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Strategic Petroleum Reserve (United States)

The Strategic Petroleum Reserve (SPR) is an emergency stockpile of petroleum maintained by the United States Department of Energy (DOE). It is the largest publicly known emergency supply in the world; its underground tanks in Louisiana and Texas have capacity for 714 Moilbbl. The United States started the petroleum reserve in 1975 to mitigate future supply disruptions as part of the international Agreement on an International Energy Program, after oil supplies were interrupted during the 1973–1974 oil embargo.

The current inventory is displayed on the SPR's website. , the inventory was 346.8 Moilbbl. This equates to about days of oil at 2019 daily U.S. consumption levels of 20.54 Moilbbl/d or  days of oil at 2019 daily U.S. import levels of 9.141 Moilbbl/d. However, the maximum total withdrawal capability from the SPR is only 4.4 Moilbbl/d, so it would take about days to use the entire inventory. At recent market prices ($58 a barrel as of March 2021), the SPR holds over $14.6 billion in sweet crude and approximately $18.3 billion in sour crude (assuming a $15/barrel discount for sulfur content). In 2012, the total value of the crude in the SPR was approximately $43.5 billion, while the price paid for the oil was $20.1 billion (an average of $28.42 per barrel).

Since 2015 Congress has mandated sales of oil from the reserve to fund federal spending. The U.S. Department of Energy has run at least seven sales since 2017, selling 132 million barrels, or about 18.2% of what had been in the reserve.

On March 31, 2022, President Joe Biden announced that his administration would release 1 million barrels of oil per day from the reserve for the next 180 days, selling it at an average price of $96 per barrel. The 2022 release became the largest ever SPR sale and lowered the SPR to its lowest levels in 40 years. After oil prices declined during the second half of the year, in December the administration announced it would begin replenishing the SPR in early 2023, expecting to purchase oil at a lower price than it was sold, a process that would take months or years to complete. The Biden administration continued to release reserves in 2023, selling off 45% of the SPR by September 2023. The Department of Energy unveiled plans to purchase oil at a price of $79 per barrel or less, but has since purchased oil at higher prices. As of April 2024, further purchases were cancelled due to rising oil prices.

According to legislation already in place, the amount of oil in the reserve could fall to 238 million barrels by 2028. This will be a 67% reduction to the oil in the reservoir since 2010.

Facilities
The SPR management office is located in Elmwood, Louisiana, a suburb of New Orleans.

The reserve is stored at four sites on the Gulf of Mexico, each located near a major center of petrochemical refining and processing. Each site contains a number of artificial caverns created in salt domes below the surface.

Individual caverns within a site can be up to 1000 m below the surface. Average dimensions are 60 m wide and 600 m deep; capacity ranges from 6 to 37 Moilbbl. Almost $4 billion was spent on the facilities. The decision to store in caverns was made to reduce costs. The Department of Energy claims that it is approximately ten times more cost effective to store oil below the surface, with the added advantages of no leaks and a constant natural churn of the oil due to a temperature gradient in the caverns. The caverns were created by drilling down and then dissolving the salt with water.

Existing

 * Bryan Mound: Freeport, Texas. 20 caverns with a storage capacity of 254 Moilbbl with a drawdown capacity of 1.5 Moilbbl per day.
 * Big Hill: Winnie, Texas. Has a capacity of 160 Moilbbl with a drawdown capacity of 1.1 Moilbbl per day.
 * West Hackberry: Lake Charles, Louisiana. Has a capacity of 227 Moilbbl with a drawdown capacity of 1.3 Moilbbl per day.
 * Bayou Choctaw: Baton Rouge, Louisiana. Has a capacity of 76 Moilbbl with a maximum drawdown rate of 550000 oilbbl per day.

Proposed

 * Richton, Mississippi: This facility, if built as planned, would have had a capacity of 160 Moilbbl with a drawdown capacity of 1 Moilbbl per day. Former Secretary of Energy Samuel Bodman announced the creation of this site in February 2007. As of 2008, this site was facing some opposition. According to the DOE: "Activities towards the goal of expansion of the SPR to one billion barrels, as directed by Congress in the 2005 Act, were cancelled in 2011 after Congress rescinded all remaining expansion funds."

Retired

 * Weeks Island: Iberia Parish, Louisiana (decommissioned 1999): Capacity of 72 Moilbbl. This facility was a conventional room and pillar near-surface salt mine, formerly owned by Morton Salt. In 1993, a sinkhole formed on the site, allowing fresh water to intrude into the mine. Because of the mine's construction in salt deposits, fresh water would erode the ceiling, potentially causing the structure to fail. The mine was backfilled with salt-saturated brine. This process, which allowed for recovery of 98% of the petroleum stored in the facility, reduced the risk of further fresh water intrusion, and helped prevent the remaining oil from leaking into the aquifer that is located over the salt dome.

Background
The SPR was created following the 1973 energy crisis. On November 18, 1974, the United States became a signatory to the Agreement on an International Energy Program (IEP) and a founding member of the International Energy Agency that the IEP established. One of the key commitments made by the treaty's signatories is to maintain oil stocks of no less than 90 days of net imports.

Access to the reserve is determined by the conditions written into the 1975 Energy Policy and Conservation Act (EPCA), primarily to counter a severe supply interruption. The maximum removal rate, by physical constraints, is 4.4 Moilbbl/d. Oil could begin entering the marketplace 13 days after a presidential order. The Department of Energy says it has about 59 days of import protection in the SPR. This, combined with private sector inventory protection, is estimated to equal 115 days of imports.

The EPCA of December 22, 1975, made it policy for the United States to establish a reserve up to 1 billion barrels (159 million m³) of petroleum. A number of existing storage sites were acquired in 1977. Construction of the first surface facilities began in June 1977. On July 21, 1977, the first oil—approximately 412000 oilbbl of Saudi Arabian light crude—was delivered to the SPR. Fill was suspended in Fiscal Year 1995 to devote budget resources to refurbishing the SPR equipment and extending the life of the complex.

Domestic actions
On November 13, 2001, shortly after the September 11 terrorist attacks, President George W. Bush announced that the SPR would be filled, saying, "The Strategic Petroleum Reserve is an important element of our Nation's energy security. To maximize long-term protection against oil supply disruptions, I am directing the Secretary of Energy to fill the SPR up to its 700 Moilbbl capacity." The highest prior level was reached in 1994 with 592 Moilbbl. At the time of President Bush's directive, the SPR contained about 545 Moilbbl. Since the directive in 2001, the capacity of the SPR has increased by 27 Moilbbl due to natural enlargement of the salt caverns in which the reserves are stored. The Energy Policy Act of 2005 has since directed the Secretary of Energy to fill the SPR to the full 1 Goilbbl authorized capacity, a process which will require a physical expansion of the Reserve's facilities.

On August 17, 2005, the SPR reached its goal of 700 Moilbbl. Approximately 60% of the crude oil in the reserve is the less desirable sour (high sulfur content) variety. The oil delivered to the reserve is "royalty-in-kind" oil—royalties owed to the U.S. government by operators who acquire leases on the federally owned Outer Continental Shelf in the Gulf of Mexico. These royalties were previously collected as cash, but in 1998 the government began testing the effectiveness of collecting royalties "in kind"—or in other words, acquiring the crude oil itself. This mechanism was adopted when refilling the SPR began, and once filling is completed, revenues from the sale of future royalties will be paid into the federal treasury.

On April 25, 2006, President Bush announced a temporary halt to petroleum deposits to the SPR as part of a four-point program to alleviate high fuel prices.

On January 23, 2007, President Bush suggested in his State of the Union speech that Congress should approve expansion of the current reserve capacity to twice its current level.

On May 16, 2008, the U.S. Department of Energy (DOE) said it would halt all deliveries to the Strategic Petroleum Reserve sometime in July. This announcement came days after Congress voted to direct the Bush administration to do the same.

On January 2, 2009, after a sharp decline in fuel prices, DOE said that it would begin buying approximately 12000000 oilbbl of crude oil to fill the Strategic Petroleum Reserve, replenishing supplies that were sold after hurricanes Katrina and Rita in 2005. The purchase would be funded by the roughly $600 million received from those emergency sales.

On September 9, 2011, a Notice of Cancellation was published in the Federal Register after Congress rescinded funding for the expansion of the Strategic Petroleum Reserve, reversing the SPR expansion initiative previously directed under the Energy Policy Act of 2005.

On October 20, 2014, a report by the U.S. Government Accountability Office (GAO) recommended reducing the size of the Reserve. According to the report, the amount of oil held in reserve exceeds the amount required to be kept on hand given the need for imported crude oil had decreased in recent years. The report said the DOE agreed with the GAO recommendation.

On March 19, 2020, President Donald Trump directed the Department of Energy to fill the Strategic Petroleum Reserve to maximum capacity, when oil was priced at $24 per barrel. This directive was given to help support domestic oil producers given the impending economic collapse from COVID-19 and extreme drops in international oil markets. However, funding was blocked by Congress, with Senate Democratic Leader Chuck Schumer stating that Democrats had blocked a "bailout for big oil".

In 2022, the Biden administration sold 180 million barrels of oil from the Strategic Petroleum Reserve over 6 months. The sale lowered the SPR to its lowest levels in 40 years and was the largest ever release of oil from the SPR. SPR drawdowns continued into 2023, lowering the SPR by 45% from January 2021. Since the 2022 sales, the Department of Energy bought back 32.3 million barrels of oil, aiming to purchase at a maximum price of $79 per barrel. However, oil has been purchased exceeding the $79 price cap in March 2024. As of April 2024, further SPR purchases were cancelled due to rising oil prices.

Emergency sales to Israel
According to the 1975 Sinai Interim Agreement signed by the United States and Israel, as a precondition for Israel's return of the Sinai Peninsula and its associated oil reserves to Egypt, in an emergency the United States was obligated to make oil available for sale to Israel for up to five years. Israel has never invoked the agreement, however. The agreement was extended in 1979, 1994, 2004, and, most recently, in 2015 for a ten-year period.

International obligations
As a member of the International Energy Agency (IEA), the United States must stock an amount of petroleum equivalent to at least 90 days of U.S. imports. The SPR contained an equivalent to 141 days of imports as of September 2016. The United States is also obligated to contribute 43.9% of petroleum in any IEA-coordinated release.

Limitations
The Strategic Petroleum Reserve is primarily a crude petroleum reserve, not a stockpile of refined petroleum fuels such as gasoline, diesel and kerosene. Although the United States maintains some extra supply of refined petroleum fuels, e.g., the Northeast Home Heating Oil Reserve and Northeast Gasoline Supply Reserve under the aegis of the Department of Energy (DOE), the government does not maintain gasoline reserves on anything like the scale of the SPR. The SPR is intended to give the United States protection from disruptions in oil supplies. In the event of a major disruption to refinery operations, the United States would have to call on members of the International Energy Agency that stockpile refined products, and use refining capacities outside of the continental United States for relief.

There have been suggestions that the DOE should increase its supplies and stockpile both gasoline and jet fuel. Some countries and zones have a strategic reserve of both petroleum and petroleum products. In some cases, this includes a strategic reserve of jet fuel.

Former Secretary of Energy Samuel Bodman said that the Department would consider new facilities for refined products as part of an expansion of 1 to 1.5 Goilbbl.

Petroleum sales - Prior to 2015

 * 1985: Test sale—1.1 Moilbbl
 * 1990–1991: Desert Storm sale—21 Moilbbl
 * 4 Moilbbl in October 1990 test sale
 * 17 Moilbbl in January 1991 presidentially ordered drawdown
 * 1996–1997: 28 Moilbbl non-emergency sales for deficit reduction
 * July–August 2000: 2.8 Moilbbl to supply the Northeast Home Heating Oil Reserve.
 * September–October 2000: 30 Moilbbl in response to a concern over low distillate levels in the northeastern U.S.
 * 2005 Hurricane Katrina sale: 11 Moilbbl—Katrina shut down 95% of crude production and 88% of natural gas output in the Gulf of Mexico. This amounted to a quarter of total U.S. output.  About 735 oil and natural gas rigs and platforms had been evacuated due to the hurricane.
 * 2011 Arab Spring sale: 30 Moilbbl—non-emergency sale to offset disruptions caused by political upheaval in Libya and elsewhere in the Middle East. The amount was matched by IEA countries, for a total of 60 Moilbbl released from stockpiles around the world.

Petroleum exchanges and loans
''Note: Loans are made on a case-by-case basis to alleviate supply disruptions. Once conditions return to normal, the loan is returned to the SPR with additional oil as interest.''
 * April–May 1996: 900000 oilbbl lent to ARCO to alleviate pipeline blockage.
 * August 1998: 11 Moilbbl lent to PEMEX in return for 8.5 Moilbbl of higher-quality crude.
 * June 2000: 1 Moilbbl lent to Citgo and Conoco in response to shipping channel blockage.
 * October 2002: 296000 oilbbl lent to Shell Pipeline Company in advance of Hurricane Lili.
 * September–October 2004: 5.4 Moilbbl lent to Astra Oil, ConocoPhillips, Placid Refining, Shell Oil Company, and Premcor after Hurricane Ivan.
 * September–October 2005: 9.8 Moilbbl lent to ExxonMobil, Placid Refining, Valero, BP, Marathon Oil, and TotalEnergies after Hurricane Katrina. Purchases of crude oil would then resume in January 2009 using revenues available from the 2005 Hurricane Katrina emergency sale. The DOE purchased 10.7 Moilbbl at a cost of $553 million.
 * January–February 2006: 767000 oilbbl lent to Total Petrochemicals USA due to closure of the Sabine–Neches Waterway to deep-draft vessels after a barge accident in the channel.
 * June 2006: 750000 oilbbl of sour crude lent to ConocoPhillips and Citgo due to the closure for several days of the Calcasieu Ship Channel caused by the release of a mixture of storm water and oil. Repaid in early October 2006.
 * September 2008: 630000 oilbbl lent to Citgo, Placid Refining, and Marathon Oil due to disruptions from Hurricane Gustav.
 * August 2017: 200,000 barrels of sweet crude and 300,000 barrels of sour crude were lent to Phillips 66 due to disruptions from Hurricane Harvey.

Drawdowns since 2015
Since 2015, Congress has been selling the oil in the reserve to fund the deficit, in unpublicized sales. The U.S. Department of Energy has run seven sales since 2017, selling more than 132 million barrels, or about 18.2% of what had been in the reserve.

According to legislation already in place, the amount of oil in the reserve could fall to as little as 238 million barrels by 2028. This will be a 67% reduction of oil in the reserve since 2010.

The legislation is summarized below:

 * The Bipartisan Budget Act (Section 404), enacted in 2015, includes authorization for funding an SPR modernization program to support improvements deemed necessary to preserve the long-term integrity and utility of SPR's infrastructure by selling up to $2 billion worth of SPR crude oil in fiscal years 2017 through 2020. Although the estimated volumes presented in the chart above are based on an assumed oil price of $50 per barrel, the actual final sales volumes will depend on how SPR decides to allocate the sales volumes across those fiscal years and the actual price of crude oil at the time of the sales. For the Section 404 sales, SPR must get an appropriation from Congress to approve its requested sales revenue target.
 * Another section of the Bipartisan Budget Act (Section 403), enacted in 2015, mandates SPR crude oil sales for fiscal years 2018 through 2025 on a volumetric basis, rather than on a dollar basis, as specified in Section 404. The revenues from sales authorized under section 403 will be deposited into the general fund of the U.S. Department of the Treasury.
 * The Fixing America’s Surface Transportation Act, enacted in December 2015, calls for SPR sales totaling 66 million barrels from fiscal years 2023 through 2025.
 * The 21st Century Cures Act, enacted in December 2016, calls for the sale of 25 million barrels of SPR crude oil for fiscal years 2017 through 2019. The first portion of these sales is expected in late spring 2017.
 * In December 2016, the DOE announced it would begin the sale of 190 Moilbbl in January 2017.
 * The Tax Cuts and Jobs Act of 2017, enacted in December 2017, calls for the sale of 7 million barrels over the two-year period of FY 2026 through FY 2027.
 * The Bipartisan Budget Act of 2018, enacted in February 2018, calls for the sale of 30 million barrels over the four-year period of FY 2022 through FY 2025, 35 million barrels in FY 2026, and 35 million barrels in FY 2027.
 * In November 2021, the White House announced the release of 50 Moilbbl to address high gasoline prices.
 * On March 1, 2022, President Biden announced the release of 30 million barrels of oil from the reserve in response to Russia's invasion of Ukraine.
 * On March 31, 2022, President Biden announced that his administration would release 1 million barrels of oil per day from the reserve for the next 180 days.


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Acute accent

The acute accent, ◌́ , is a diacritic used in many modern written languages with alphabets based on the Latin, Cyrillic, and Greek scripts. For the most commonly encountered uses of the accent in the Latin and Greek alphabets, precomposed characters are available.

History
An early precursor of the acute accent was the apex, used in Latin inscriptions to mark long vowels.

Ancient Greek
The acute accent was first used in the polytonic orthography of Ancient Greek, where it indicated a syllable with a high pitch. In Modern Greek, a stress accent has replaced the pitch accent, and the acute marks the stressed syllable of a word. The Greek name of the accented syllable was and is (oxeîa, Modern Greek oxía) "sharp" or "high", which was calqued (loan-translated) into Latin as  "sharpened".

Stress
The acute accent marks the stressed vowel of a word in several languages:
 * Asturian
 * Blackfoot uses acute accents to show the place of stress in a word, for example, soyópokistsi.
 * Bulgarian: stress, which is variable in Bulgarian, is not usually indicated in Bulgarian except in dictionaries and sometimes in homonyms that are distinguished only by stress. However, Bulgarian usually uses the grave accent to mark the vowel in a stressed syllable, unlike Russian and Ukrainian, which use the acute accent.
 * Catalan uses it in stressed vowels: é, í, ó, ú.
 * Dutch uses it to mark stress (vóórkomen – voorkómen, meaning occur and prevent respectively) or a more closed vowel (hé – hè, equivalent to English hey and heh) if it is not clear from context. Sometimes, it is simply used for disambiguation, as in één – een, meaning "one" and "a(n)".
 * Galician
 * Hopi has acute to mark a higher tone.
 * Irish uses the accent, called a síneadh fada in Irish, to indicate a long vowel. It is commonly referred to simply as a fada.
 * Italian The accent is used to indicate the stress in a word, or whether the vowel is "open" or "wide", or "closed", or "narrow". For example, pèsca "peach" ("open" or "wide" vowel, as in "pen") and pésca  "fishing" ("closed" or "narrow" vowel, as in "pain"). However, in some regional accents, these words can be pronouned the same way, or even with opposite values.
 * Lakota. For example, kákhi "in that direction" but kakhí "take something to someone back there".
 * Leonese uses it for marking stress or disambiguation.
 * Modern Greek marks the stressed vowel of every polysyllabic word: ά (á), έ (é), ή (í), ί (í), ό (ó), ύ (í), ώ (ó).
 * Navajo where the acute marks a higher tone.
 * Norwegian, Swedish and Danish use the acute accent to indicate that a terminal syllable with the e is stressed and is often omitted if it does not change the meaning: armen (first syllable stressed) means "the arm" while armé(e)n means "the army"; ide (first syllable stressed) means "bear's den" in Swedish, while idé means "idea". Also stress-related are the different spellings of the words en/én and et/ét (the indefinite article and the word "one" in Danish and Norwegian). In Norwegian, however, the neuter word "one" is spelled ett. Then, the acute points out that there is one and only one of the object, which derives from the obsolete spelling(s) een and eet. Some loanwords, mainly from French, are also written with the acute accent, such as Norwegian and Swedish kafé and Danish café (also cafe).
 * Occitan
 * Portuguese: á, é, í, ó, ú. It may also indicate height (see below).
 * Russian. Syllabic stress is irregular in Russian, and in reference and teaching materials (dictionaries and books for children or foreigners), stress is indicated by an acute accent above the stressed vowel, e.g. соба́ка (, dog), as follows: а́, е́, и́, о́, у́, ы́, э́, ю́, я́. The acute accent can be used both in the Cyrillic and sometimes in the romanised text.
 * Spanish marks stressed syllables in polysyllabic words that deviate from the standardized stress patterns. In monosyllabic words, it is used to distinguish homophones, e.g.: el (the) and él (he).
 * Tagalog dictionaries including other Philippine languages use the acute accent to mark a vowel in a syllable with lexical stress (Diín) and avoid ambiguity. Combinations include á, í, ó, and ú while é is the rarest one. Since they are not part of the official alphabet, these vowels do not affect the order of each letter. Vowels with a stress at the first syllable are left unwritten and serves as the default word. For example, baka (cow) and baká (maybe).
 * Ukrainian: sometimes added to mark syllabic stress, when it can help to distinguish between homographs: за́мок castle vs. замо́к lock, as follows: а́, е́, и́, і́, о́, у́, ю́, я́. Commonly used in dictionaries, readers, and some children's books.
 * Welsh: word stress usually falls on the penultimate syllable, but one way of indicating stress on a final (short) vowel is by the use of the acute accent. In the Welsh orthography, it can be on any vowel: á, é, í, ó, ú, ẃ, or ý. Examples: casáu "to hate", sigarét  "cigarette", ymbarél  "umbrella".

Height
The acute accent marks the height of some stressed vowels in various Romance languages.
 * To mark high vowels:
 * Bislama. One of the two orthographies distinguishes é from e . The orthography after 1995 does not distinguish these sounds, and has no diacritics.
 * Catalan. The acute marks the quality of the vowels é (as opposed to è ), and ó  (as opposed to ò ).
 * French. The acute is used on é. It is known as accent aigu, in contrast to the accent grave which is the accent sloped the other way. It distinguishes é from è, ê , and e . Unlike in other Romance languages, the accent marks do not imply stress in French.
 * Italian. The acute accent (sometimes called accento chiuso, "closed accent" in Italian) is compulsory only in words of more than one syllable stressed on their final vowel (and a few other words). Words ending in stressed -o are never marked with an acute accent (ó), but with a grave accent (ò). Therefore, only é and è are normally contrasted, typically in words ending in -ché, such as perché ("why/because"); in the conjugated copula è ("is"); in ambiguous monosyllables such as né ('neither') vs. ne ('of it') and sé ('itself') vs. se ('if'); and some verb forms, e.g. poté ("he/she/it could" (past tense)). The symbol ó can be used in the body of a word for disambiguation, for instance between bótte ("barrel") and bòtte ("beating"), though this is not mandatory: in fact standard Italian keyboards lack a dedicated ó key.
 * Occitan. The acute marks the quality of the vowels é (as opposed to è ), ó  (as opposed to ò ) and á  (as opposed to à ).
 * Scottish Gaelic (a Celtic rather than Romance language) uses/used a system in which é is contrasted with è  and ó  with ò . Both the grave and acute indicate length; é/è and ó/ò are thus contrasted with e  and o  respectively. Besides, á appears in the words á, ám  and ás  in order to distinguish them from  a , am  and as  respectively.  The other vowels (i and u) only appear either without an accent or with a grave. Since the 1980s the SQA (which sets school standards and thus the de facto standard language) and most publishers have abandoned the acute accent, using grave accents in all situations (analogous to the use of the acute in Irish). However, universities, some publishers and many speakers continue to use acute accents.
 * To mark low vowels:
 * Portuguese. The vowels á, é and ó  are stressed low vowels, in opposition to â , ê  and ô  which are stressed high vowels.  However, the accent is only used in words whose stressed syllable is in an unpredictable location within the word:  where the location of the stressed syllable is predictable, no accent is used, and the height of the stressed vowel cannot then usually be determined solely from the word's spelling.

Long vowels

 * Arabic and Persian: $\langleá, í, ú\rangle$ were used in western transliteration of Islamic language texts from the 18th to early 20th centuries. Representing the long vowels, they are typically transcribed with a macron today except in Bahá'í orthography.
 * Classical Latin: sometimes used to represent the apex in modern orthography.
 * Czech: $\langleá, é, í, ó, ú, ý\rangle$ are the long versions of $\langlea, e, i, o, u, y\rangle$. The accent is known as čárka. To indicate a long $\langleu\rangle$ in the middle or at the end of a word, a kroužek ("ring") is used instead, to form $\langleů\rangle$.
 * Hungarian: $\langleí, ó, ú\rangle$ are the long equivalents of the vowels $\langlei, o, u\rangle$. $\langleő, ű\rangle$ (see double acute accent) are the long equivalents of $\langleö, ü\rangle$. Both types of accents are known as hosszú ékezet (hosszú means long). The letters $\langleá\rangle$ and $\langleé\rangle$ are two long vowels but they are also distinct in quality, rather than being the long equivalents of $\langlea\rangle$ and $\langlee\rangle$ (see below in Letter extension).
 * Irish: $\langleá, é, í, ó, ú\rangle$ are the long equivalents of the vowels $\langlea, e, i, o, u\rangle$, the accent affects pronunciation and meaning, e.g. Seán ("John") but sean ("old"). The accent is known as a (síneadh) fada ("long (sign)"), which is also used in Hiberno-English.
 * Old Norse: are the long versions of . Sometimes, $\langleǿ\rangle$ is used as the long version of $\langleø\rangle$, but $\langleœ\rangle$ is used more often. Sometimes, the short-lived Old Icelandic long $\langleǫ\rangle$ (also written $\langleö\rangle$) is written using an acute-accented form, $\langleǫ́\rangle$, or a version with a macron, $\langleǭ\rangle$, but usually it is not distinguished from $\langleá\rangle$ from which it is derived by u-mutation.
 * Slovak: the acute accent is called dĺžeň in Slovak. In addition to the long vowels $\langleá, é, í, ó, ú, ý\rangle$, dĺžeň is used to mark syllabic consonants $\langleŕ, ĺ\rangle$, which are the long counterparts of syllabic $\langler, l\rangle$.

Short vowels

 * Ligurian: in the official orthography, é is used for short, and ó is used for short.

Palatalization
A graphically similar, but not identical, mark is indicative of a palatalized sound in several languages.

In Polish, such a mark is known as a kreska ("stroke") and is an integral part of several letters: four consonants and one vowel. When appearing in consonants, it indicates palatalization, similar to the use of the háček in Czech and other Slavic languages (e.g. sześć "six"). However, in contrast to the háček which is usually used for postalveolar consonants, the kreska denotes alveolo-palatal consonants. In traditional Polish typography, the kreska is more nearly vertical than the acute accent, and placed slightly right of center. A similar rule applies to the Belarusian Latin alphabet Łacinka. However, for computer use, Unicode conflates the codepoints for these letters with those of the accented Latin letters of similar appearance.

In Serbo-Croatian, as in Polish, the letter $⟨ć⟩$ is used to represent a voiceless alveolo-palatal affricate.

In the romanization of Macedonian, $⟨ǵ⟩$ and $⟨ḱ⟩$ represent the Cyrillic letters $⟨ѓ⟩$ (Gje) and $⟨ќ⟩$ (Kje), which stand for palatal or alveolo-palatal consonants, though $⟨gj⟩$ and $⟨kj⟩$ (or $⟨đ⟩$ and $⟨ć⟩$) are more commonly used for this purpose. The same two letters are used to transcribe the postulated Proto-Indo-European phonemes and.

uses the acute for palatalization as in Polish: $⟨ć dź ń⟩$. Lower also uses $⟨ŕ ś ź⟩$, and Lower  previously used $⟨ḿ ṕ ẃ⟩$ and $⟨b́ f́⟩$, also written as $⟨b' f'⟩$; these are now spelt as $⟨mj pj wj⟩$ and $⟨bj fj⟩$.

Tone
In the Quốc Ngữ system for Vietnamese, the Yale romanization for Cantonese, the Pinyin romanization for Mandarin Chinese, and the Bopomofo semi-syllabary, the acute accent indicates a rising tone. In Mandarin, the alternative to the acute accent is the number 2 after the syllable: lái = lai2. In Cantonese Yale, the acute accent is either tone 2, or tone 5 if the vowel(s) are followed by 'h' (if the number form is used, 'h' is omitted): má = ma2, máh = ma5.

In African languages and Athabaskan languages, it frequently marks a high tone, e.g., Yoruba apá 'arm', Nobiin féntí 'sweet date', Ekoti kaláwa 'boat', Navajo t’áá 'just'.

The acute accent is used in Serbo-Croatian dictionaries and linguistic publications to indicate a high-rising accent. It is not used in everyday writing.

Disambiguation
The acute accent is used to disambiguate certain words which would otherwise be homographs in the following languages:

 * Catalan. Examples: són "they are" vs. son "tiredness", més "more" vs. mes "month".
 * Danish. Examples: én "one" vs. en "a/an"; fór "went" vs. for "for"; véd "know(s)" vs. ved "by"; gǿr "bark(s)" vs. gør "do(es)"; dǿr "die(s)" vs. dør "door"; allé "alley" vs. alle "everybody". Furthermore, it is also used for the imperative form of verbs ending in -ere, which lose their final e and might be mistaken for plurals of a noun (which most often end in -er): analysér is the imperative form of at analysere "to analyse", analyser is "analyses", plural of the noun analyse "analysis". Using an acute accent is always optional, never required.
 * Dutch. Examples: één "one" vs. een "a/an"; vóór "before" vs. voor "for"; vóórkomen "to exist/to happen" vs. voorkómen "to prevent/to avoid". Using an acute accent is mostly optional.
 * Modern Greek. Although all polysyllabic words have an acute accent on the stressed syllable, in monosyllabic words the presence or absence of an accent may disambiguate. The most common case is η, the feminine definite article ("the"), versus ή, meaning "or". Other cases include που ("who"/"which") versus πού ("where") and πως ("that", as in "he told me that...") versus πώς ("how").
 * Norwegian. It is used to indicate stress on a vowel otherwise not expected to have stress. Most words are stressed on the first syllable and diacritical marks are rarely used. Although incorrect, it is frequently used to mark the imperative form of verbs ending in -ere as it is in Danish: kontrollér is the imperative form of "to control", kontroller is the noun "controls". The simple past of the verb å fare, "to travel", can optionally be written fór, to distinguish it from for (preposition "for" as in English), fôr "feed" n./"lining", or fòr (only in Nynorsk) "narrow ditch, trail by plow" (all the diacritics in these examples are optional. )
 * Russian. Acute accents (technically, stress marks) are used in dictionaries to indicate the stressed syllable. They may also be optionally used to disambiguate both between minimal pairs, such as за́мок (read as zámak, means "castle") and замо́к (read as zamók, means "lock"), and between question words and relative pronouns such as что ("what", stressed, or "that", unstressed), similarly to Spanish. This is rare, however, as usually meaning is determined by context and no stress mark is written. The same rules apply to Ukrainian, Rusyn, Belarusian and Bulgarian.
 * Spanish. Covers various question word / relative pronoun pairs where the first is stressed and the second is a clitic, such as cómo (interrogative "how") and como (non-interrogative "how", comparative "like", "I eat" ), differentiates qué (what) from que (that), and some other words such as tú "you" and tu "your," té "tea" and te "you" (direct/indirect object), él "he/him" and el ("the", masculine). This usage of the acute accent is called tilde diacrítica.

Emphasis

 * In Danish, the acute accent can also be used for emphasis, especially on the word der (there), as in Der kan ikke være mange mennesker dér, meaning "There can't be many people there" or Dér skal vi hen meaning "That's where we're going".
 * In Dutch, the acute accent can also be used to emphasize an individual word within a sentence. For example, Dit is ónze auto, niet die van jullie, "This is our car, not yours." In this example, ónze is merely an emphasized form of onze. Also in family names like Piét, Piél, Plusjé, Hofsté.  The IJ digraph can be stressed with íj́ but is usually stressed as íj for technical reasons.
 * In the Armenian script emphasis on a word is marked by an acute accent above the word's stressed vowel; it is traditionally grouped with the Armenian question and exclamation marks which are also diacritics applied to the stressed vowel.

Letter extension

 * In Faroese, the acute accent is used on five of the vowels (a, i, o, u and y), but these letters, á, í, ó, ú and ý are considered separate letters with separate pronunciations.
 * á: long, short and before :
 * í/ý: long, short
 * ó: long, or , short: , except Suðuroy:
 * When ó is followed by the skerping -gv, it is pronounced, except in Suðuroy where it is
 * ú: long, short
 * When ú is followed by the skerping -gv, it is pronounced
 * In Hungarian, the acute accent marks a difference in quality on two vowels, apart from vowel length:
 * The (short) vowel a is open back rounded (ɒ), but á is open front unrounded (a) (and long).
 * Similarly, the (short) vowel e is open-mid front unrounded (ɛ), while (long) é is close-mid front unrounded (e).
 * Despite this difference, in most of the cases, these two pairs are arranged as equal in collation, just like the other pairs (see above) that only differ in length.
 * In Icelandic the acute accent is used on all 6 of the vowels (a, e, i, o, u and y), and, like in Faroese, these are considered separate letters.Icelandic Text Extract.jpg.]]
 * á:
 * é: long, short
 * í/ý:
 * ó:
 * ú:
 * All can be either short or long, but the pronunciation of é is not the same short and long.
 * Etymologically, vowels with an acute accent in these languages correspond to their Old Norse counterparts, which were long vowels but in many cases have become diphthongs. The only exception is é, which in Faroese has become æ.
 * In Kashubian, Polish, and Sorbian, the acute on "ó", historically used to indicate a lengthening of "o", now indicates higher pronunciation, and , respectively.
 * In Turkmen, the letter ý is a consonant:.

Other uses

 * In some Basque texts predating Standard Basque, the letters $⟨r⟩$ and $⟨l⟩$ carry acute accents (an invention by Sabino Arana ), which are otherwise indicated by double letters. In such cases, $⟨ŕ⟩$ is used to represent $⟨rr⟩$ (a trilled $⟨r⟩$, this spelling is used even at the end of a syllable, to differentiate from -$⟨r⟩$-, an alveolar tap –  in Basque  in word-final positions is always trilled) and $⟨ĺ⟩$ for $⟨ll⟩$ (a palatalized ).
 * In transliterating texts written in Cuneiform, an acute accent over the vowel indicates that the original sign is the second representing that value in the canonical lists. Thus su is used to transliterate the first sign with the phonetic value, while sú transliterates the second sign with the value.
 * In Emilian, é ó denote both length and height, representing [e, o].
 * In Indonesian dictionaries, $⟨é⟩$ is used to represent, while $⟨e⟩$ is used to represent.
 * In Northern Sámi, an acute accent was placed over the corresponding Latin letter to represent the letters peculiar to this language (Áá, Čč, Đđ, Ŋŋ, Šš, Ŧŧ, Žž) when typing when there was no way of entering these letters correctly otherwise.
 * Many Norwegian words of French origin retain an acute accent, such as allé, kafé, idé, komité. Popular usage can be sketchy and often neglects the accent, or results in the grave accent erroneously being used in its place. Likewise, in Swedish, the acute accent is used only for the letter $⟨e⟩$, mostly in words of French origin and in some names. It is used both to indicate a change in vowel quantity as well as quality and that the stress should be on this, normally unstressed, syllable. Examples include café ("café") and resumé ("résumé", noun). There are two pairs of homographs that are differentiated only by the accent: armé ("army") versus arme ("poor; pitiful", masculine gender) and idé ("idea") versus ide ("winter quarters").
 * $⟨Ǵǵ⟩$ and $⟨Źź⟩$ are used in Pashto in the Latin alphabet, equivalent to ږ and ځ, respectively.
 * In Romagnol, é ó denote both length and height, representing [eː, oː].

English
As with other diacritical marks, a number of (usually French) loanwords are sometimes spelled in English with an acute accent as used in the original language: these include attaché, blasé, canapé, cliché, communiqué, café, décor, déjà vu, détente, élite, entrée, exposé, mêlée, fiancé, fiancée, papier-mâché, passé, pâté, piqué, plié, repoussé, résumé, risqué, sauté, roué, séance, naïveté and touché. Retention of the accent is common only in the French ending é or ée, as in these examples, where its absence would tend to suggest a different pronunciation. Thus the French word résumé is commonly seen in English as resumé [sic], with only one accent (but also with both or none).

Acute accents are sometimes added to loanwords where a final e is not silent, for example, maté from Spanish mate, the Maldivian capital Malé, saké from Japanese sake, and Pokémon from the Japanese compound for pocket monster, the last three from languages which do not use the Roman alphabet, and where transcriptions do not normally use acute accents.

For foreign terms used in English that have not been assimilated into English or are not in general English usage, italics are generally used with the appropriate accents: for example, coup d'état, pièce de résistance, crème brûlée and ancien régime.

The acute accent is sometimes (though rarely) used for poetic purposes:

 * It can mark stress on an unusual syllable: for example, caléndar to indicate (rather than the standard ).
 * It can disambiguate stress where the distinction is metrically important: for example, rébel (as opposed to rebél), or áll trádes, to show that the phrase is pronounced as a spondee, rather than the more natural iamb.
 * It can indicate the sounding of an ordinarily silent letter: for example, pickéd to indicate the pronunciation, rather than standard (the grave accent is more common for this last purpose).

The layout of some European PC keyboards, combined with problematic keyboard-driver semantics, causes some users to use an acute accent or a grave accent instead of an apostrophe when typing in English (e.g. typing or  instead of John's).

Typographic form
Western typographic and calligraphic traditions generally design the acute accent as going from top to bottom. French even has the definition of acute is the accent «qui va de droite à gauche» ("which goes from right to left"), meaning that it descends from top right to lower left.

In Polish, the kreska diacritic is used instead, which usually has a different shape and style compared to other European languages. It features a more vertical steep form and is moved more to the right side of center line than acute. As Unicode does not differentiate the kreska from acute, letters from Western (computer) fonts and Polish fonts had to share the same set of code points, which make designing the conflicting character (i.e. o acute, $⟨ó⟩$) more troublesome. OpenType tried to solve this problem by giving language-sensitive glyph substitution to designers such that the font would automatically switch between Western $⟨ó⟩$ and Polish $⟨ó⟩$ based on language settings. New computer fonts are sensitive to this issue and their design for the diacritics tends toward a more "universal design" so that there will be less need for localization, for example Roboto and Noto typefaces.

Pinyin uses the acute accent to mark the second tone (rising or high-rising tone), which indicates a tone rising from low to high, causing the writing stroke of acute accent to go from lower left to top right. This contradicts the Western typographic tradition which makes designing the acute accent in Chinese typefaces a problem. Designers approach this problem in 3 ways: either keep the original Western form of going top right (thicker) to bottom left (thinner) (e.g. Arial/Times New Roman), flip the stroke to go from bottom left (thicker) to top right (thinner) (e.g. Adobe HeiTi Std/SimSun), or just make the accents without stroke variation (e.g. SimHei).

Unicode
Unicode encodes a number of cases of "letter with acute accent" as precomposed characters and these are displayed below. In addition, many more symbols may be composed using the combining character facility ( and ) that may be used with any letter or other diacritic to create a customised symbol but this does not mean that the result has any real-world application and are not shown in the table. Á|Áá · Ấ|Ấấ · Ắ|Ắắ · Ǻ|Ǻǻ · Ą́ą́ · Ǽ|Ǽǽ · Ć|Ćć · Ḉ|Ḉḉ · É|Éé · Ế|Ếế · Ḗ|Ḗḗ · Ǵ|Ǵǵ · Í|Íí · Ḯ|Ḯḯ · Ḱ|Ḱḱ · Ĺ|Ĺĺ · Ḿ|Ḿḿ · Ń|Ńń · Ó|Óó · Ố|Ốố · Ớ|Ớớ · Ṍ|Ṍṍ · Ṓ|Ṓṓ · Ǿ|Ǿǿ · Ṕ|Ṕṕ · Ŕ|Ŕŕ · Ś|Śś · Ṥ|Ṥṥ · Ú|Úú · Ǘ|Ǘǘ · Ứ|Ứứ · Ṹ|Ṹṹ · Ẃ|Ẃẃ · Ý|Ýý · ŹźΆ|Άά · Έ|Έέ · Ή|Ήή · Ί|Ίί · Ό|Όό · Ύ|Ύύ · ϓЃ|Ѓѓ · Ќ|Ќќ

Microsoft Windows
On Windows computers with US keyboard mapping, letters with acute accents can be created by holding down the alt key and typing in a three-number code on the number pad to the right of the keyboard before releasing the Alt key. Before the appearance of Spanish keyboards, Spanish speakers had to learn these codes if they wanted to be able to write acute accents, though some preferred using the Microsoft Word spell checker to add the accent for them. Some young computer users got in the habit of not writing accented letters at all. The codes (which come from the IBM PC encoding) are:
 * 160 for á
 * 130 for é
 * 161 for í
 * 162 for ó
 * 163 for ú

On most non-US keyboard layouts (e.g. Spanish, Hiberno-English), these letters can also be made by holding AltGr (or Ctrl+Alt with US international mapping) and the desired letter. Individual applications may have enhanced support for accents.

macOS
On macOS computers, an acute accent is placed on a vowel by pressing Option and then the vowel, which can also be capitalised; for example, á is formed by pressing Option and then a, and Á is formed by pressing Option and then Shift.

Keyboards
Because keyboards have only a limited number of keys, US English keyboards do not have keys for accented characters. The concept of dead key, a key that modified the meaning of the next key press, was developed to overcome this problem. This acute accent key was already present on typewriters where it typed the accent without moving the carriage, so a normal letter could be written on the same place. The US-International layout provides this function: is a dead key so appears to have no effect until the next key is pressed, when it adds the desired accute accent.

Computers sold in Europe (including UK) have an ('alternate graphic') key which adds a third and (with the Shift key) fourth effect to most keys. Thus produces á and  produces Á.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Acute accent


text:
Double acute accent

The double acute accent ( ◌̋ ) is a diacritic mark of the Latin and Cyrillic scripts. It is used primarily in Hungarian or Chuvash, and consequently it is sometimes referred to by typographers as hungarumlaut. The signs formed with a regular umlaut are letters in their own right in the Hungarian alphabet—for instance, they are separate letters for the purpose of collation. Letters with the double acute, however, are considered variants of their equivalents with the umlaut, being thought of as having both an umlaut and an acute accent.

History
Length marks first appeared in Hungarian orthography in the 15th-century Hussite Bible. Initially, only á and é were marked, since they are different in quality as well as length. Later í, ó, ú were marked as well.

In the 18th century, before Hungarian orthography became fixed, u and o with umlaut + acute (ǘ, ö́) were used in some printed documents. 19th century typographers introduced the double acute as a more aesthetic solution.

Hungarian
In Hungarian, the double acute is thought of as the letter having both an umlaut and an acute accent. Standard Hungarian has 14 vowels in a symmetrical system: seven short vowels (a, e, i, o, ö, u, ü) and seven long ones, which are written with an acute accent in the case of á, é, í, ó, ú, and with the double acute in the case of ő, ű. Vowel length has phonemic significance in Hungarian, that is, it distinguishes different words and grammatical forms.

Slovak
At the beginning of the 20th century, the letter A̋ (A with double acute) was used in Slovak as a long variant of the short vowel Ä (A with diaeresis), representing the vowel in dialect and some loanwords. The letter is still used for this purpose in Slovak phonetic transcription systems.

Handwriting
In handwriting in German and Swedish, the umlaut is sometimes written similarly to a double acute. In the Swedish alphabet, Å, Ä and Ö are letters in their own right.

Chuvash
The Chuvash language written in the Cyrillic script uses a double-acute Ӳ, ӳ as a front counterpart of Cyrillic letter У, у  (see Chuvash vowel harmony), likely after the analogy of handwriting in Latin script languages. In other minority languages of Russia (Khakas, Mari, Altai, and Khanty), the umlauted form Ӱ is used instead.

Faroese
Classical Danish handwriting uses "ó" for "ø", which becomes a problem when writing Faroese in the same tradition, as "ó" is a part of the Faroese alphabet. Thus ő is sometimes used for ø in Faroese.

International Phonetic Alphabet
The IPA and many other phonetic alphabets use two systems to indicate tone: a diacritic system and an adscript system. In the diacritic system, the double acute represents an extra high tone.

One may encounter this use as a tone sign in some IPA-derived orthographies of minority languages, such as in the North American Native Tanacross (Athapascan). In line with the IPA usage it denotes the extra-high tone.

Unicode
Unicode encodes a number of cases of "letter with double acute" as precomposed characters and these are displayed below. In addition, many more symbols may be composed using the combining character facility that may be used with any letter or other diacritic to create a customised symbol but this does not mean that the result has any real-world application and thus are not shown in the table.

A̋a̋ · Őő · Űű Ӳӳ

Technical notes
O and U with double acute accents are supported in the Code page 852, ISO 8859-2, and Unicode character sets.

Code page 852
Some of the box-drawing characters of the original DOS code page 437 were sacrificed in order to put in more accented letters (all printable characters from ISO 8859-2 are included).

ISO 8859-2
In ISO 8859-2, the characters Ő, ő, Ű, and ű take the place of some similar-looking (but distinct, especially at bigger font sizes) letters of ISO 8859-1.

Unicode
All occurrences of "double acute" in character names in the Unicode 9.0 standard:

LaTeX Input
In LaTeX, the double acute accent is typeset with the \H{} (mnemonic for "Hungarian") command. For example, the name Paul Erdős (in his native Hungarian: Erdős Pál) would be typeset as

X11 Input
In modern X11 systems (or utilities such as WinCompose on Windows systems), the double acute can be typed by pressing the Compose key followed by = (the equal sign) and desired letter (o or u).

title: Double acute accent


text:
Twin study

Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. Twin research is considered a key tool in behavioral genetics and in related fields, from biology to psychology. Twin studies are part of the broader methodology used in behavior genetics, which uses all data that are genetically informative – siblings studies, adoption studies, pedigree, etc. These studies have been used to track traits ranging from personal behavior to the presentation of severe mental illnesses such as schizophrenia.

Twins are a valuable source for observation because they allow the study of environmental influence and varying genetic makeup: "identical" or monozygotic (MZ) twins share essentially 100% of their genes, which means that most differences between the twins (such as height, susceptibility to boredom, intelligence, depression, etc.) are due to experiences that one twin has but not the other twin. "Fraternal" or dizygotic (DZ) twins share only about 50% of their genes, the same as any other sibling. Twins also share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) because they are born into the same family. The presence of a given genetic or phenotypic trait in only one member of a pair of identical twins (called discordance) provides a powerful window into environmental effects on such a trait.

Twins are also useful in showing the importance of the unique environment (specific to one twin or the other) when studying trait presentation. Changes in the unique environment can stem from an event or occurrence that has only affected one twin. This could range from a head injury or a birth defect that one twin has sustained while the other remains healthy.

The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (which is found for most traits), this implies that genes play an important role in these traits. By comparing many hundreds of families with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior.

Modern twin studies have concluded that all studied traits are partly influenced by genetic differences, with some characteristics showing a stronger influence (e.g. height), others an intermediate level (e.g. personality traits) and some more complex heritabilities, with evidence for different genes affecting different aspects of the trait – as in the case of autism.

History
Twins have been of interest to scholars since early civilization, including the early physician Hippocrates (5th century BCE), who attributed different diseases in twins to different material circumstances, and the stoic philosopher Posidonius (1st century BCE), who attributed such similarities to shared astrological circumstances.

Gustav III, King of Sweden was the first to commission a medical study using identical twins. Gustav's father, Adolph Frederick had been an opponent of stimulating drinks such as tea and coffee, signing the Misuse and Excesses Tea and Coffee Drinking Edict in 1757. Both Gustav III and his father had read and been strongly influenced by a 1715 treatise from a French physician on the dangers of what would later be identified as caffeine in tea and coffee. After assuming the throne in 1771 the king became strongly motivated to demonstrate to his subjects that coffee and tea had deleterious effects on human health. To this end he offered to commute the death sentences of a pair of twin murderers if they participated in a primitive clinical trial.

Both condemned men agreed and subsequently spent the rest of their lives in prison fulfilling the king's demands: that one twin drink three pots of coffee every day and the other three pots of tea. The tea drinking twin died first at the age of 83, long after Gustav III, who was assassinated in 1792. The age of the coffee-drinking twin at his death is unknown, as both doctors assigned by the king to monitor this study predeceased him. The ban on coffee and tea in Sweden was lifted in 1823.

A more recent study is from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior. Galton, however, was unaware of the difference between identical and DZ twins. This factor was still not understood when the first study using psychological tests was conducted by Edward Thorndike (1905) using fifty pairs of twins. This paper was an early statement of the hypothesis that family effects decline with age. His study compared twin pairs age 9–10 and 13–14 to normal siblings born within a few years of one another.

Thorndike incorrectly reasoned that his data supported for there being one, not two, twin types. This mistake was repeated by Ronald Fisher (1919), who argued "The preponderance of twins of like sex, does indeed become a new problem, because it has been formerly believed to be due to the proportion of identical twins. So far as I am aware, however, no attempt has been made to show that twins are sufficiently alike to be regarded as identical really exist in sufficient numbers to explain the proportion of twins of like sex."

An early, and perhaps first, study understanding the distinction is from the German geneticist Hermann Werner Siemens in 1924. Chief among Siemens' innovations was the polysymptomatic similarity diagnosis. This allowed him to account for the oversight that had stumped Fisher, and was a staple in twin research prior to the advent of molecular markers.

Wilhelm Weinberg and colleagues in 1910 used the identical-DZ distinction to calculate respective rates from the ratios of same- and opposite-sex twins in a maternity population. They partitioned co-variation amongst relatives into genetic and environmental elements, anticipating the later work of Fisher and Wright, including the effect of dominance on similarity of relatives, and beginning the first classic-twin studies.

A study conducted by Darrick Antell and Eva Taczanowski found that "twins showing the greatest discrepancies in visible aging signs also had the greatest degree of discordance between personal lifestyle choices and habits", and concluded that "the genetic influences on aging may be highly overrated, with lifestyle choices exerting far more important effects on physical aging."

Examples
Examples of prominent twin studies include the following:
 * Maudsley Bipolar Twin Study
 * Minnesota Twin Family Study
 * Twins Early Development Study
 * NASA Twins Study

Methods
The power of twin designs arises from the fact that twins may be either identical (monozygotic (MZ), i.e. developing from a single fertilized egg and therefore sharing all of their polymorphic alleles) or fraternal (dizygotic (DZ), i.e. developing from two fertilized eggs and therefore sharing on average 50% of their alleles, the same level of genetic similarity found in non-twin siblings). These known differences in genetic similarity, together with a testable assumption of equal environments for identical and fraternal twins, creates the basis for the design of twin studies aimed at estimating the overall effects of genes and environment on a phenotype.

The basic logic of the twin study can be understood with very little mathematical knowledge beyond an understanding of the concepts of variance and thence derived correlation.

Classical twin method
Like all behavior genetic research, the classical twin study begins by assessing the variance of behavior (called a phenotype by geneticists) in a large group, and attempts to estimate how much of this is due to: Typically these three components are called A (additive genetics) C (common environment) and E (unique environment); hence the acronym ACE. It is also possible to examine non-additive genetics effects (often denoted D for dominance (ADE model); see below for more complex twin designs).
 * genetic effects (heritability)
 * shared environment – events that happen to both twins, affecting them in the same way
 * unshared, or unique, or nonshared environment – events that occur to one twin but not the other, or events that affect either twin in a different way.

The ACE model indicates what proportion of variance in a trait is heritable, versus the proportion due to a shared environment or unshared environment. Research is typically carried out using Structural equation modeling (SEM) programs such as OpenMx capable in principle of handling all sorts of complex pedigrees. However the core logic underlying such programs is the same as the one underlying the twin design described here.

Monozygotic (identical – MZ) twins raised in a family share 100% of their genes, and all of their shared environment. Any differences arising between them in these circumstances are random (i.e. due to environmental effects unique to each twin). The correlation between identical twins provides an estimate of A + C. Dizygotic (DZ) twins also share C, but share, on average only 50% of their genes: so the correlation between fraternal twins is a direct estimate of ½A+C. If we denote with r the correlation, we can define rmz and rdz as the correlations of a trait among identical and fraternal twins, respectively. For any particular trait, then:

 * rmz = A + C

 * rdz = ½A + C

Stated again, the difference between these two sums then allows us to solve for A and C (and as a consequence, for E). As the difference between the identical and fraternal correlations is due entirely to a halving of the genetic similarity, the additive genetic effect A is twice the difference between the identical and fraternal correlations:

 * A = 2 (rmz − rdz)

given the estimate for A, the one for C can be derived, for instance, from the first equation:

 * C = rmz − A

Finally, since the trait correlation among identical twins reflects the full contribution of A and C, the residual variation E can be estimated by subtracting this correlation from 1

 * E = 1 − rmz.

To summarize therefore, the additive genetic factor A is twice the difference between MZ and DZ twin correlations (this is known as Falconer's formula), C is the MZ twin correlation minus this estimate of A, and the random (unique) factor E is (1 - rmz), i.e. MZ twins differ due to unique environments only (Jinks & Fulker, 1970; Plomin, DeFries, McClearn, & McGuffin, 2001).

Underestimation of the genetical effect
The effect of genes may be much higher than what is predicted by naïve twin models, because they assume that non-identical twins are just half closer to identical twins in genetical similarity than random people. Due to assortative mating, they are even more similar, as parents are genetically more similar to each other than random people. Taking this into account (NTFD method), Wolfram and Morris showed that the number of years in education was just 26% affected by the shared environment, not 43%.

Modern modeling
Beginning in the 1970s, research transitioned to modeling genetic, environmental effects using maximum likelihood methods (Martin & Eaves, 1977). While computationally much more complex, this approach has numerous benefits rendering it almost universal in current research.

An example structural model (for the heritability of height among Danish males) is shown:

Model A on the left shows the raw variance in height. This is useful as it preserves the absolute effects of genes and environments, and expresses these in natural units, such as mm of height change. Sometimes it is helpful to standardize the parameters, so each is expressed as percentage of total variance. Because we have decomposed variance into A, C, and E, the total variance is simply A + C + E. We can then scale each of the single parameters as a proportion of this total, i.e., Standardised–A = A/(A + C + E). Heritability is the standardised genetic effect.

Model comparison
A principal benefit of modeling is the ability to explicitly compare models: Rather than simply returning a value for each component, the modeler can compute confidence intervals on parameters, but, crucially, can drop and add paths and test the effect via statistics such as the AIC. Thus, for instance to test for predicted effects of family or shared environment on behavior, an AE model can be objectively compared to a full ACE model. For example, we can ask of the figure above for height: Can C (shared environment) be dropped without significant loss of fit? Alternatively, confidence intervals can be calculated for each path.

Multi-group and multivariate modeling
Multivariate modeling can give answers to questions about the genetic relationship between variables that appear independent. For instance: do IQ and long-term memory share genes? Do they share environmental causes? Additional benefits include the ability to deal with interval, threshold, and continuous data, retaining full information from data with missing values, integrating the latent modeling with measured variables, be they measured environments, or, now, measured molecular genetic markers such as SNPs. In addition, models avoid constraint problems in the crude correlation method: all parameters will lie, as they should, between 0–1 (standardized).

Multivariate, and multiple-time wave studies, with measured environment and repeated measures of potentially causal behaviours are now the norm. Examples of these models include extended twin designs, simplex models, and growth-curve models.

SEM programs such as OpenMx and other applications suited to constraints and multiple groups have made the new techniques accessible to reasonably skilled users.

Modeling the environment: MZ discordant designs
As MZ twins share both their genes and their family-level environmental factors, any differences between MZ twins reflect E: the unique environment. Researchers can use this information to understand the environment in powerful ways, allowing epidemiological tests of causality that are otherwise typically confounded by factors such as gene–environment covariance, reverse causation and confounding.

An example of a positive MZ discordant effect is shown below on the left. The twin who scores higher on trait 1 also scores higher on trait 2. This is compatible with a "dose" of trait 1 causing an increase in trait 2. Of course, trait 2 might also be affecting trait 1. Disentangling these two possibilities requires a different design (see below for an example). A null result is incompatible with a causal hypothesis.

Take for instance the case of an observed link between depression and exercise (See Figure above on right). People who are depressed also reporting doing little physical activity. One might hypothesise that this is a causal link: that "dosing" patients with exercise would raise their mood and protect against depression. The next figure shows what empirical tests of this hypothesis have found: a null result.

Longitudinal discordance designs

As may be seen in the next Figure, this design can be extended to multiple measurements, with consequent increase in the kinds of information that one can learn. This is called a cross-lagged model (multiple traits measured over more than one time).

In the longitudinal discordance model, differences between identical twins can be used to take account of relationships among differences across traits at time one (path A), and then examine the distinct hypotheses that increments in trait1 drive subsequent change in that trait in the future (paths B and E), or, importantly, in other traits (paths C & D). In the example, the hypothesis that the observed correlation where depressed persons often also exercise less than average is causal, can be tested. If exercise is protective against depression, then path D should be significant, with a twin who exercises more showing less depression as a consequence.

Assumptions
It can be seen from the modeling above, the main assumption of the twin study is that of equal family environments, also known as the equal environments assumption. A special ability to test this assumption occurs where parents believe their twins to be non-identical when in fact they are genetically identical. Studies of a range of psychological traits indicate that these children remain as concordant as MZ twins raised by parents who treated them as identical.

Molecular genetic methods of heritability estimation have tended to produce lower estimates than classical twin studies due to modern SNP arrays not capturing the influence of certain types of variants (e.g., rare variants or repeat polymorphsisms), though some have suggested it is because twin studies overestimate heritability. A 2016 study determined that the assumption that the prenatal environment of twins was equal was largely tenable. Researchers continue to debate whether or not the equal environment assumption is valid.

Measured similarity: A direct test of assumptions in twin designs
A particularly powerful technique for testing the twin method was reported by Visscher et al. Instead of using twins, this group took advantage of the fact that while siblings on average share 50% of their genes, the actual gene-sharing for individual sibling pairs varies around this value, essentially creating a continuum of genetic similarity or "twinness" within families. Estimates of heritability based on direct estimates of gene sharing confirm those from the twin method, providing support for the assumptions of the method.

Sex differences
Genetic factors, including both gene expression and the range of gene × environment interactions, may differ between the sexes. Fraternal opposite sex twin pairs are invaluable in explicating these effects.

In an extreme case, a gene may only be expressed in one sex (qualitative sex limitation). More commonly, the effects of particular alleles may depend on the sex of the individual. A gene might cause a change of 100 g in weight in males, but perhaps 150 g in females – a quantitative gene effect.

Environments may impact on the ability of genes to express themselves and may do this via sex differences. For instance, genes affecting voting behavior would have no effect in females if females are excluded from the vote. More generally, the logic of sex-difference testing can extend to any defined sub-group of individuals. In cases such as these, the correlation for same and opposite sex DZ twins will differ, betraying the effect of the sex difference.

For this reason, it is normal to distinguish three types of fraternal twins. A standard analytic workflow would involve testing for sex-limitation by fitting models to five groups, identical male, identical female, fraternal male, fraternal female, and fraternal opposite sex. Twin modeling thus goes beyond correlation to test causal models involving potential causal variables, such as sex.

Gene × environment interactions
Gene effects may often be dependent on the environment. Such interactions are known as G×E interactions, in which the effects of a gene allele differ across different environments. Simple examples would include situations where a gene multiplies the effect of an environment: perhaps adding 1 inch to height in high nutrient environments, but only half an inch to height in low-nutrient environments. This is seen in different slopes of response to an environment for different genotypes.

Often researchers are interested in changes in heritability under different conditions: In environments where alleles can drive large phenotypic effects (as above), the relative role of genes will increase, corresponding to higher heritability in these environments.

A second effect is G × E correlation, in which certain alleles tend to accompany certain environments. If a gene causes a parent to enjoy reading, then children inheriting this allele are likely to be raised in households with books due to GE correlation: one or both of their parents has the allele and therefore will accumulate a book collection and pass on the book-reading allele. Such effects can be tested by measuring the purported environmental correlate (in this case books in the home) directly.

Often the role of environment seems maximal very early in life, and decreases rapidly after compulsory education begins. This is observed for instance in reading as well as intelligence. This is an example of a G*Age effect and allows an examination of both GE correlations due to parental environments (these are broken up with time), and of G*E correlations caused by individuals actively seeking certain environments.

Norms of reaction
Studies in plants or in animal breeding allow the effects of experimentally randomized genotypes and environment combinations to be measured. By contrast, human studies are typically observational. This may suggest that norms of reaction cannot be evaluated.

As in other fields such as economics and epidemiology, several designs have been developed to capitalise on the ability to use differential gene-sharing, repeated exposures, and measured exposure to environments (such as children social status, chaos in the family, availability and quality of education, nutrition, toxins etc.) to combat this confounding of causes. An inherent appeal of the classic twin design is that it begins to untangle these confounds. For example, in identical and fraternal twins shared environment and genetic effects are not confounded, as they are in non-twin familial studies. Twin studies are thus in part motivated by an attempt to take advantage of the random assortment of genes between members of a family to help understand these correlations.

While the twin study tells us only how genes and families affect behavior within the observed range of environments, and with the caveat that often genes and environments will covary, this is a considerable advance over the alternative, which is no knowledge of the different roles of genes and environment whatsoever. Twin studies are therefore often used as a method of controlling at least one part of this observed variance: Partitioning, for instance, what might previously have been assumed to be family environment into shared environment and additive genetics using the experiment of fully and partly shared genomes in twins. Additional information is available outside the classic twin design. Adoption designs are a form of natural experiment that tests norms of reaction by placing the same genotype in different environments. Association studies, e.g., allow direct study of allelic effects. Mendelian randomization of alleles also provides opportunities to study the effects of alleles at random with respect to their associated environments and other genes.

Extended twin designs and more complex genetic models
The basic or classical twin-design contains only identical and fraternal twins raised in their biological family. This represents only a sub-set of the possible genetic and environmental relationships. It is fair to say, therefore, that the heritability estimates from twin designs represent a first step in understanding the genetics of behavior.

The variance partitioning of the twin study into additive genetic, shared, and unshared environment is a first approximation to a complete analysis taking into account gene–environment covariance and interaction, as well as other non-additive effects on behavior. The revolution in molecular genetics has provided more effective tools for describing the genome, and many researchers are pursuing molecular genetics in order to directly assess the influence of alleles and environments on traits.

An initial limitation of the twin design is that it does not afford an opportunity to consider both Shared Environment and Non-additive genetic effects simultaneously. This limit can be addressed by including additional siblings to the design.

A second limitation is that gene–environment correlation is not detectable as a distinct effect unless it is added to the model. Addressing this limit requires incorporating adoption models, or children-of-twins designs, to assess family influences uncorrelated with shared genetic effects.

Continuous variables and ordinal variables
While concordance studies compare traits either present or absent in each twin, correlational studies compare the agreement in continuously varying traits across twins.

Criticism
The twin method has been subject to criticism from statistical genetics, statistics and psychology, with some researchers, such as Burt & Simons (2014), arguing that conclusions reached via this method are ambiguous or meaningless. Core elements of these criticisms and their rejoinders are listed below.

Criticisms of fundamental assumptions
Critics of twin studies argue that they are based on false or questionable assumptions, including that monozygotic twins share 100% of their genes and the equal environments assumption. On this basis, critics contend that twin studies tend to generate inflated or deflated estimates of heritability due to biological confounding factors and consistent underestimation of environmental variance. Other critics take a more moderate stance, arguing that the equal environments assumption is typically inaccurate, but that this inaccuracy tends to have only a modest effect on heritability estimates.

Criticisms of statistical methods
Peter Schonemann criticized methods for estimating heritability developed in the 1970s. He has also argued that the heritability estimate from a twin study may reflect factors other than shared genes. Using the statistical models published in Loehlin and Nichols (1976), the narrow HR-heritability of responses to the question "did you have your back rubbed" has been shown to work out to .92 heritable for males and .21 heritable for females, and the question "Did you wear sunglasses after dark?" is 130% heritable for males and 103% for females. Critics also contend that the concept of "heritability" estimated in twin studies is merely a statistical abstraction with no relationship to the underlying DNA and that the statistical underpinnings of twin research are invalid. Statistical critiques argue that heritability estimates used for most twin studies rest on restrictive assumptions that are usually not tested, and if they are, they are often contradicted by the data.

Responses to statistical critiques
Before computers, statisticians used methods that were computationally tractable, at the cost of known limitations. Since the 1980s these approximate statistical methods have been discarded. Modern twin methods based on structural equation modeling are not subject to the limitations and heritability estimates such as those noted above are mathematically impossible. Critically, the newer methods allow for explicit testing of the role of different pathways and incorporation and testing of complex effects.

Sampling: Twins as representative members of the population
Results of twin studies cannot be automatically generalized beyond the population they come from. It is therefore important to understand the particular sample studied, and the nature of twins themselves. Twins are not a random sample of the population, and they differ in their developmental environment. In this sense they are not representative.

For example: Dizygotic (DZ) twin births are affected by many factors. Some women frequently produce more than one egg at each menstrual period and are therefore more likely to have twins. This tendency may run in the family either on the mother's or father's side, and often runs through both. Women over the age of 35 are more likely to produce two eggs. Women who have three or more children are also likely to have dizygotic twins. Artificial induction of ovulation and in vitro fertilization-embryo replacement can also give rise to fraternal and identical twins.

Response to representativeness of twins
However, twins differ very little from non-twin siblings. Measured studies on the personality and intelligence of twins suggest that they have scores on these traits very similar to those of non-twins (for instance Deary et al. 2006).

Separated twin pairs as representative of other twins
Separated twin pairs, identical or fraternal, are generally separated by adoption. This makes their families of origin non-representative of typical twin families in that they give up their children for adoption. The families they are adopted to are also non-representative of typical twin families in that they are all approved for adoption by children's protection authorities and that a disproportionally large fraction of them have no biological children. Those who volunteer to studies are not even representative of separated twins in general since not all separated twins agree to be part of twin studies.

Detection problems
There can be some issues of undetected behaviors in the case of behaviors that many people keep secret presently or in their earlier lives. They may not be as willing to reveal behaviors that are discriminated against or stigmatized. If environment played no role in the actual behavior, skewed detection would still make it look like it played a role. For environment to appear to have no role in such cases, there would have to be either a counterproductivity of intolerance in the sense of intolerance causing the behavior it is bigoted against, or a flaw in the study that makes the results scientifically useless. Even if environment plays no role, the numbers would still be skewed.

Pairwise concordance
For a group of twins, pairwise concordance is defined as C/(C+D), where C is the number of concordant pairs and D is the number of discordant pairs.

For example, a group of 10 twins have been pre-selected to have one affected member (of the pair). During the course of the study four other previously non-affected members become affected, giving a pairwise concordance of 4/(4+6) or 4/10 or 40%.

Probandwise concordance
For a group of twins in which at least one member of each pair is affected, probandwise concordance is a measure of the proportion of twins who have the illness who have an affected twin and can be calculated with the formula of 2C/(2C+D), in which C is the number of concordant pairs and D is the number of discordant pairs.

For example, consider a group of 10 twins that have been pre-selected to have one affected member. During the course of the study, four other previously non-affected members become affected, giving a probandwise concordance of 8/(8+6) or 8/14 or 57%.

Critical accounts

 * Kamin, L. J. (1974). The Science and Politics of I.Q. Potomac, MD: Lawrence Erlbaum Associates.
 * Joseph, J. (2003). The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope. PCCS Books.
 * This book has been critically reviewed for the American Psychological Association. Hanson, D. R. (2005). 'The Gene Illusion Confusion: A review of The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope by Jay Joseph' [Electronic Version]. PsycCritiques, 50, e14.
 * Joseph, J. (2003). The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope. PCCS Books.
 * This book has been critically reviewed for the American Psychological Association. Hanson, D. R. (2005). 'The Gene Illusion Confusion: A review of The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope by Jay Joseph' [Electronic Version]. PsycCritiques, 50, e14.
 * This book has been critically reviewed for the American Psychological Association. Hanson, D. R. (2005). 'The Gene Illusion Confusion: A review of The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope by Jay Joseph' [Electronic Version]. PsycCritiques, 50, e14.

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Academic bodies
Several academic bodies exist to support behavior genetic research, including the Behavior Genetics Association, the International Society for Twin Studies, and the International Behavioural and Neural Genetics Society. Behavior genetic work also features prominently in several more general societies, for instance the International Society of Psychiatric Genetics.

Journals
Prominent specialist journals in the field include Behavior Genetics, Genes, Brain and Behavior, and Twin Research and Human Genetics.

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