738***	-1.305	0.714ns	significant
	Log likelihood			-436.375
	No. of obs.			435
	Pseudo R²			0.0867

Notes:

The models all pass the test of the independence of irrelevant alternatives (IIA). ***Significant at 1%. **Significant at 5%. *Significant at 10%. ns: non-significant.

Table 5 .

 5 Descriptive statistics of barriersNotes: We calculated the mean of each perceived barrier and compared the scores for EI, TI and NI SMEs. For example, the mean for barriers' intensity (ranging from 0 to 45) is 19.894 for EI SMEs which scores higher than for TI and NI ones. T-test, previously made, indicate that differences are statistically significant (except between TI and NI SMEs for Market-related barriers' intensity).

	Outcomes

EI Firms, Means (SD) TI Firms, Means (SD) NI Firms, Means (SD)

	Barriers' intensity	19.894 (9.628)	17.417 (10.225)	14.302 (10.395)
	Barriers' number	7.099 (3.032)	6.451 (3.319)	5.497 (3.506)
	Knowledge barriers' intensity	8.873 (4.639)	7.688 (4.651)	6.443 (4.483)
	Knowledge barriers' number	3.204 (1.366)	2.938 (1.511)	2.523 (1.553)
	Financial barriers' intensity	6.697 (4.241)	5.958 (4.455)	4.416 (4.454)
	Financial barriers' number	2.324 (1.133)	2.125 (1.234)	1.738 (1.358)
	Market-related barriers' intensity	4.324 (2.665)	3.771 (2.944)	3.443 (2.974)
	Market-related barriers' number	1.570 (0.766)	1.389 (0.870)	1.235 (0.881)
	Number of observations	142	144	149

Table 6 .

 6 Comparison of the perceived barriers (ATET)

	Notes: The standard deviations, in brackets, are robust.
	***Significant at 1%. **Significant at 5%. *Significant at 10%. ns: non-significant.

  Taken together, these results provide strong support for H1, insofar as barriers are perceived as more numerous by environmentally innovative SMEs than by technologically innovative or non-innovating SMEs. In contrast, we find support for H2 for the aggregate results but only partially, because barriers are not all perceived more intensely by environmentally innovative SMEs compared with the two other groups of SMEs. That is, EI SMEs always perceive barriers more intensely than non-innovative SMEs but not in comparison with TI SMEs. Thus, the results reveal a double specificity of perceived barriers between EI SMEs and other firms. Differences in the perceptions of EI SMEs are a matter of intensity and number compared with those of NI SMEs, but they differ mainly in number compared with TI SMEs. These results reflect a robust test of ATET estimates, which provide the net, average perceptions of barriers by EI SMEs compared with two control group SMEs (TI and NI).

		Barriers'	Barrier's	Knowledge	Knowledge	Financial	Financial	Market-	Market-
		intensity	number	barriers'	barriers'	barriers'	barriers'	related	related
				intensity	number	intensity	number	barriers'	barriers'
								intensity	number
	EI vs. NI 4.686***	1.224***	2.094***	0.543***	1.930***	0.452***	0.662*	0.229**
		(1.497)	(0.469)	(0.683)	(0.208)	(0.659)	(0.173)	(0.403)	(0.114)
	EI vs.	3.064*	1.164**	1.377*	0.562**	1.017ns	0.337*	0.670ns	0.264**
	TI	(1.649)	(0.514)	(0.724)	(0.226)	(0.686)	(0.189)	(0.418)	(0.129)
	TI vs. NI 2.287*	0.556ns	0.921ns	0.275ns	1.290**	0.244ns	0.076ns	0.037ns
		(1.286)	(0.420)	(0.580)	(0.186)	(0.604)	(0.161)	(0.369)	(0.108)

Such perceptions may be subject to decision-making biases though, especially under uncertainty[START_REF] Busenitz | Biases and Heuristics in Strategic Decision-Making: Differences between Entrepreneurs and Managers in Large Organizations[END_REF].

This classification is now well accepted, but it raises some challenges, because regulation may support both supply (by improving infrastructure and/or public R&D) and market forces, such as through public procurement[START_REF] Rennings | Increasing Energy and Resource Efficiency through Innovation -An Explorative Analysis Using Innovation Survey Data[END_REF].

A discussion of the determinants of obstacles to innovation is beyond the scope of this study (see[START_REF] Baldwin | Impediments to Advanced Technology Adoption for Canadian Manufacturers[END_REF][START_REF] D'este | What Hampers Innovation? Revealed Barriers versus Deterring Barriers[END_REF][START_REF] Galia | Complementarities between Obstacles to Innovation: Evidence from France[END_REF][START_REF] Iammarino | The Perception of Obstacles to Innovation. Foreign Multinationals and Domestic Firms in Italy[END_REF][START_REF] Schneider | On Young Highly Innovative Companies: Why They Matter and How (not) to Policy Support Them[END_REF][START_REF] Tourigny | Impediments to Innovation Faced by Canadian Manufacturing Firms[END_REF].

Because the CIS surveys do not provide all necessary information (e.g., CIS 2006 only provides information on barriers, CIS 2008 only provides EI information), we conducted this unique survey to gather both pieces of information simultaneously from firms located in the Rhône-Alpes region.

Accounting for government support may introduce an endogeneity bias, because subsidized firms may have characteristics that make them distinct from other firms[START_REF] Mairesse | Using Innovations Surveys for Econometric Analysis[END_REF], and other firms may not be recipients during the sample period, which would introduce an additional bias. Moreover, SMEs receive only 9% of all public subsidies dedicated to R&D by the French government[START_REF] Lhuillery | Evaluation de l'Impact des Aides Directes et Indirectes à la R&D en France[END_REF]. We therefore chose to not introduce government support for innovation.

7 Here, "belonging to a cluster" means being a dues-paying member. French clusters are unique, in that they are more than just geographic concentrations (cf.[START_REF] Porter | Location, Competition, and Economic Development: Local Clusters in a Global Economy[END_REF]. Created by the French government in 2005, they are led by an organization or formal governance. Even if geographically speaking, a firm appears in the cluster's territory, it belongs to that cluster only if it pays a membership fee; in return, it benefits from a series of actions determined by the cluster's governance related to innovation, human resource programs, commercial development, and so on. We thank an anonymous reviewer for suggesting this explanation.

We thank an anonymous reviewer for this remark.