Cousin marriage



Cousin marriage is a marriage between two cousins (cf.consanguinity). In various jurisdictions and cultures, such marriages range from being considered ideal and actively encouraged, to being uncommon but still legal, to being seen as incest and legally prohibited.

Such marriages are often highly stigmatized today in the West, but marriages between first and second cousins nevertheless account for over 10% of marriages worldwide. They are particularly common in the Middle East, where in some nations they account for over half of all marriages.

Cousin marriage has existed in many cultures throughout history. Frequently, only particular kinds of cousin marriage have been allowed, such as between cross cousins. Cousin marriage has also featured prominently in the field of anthropology, notably in alliance theory.

Supporters of accepting or legalizing first-cousin marriage today may view its genetic risk to offspring as small, compare bans on it to anti-miscegenation laws, or view them as discrimination or eugenics. Opponents may view the increased genetic risk as large, possibly when such marriages are repeated over many generations, and focus on their potential lack of social approval.

History
According to Professor Robin Fox of Rutgers University, it is likely that 80% of all marriages in history have been between second cousins or closer. It is generally accepted that the founding population of homo sapiens was small, anywhere from 700 to 10,000 individuals, and combined with the population dispersal caused by a hunter-gather existence, a certain amount of inbreeding would have been inevitable. Rates of first-cousin marriage in the United States, Europe, and other Western countries like Brazil have declined since the 19th century, though even during that period they were not more than 3.63 percent of all unions in Europe. But in many other world regions cousin marriage is still strongly favored: in the Middle East some countries have seen the rate rise over previous generations, and one study finds quite stable rates among Indian Muslims over the past four decades.

Cousin marriage has often been chosen to keep cultural values intact through several generations, ensure the compatibility of spouses, and preserve familial wealth, sometimes via advantages relating to dowry or bride price. Other reasons may include geographic proximity, tradition, strengthening of family ties, maintenance of family structure, a closer relationship between the wife and her in-laws, greater marital stability and durability, ease of prenuptial negotiations, enhanced female autonomy, the desire to avoid hidden health problems and other undesirable traits in a lesser-known spouse, and romantic love. Lower domestic violence and divorce rates have also been claimed. Many such marriages are arranged and facilitated by other extended family members.

United States
Cousin marriage was legal in all US states in the Union prior to the Civil War. However, according to Kansas sociology professor Martin Ottenheimer, after the Civil War the main purpose of marriage prohibitions was increasingly seen as less maintaining the social order and upholding religious morality and more as safeguarding the creation of fit offspring. Indeed, writers such as Noah Webster and ministers like Philip Milledoler and Joshua McIlvaine helped lay the groundwork for such viewpoints well before 1860. This led to a gradual shift in concern from affinal unions, like those between a man and his deceased wife's sister, to consanguineous unions. By the 1870s, Lewis Henry Morgan was writing about "the advantages of marriages between unrelated persons" and the necessity of avoiding "the evils of consanguine marriage," withdrawal from which would "increase the vigor of the stock." Cousin marriage to Morgan, and more specifically parallel-cousin marriage, was a remnant of a more primitive stage of human social organization. Morgan himself had married his mother's brother's daughter in 1851.

In 1846 the Governor of Massachusetts appointed a commission to study "idiots" in the state which implicated cousin marriage as being responsible for idiocy. Within the next two decades numerous reports appeared coming to similar conclusions, including for example by the Kentucky Deaf and Dumb Asylum, which concluded that cousin marriage resulted in deafness, blindness, and idiocy. Perhaps most important was the report of physician S.M. Bemiss for the American Medical Association, which concluded "that multiplication of the same blood by in-and-in marrying does incontestably lead in the aggregate to the physical and mental depravation of the offspring." Despite being contradicted by other studies like those of George Darwin and Alan Huth in England and Robert Newman in New York, the report's conclusions were widely accepted.

These developments led to thirteen states and territories passing cousin marriage prohibitions by the 1880s. Though contemporaneous, the eugenics movement did not play much direct role in the bans, and indeed George Louis Arner in 1908 considered them a clumsy and ineffective method of eugenics, which he thought would eventually be replaced by more refined techniques. Ottenheimer considers both the bans and eugenics to be "one of several reactions to the fear that American society might degenerate." In any case, by the period up until the mid-1920s the number of bans had more than doubled. Since that time, the only three states to successfully add this prohibition are Kentucky in 1943, Maine in 1985, and Texas in 2005. The NCCUSL unanimously recommended in 1970 that all such laws should be repealed, but no state has dropped its prohibition since the mid-1920s.

Europe
Only Austria, Hungary, and Spain banned cousin marriage throughout the 19th century, with dispensations being available from the government in the last two countries. But the Swedish Church, though Protestant, had banned first-cousin marriage until 1680 and required dispensation until 1844. England maintained a small but stable proportion of cousin marriages for centuries, with proportions in 1875 estimated by George Darwin at 3.5 percent for the middle classes and 4.5 percent for the nobility, though this has declined to under 1 percent in the 20th century. Queen Victoria and Prince Albert were a preeminent example.

The 19th century academic debate on cousin marriage evolved differently in Europe than it did in America. Despite the writings of Scottish deputy commissioner for lunacy Arthur Mitchell that cousin marriage had injurious effects on offspring, these conclusions were largely contradicted by researchers like Alan Huth and George Darwin. (At one point Mitchell had claimed that inbreeding in Scottish fishing communities led to a lower average hat size of six and seven-eighths, a quarter inch less than their more outbred neighbors.) In fact, Mitchell's own data did not support his hypotheses, prompting him to later speculate that the dangers of consanguinity might be partly overcome by proper living. Later studies by George Darwin found only much smaller effects that closely resemble those estimated today, and perhaps in response to his son's work, Charles Darwin eventually withdrew some earlier musings that cousin marriage might pose an evolutionary risk. In the end, when a question about cousin marriage was considered in 1871 for the census, according to George Darwin it was rejected "amid the scornful laughter of the House, on the grounds that the idle curiosity of philosophers was not to be satisfied."

First-cousin marriage was legal in ancient Rome from at least the Second Punic War (218–201 BC) to its ban by the Christian emperor Theodosius I in 381 AD in the west and until after Justinian (d. 565 AD) in the east. However whether the incidence of such marriages was low or high has been debated. Anthropologist Jack Goody advanced the position that cousin marriage was a typical pattern in Rome based on the marriage of four children of Emperor Constantine to their first cousins and what he considers the dubious nature of writings by Plutarch and Livy indicating the proscription of cousin marriage in the early Republic. Professors Brent Shaw and Richard Saller, however, counter in their more comprehensive treatment that cousin marriages were never habitual or preferred in the western empire: for example, in one set of six stemma belonging to Roman aristocrats in the two centuries after Octavian, out of 33 marriages none were between first or second cousins. Shaw, Saller and Goody mutually agree that such marriages certainly carried no social stigma in the late Republic and early Empire. They cite the example of Cicero attacking Mark Antony, who married his father's brother's daughter, and note that if Cicero could have cast aspersions on Antony using this fact he surely would have. Instead the attack was exclusively directed against Antony's divorce.

Shaw and Saller propose in their thesis of low cousin marriage rates that as families from different regions were incorporated into the imperial Roman nobility, exogamy was necessary to accommodate them and avoid destabilizing the Roman social structure. Their data from tombstones further indicates that in most of the western empire parallel-cousin marriages were also not widely practiced among commoners. Spain and Noricum were exceptions to this rule, but even there the percentages did not rise above ten percent. They further point out that since property belonging to the nobility was typically fragmented, keeping current assets in the family offered no special advantage compared with acquiring it by intermarriage. Jack Goody claimed that early Catholic marriage rules forced a sharp change from earlier norms in order to deny heirs to the wealthy and therefore increase the chance they would will their property to the Church. But Shaw and Saller believe the Church often merely took the place of the earlier position of the emperor in acquiring the inheritance of aristocrats who lacked heirs, instead averring that the Catholic injunctions against cousin marriage were due more to ideology than any conscious desire to acquire wealth.

For some prominent examples of cousin marriages in ancient Rome, such as the marriage of Octavian's daughter to his sister's son, see the Julio-Claudian family tree. Marcus Aurelius also married his maternal first cousin Faustina the Younger and had 13 children. Cousin marriage was more frequent in Ancient Greece, and in fact marriages with the niece were also permitted there, one example of which was King Leonidas I of Sparta who married his half-niece. A Greek woman who became epikleros, or heiress with no brothers, was obliged to marry her father's nearest male kin if she had not yet married and given birth to a male heir; first in line would be either her father's brothers or their sons, followed by her father's sisters' sons. According to Goody, cousin marriage was also not forbidden in the newly Christian and presumably pre-Christian Ireland, where an heiress was also obligated to marry a paternal cousin. From the 7th century the Irish Church only recognized four degrees of prohibited kinship, and civil law fewer. This persisted until after the Norman conquests and the synod at Cashel in 1101. In contrast, contemporary British law was based on official Catholic policy, and Anglo-Norman clergy often became disgusted with the Irish "law of fornication." Finally, Edward Westermarck states that marriage among the ancient Teutons was apparently prohibited only in the ascending and descending lines and among siblings.

Middle East
Cousin marriage has been least allowed throughout the Middle East for all recorded history. Anthropologists have debated the significance of the practice; some view it as the defining feature of the Middle Eastern kinship system while others note that overall rates of cousin marriage have varied sharply between different Middle Eastern communities. There is very little numerical evidence of rates of cousin marriage in the past. In many cases there is not only a preference but a right to marry the father's brother's daughter, wherein if the girl's family wishes to marry her to anyone else they must first get the permission of the father's brother's son.

Raphael Patai reports that in central Arabia no relaxation of a man's right to the FBD seems to have taken place in the past hundred years before his 1962 work. Here the girl is not forced to marry her bint 'amm but she cannot marry another unless he gives consent. The force of the custom is seen in one case from Jordan when the father arranged for the marriage of his daughter to an outsider without obtaining the consent of her ibn 'amm. When the marriage procession progressed with the bride toward the house of the bridegroom, the ibn 'amm rushed forward, snatched away the girl, and forced her into his own house. This was regarded by all as a lawful marriage. In Iraq the right of the cousin has also traditionally been followed and a girl breaking the rule without the consent of the ibn 'amm could have ended up murdered by him. The Syrian city of Aleppo during the 19th century featured a rate of cousin marriage among the elite of 24% according to one estimate, a figure that masked widespread variation: some leading families had none or only one cousin marriage, while others had rates approaching 70%. Cousin marriage rates were highest among women, merchant families, and older well-established families.

In-marriage was less frequent in the late pre-Islamic Hijaz than in ancient Egypt. It existed in Medina during Muhammad's time but at less than today's rates. In Egypt estimates from the late 19th and early 20th century state variously that either 80 percent of fellahin married first cousins or two-thirds married them if they existed. One source from the 1830s states that cousin marriage was less common in Cairo than in other areas. In traditional Palestine, if a girl had no ibn 'amm (father's brother's son) or he renounced his right to her, the next in line was traditionally the ibn khal (mother's brother's son) and then other relatives. Raphael Patai however reported that this custom loosened in the years preceding his 1947 study. In ancient Persia the Achaemenid kings habitually married their cousins and nieces, while between the 1940s and 1970s the percentage of Iranian cousin marriages increased from 34 to 44%. Cousin marriage among native Middle Eastern Jews is generally far higher than among the European Ashkenazim, who assimilated European marital practices after the diaspora.

According to anthropologist Ladislav Holy, cousin marriage is not an independent phenomenon but rather one expression of a wider Middle Eastern preference for agnatic solidarity, or solidarity with one's father's lineage. According to Holy the oft-quoted reason for cousin marriage of keeping property in the family is, in the Middle Eastern case, just one specific manifestation of keeping intact a family's whole "symbolic capital." Close agnatic marriage has also been seen as a result of the conceptualization of men as responsible for the control of the conduct of women. Honor is another reason for cousin marriage: while the natal family may lose influence over the daughter through marriage to an outsider, marrying her in their kin group allows them to help prevent dishonorable outcomes like either attacks on her or her own unchaste behavior. Pragmatic reasons for the husband, such as warmer relations with his father-in-law, and those for parents of both spouses, like reduced bride price and access to the labor of the daughter's children, also contribute. Throughout Middle Eastern history cousin marriage has been both praised and discouraged by various writers and authorities.

Africa
Cousin marriage rates from most African nations outside the Middle East are unknown. It is however estimated that 35–50% of all sub-Saharan African populations either prefer or accept cousin marriages. In Nigeria, the most populous country of Africa, the three largest tribes in order of size are the Hausa, Yoruba, and Igbo. The Hausa are overwhelmingly Muslim, though followers of traditional religions do exist. Muslim Hausa practice cousin marriage preferentially, and polygyny is allowed if the husband can support multiple wives. The book Baba of Karo presents one prominent portrayal of Hausa life: according to its English coauthor, it is unknown for Hausa women to be unmarried for any great length of time after around the age of fourteen. Divorce can be accomplished easily by either the male or the female, but females must then remarry. Even for a man, lacking a spouse is looked down upon. Baba of Karo's first of four marriages was to her second cousin. She recounts in the book that her good friend married the friend's first cross cousin.

The Yoruba people are split between Islam and Christianity. A 1974 study analyzed Yoruba marriages in the town Oka Akoko, finding that among a sample of highly polygynous marriages having an average of about three wives, 51% of all pairings were consanguineous. These included not only cousin marriages but also uncle-niece unions. Reportedly it is a custom that in such marriages at least one spouse must be a relative, and generally such spouses were the preferred or favorite wives in the marriage and gave birth to more children. However, it must be emphasized that this was not a general study of Yoruba, but only of highly polygynous Yoruba residing in Oka Akoko. Finally, the Igbo people of southern Nigeria specifically prohibit both parallel- and cross-cousin marriage, though polygyny is common. Men are forbidden to marry within their own patrilineage or those of their mother or father's mother and must marry outside their own village. Igbo are almost entirely Christian, having converted heavily under colonialism.

In Ethiopia the ruling Christian Amhara people were historically rigidly opposed to cousin marriage, and could consider up to third cousins the equivalent of brother and sister, with marriage at least ostensibly prohibited out to sixth cousins. They also took affinal prohibitions very seriously. The prospect of a man marrying a former wife's "sister" was seen as incest, and conversely for a woman and her former husband's "brother." Though Muslims make up over a third of the Ethiopian population, and Islam has been present in the country since the time of Muhammad, cross-cousin marriage is very rare among most Ethiopian Muslims. In contrast to the Nigerian situation, in Ethiopia Islam cannot be identified with particular tribal groups and is found across most of them, and conversions between religions are comparatively common. But exceptions to these rules include the overwhelmingly Muslim Somali and Afar peoples, who respectively make up 6.2% and 1.73% of the population. The Afar practice a form of cousin marriage called absuma that is arranged at birth and can be forced.

China
Confucius described marriage as "the union of two surnames, in friendship and in love." In ancient China there is evidence that in some cases two clans had a longstanding arrangement wherein they would only marry members of the other clan. Some men also practiced sororate marriage, that is, a marriage to a former wife's sister or a polygynous marriage to both sisters. This would have the effect of eliminating parallel-cousin marriage as an option but would leave cross-cousin marriage acceptable. In the ancient system of the Erya dating from around the 3rd century B.C., the words for the two types of cross cousins were identical, with father's brother's children and mother's sister's children both being distinct. However, it is evident that whereas it may not have been permissible at that time, marriage with the mother's sister's children also became possible by the third century A.D. Eventually the mother's sister's children and cross cousins shared one set of terms, with only the father's brother's children retaining a separate set. This usage remains today, with biao cousins considered "outside" and paternal tang cousins being of the same house. There were also some periods in Chinese history where all cousin marriage was legally prohibited, as law codes dating from the Ming Dynasty attest. However, enforcement proved difficult and by the subsequent Qing Dynasty the former laws had been restored.

The following is a Chinese poem by Po Chu-yi (A.D. 772–846). Anthropologist Francis Hsu described mother's brother's daughter as being the most preferred type of Chinese cousin marriage, mother's sister's daughter as being tolerated, and father's sister's daughter (FZD) as being disfavored. Some writers report this last form as being nearly incestuous. One proposed explanation is that in FZD marriage the daughter does not change her surname throughout her life, so the marriage does not result in an extension of the father's kinship ties. In Chinese culture these patrilineal ties are most important in determining the closeness of a relation. In the case of the MZD marriage there are no such ties and consequently this may not even be viewed as cousin marriage. Finally, one reason that MBD marriage is often most common may be the typically greater emotional warmth between a man and his mother's side of the family. Later analyses have found regional variation in these patterns: in some rural areas where cousin marriage is still common, MBD is not preferred but merely acceptable, similar to MZD. By the early to mid-twentieth century, anthropologists described cross-cousin marriage in China as "still permissible...but...generally obsolete" or as "permitted but not encouraged."

Current status
[[File:CousinMarriageWorld.svg|thumb||| Laws regarding first-cousin marriage around the world1 {{legend|#000099|First-cousin marriage}} {{legend|#ec8028|Legality dependent on religion or culture2}} {{legend|#FF0000|Statute bans first-cousin marriage}} {{legend|#b9b9b9|No data}}

1For information on US states see the map below.

2See sections on India and Hinduism.

]] Slightly over 10% of all marriages worldwide are estimated to be between second cousins or closer. As of 2001, here is one estimate for the percentages of world population living in countries with various levels of consanguineous marriage: less than 1% consanguinity, 18%, 1–10% consanguinity, 47%, 10–50+% consanguinity, 17%, and unknown, 18%. The overall rate appears to be declining.

Middle East
The Middle East has uniquely high rates of cousin marriage among the world's regions. Certain Middle Eastern countries, including Pakistan and Saudi Arabia, have rates of marriage to first or second cousins that may exceed 50%. Iraq was estimated in one study to have a rate of 33%, and figures for Iran and Afghanistan have been estimated in the range of 30–40%. Though on the lower end, Egypt and Turkey nevertheless have rates above 20%.

All states in the Persian Gulf currently require advance genetic screening for all prospective married couples. Qatar was the last Gulf nation to institute mandatory screening in 2009, mainly to warn related couples who are planning marriage about any genetic risks they may face. The current rate of cousin marriage there is 54%, an increase of 12–18% over the previous generation. A report by the Dubai-based Centre for Arab Genomic Studies (CAGS) in September 2009 found that Arabs have one of the world's highest rates of genetic disorders, nearly two-thirds of which are linked to consanguinity. Research from CAGS and others suggests consanguinity is declining in Lebanon and Egypt and among Palestinians, but is increasing in Morocco, Mauritania and Sudan.

Dr. Ahmad Teebi, a genetics and pediatrics professor at Weill Cornell Medical College in Qatar, links the increase in cousin marriage in Qatar and other Gulf states to tribal tradition and the region’s expanding economies. “Rich families tend to marry rich families, and from their own – and the rich like to protect their wealth,” he said. “So it’s partly economic, and it’s also partly cultural.” In regard to the higher rates of genetic disease in these societies, he says: "It's certainly a problem," but also that "The issue here is not the cousin marriage, the issue here is to avoid the disease."

In Pakistan there is the concept of biradari or "brotherhood," whose members may or may not be related. Each biradari usually has an associated caste (zat) name. It has been proposed that one of the features underlying cousin marriage in Pakistan is caste endogamy. According to this interpretation, marrying within the extended family allows Muslim Pakistanis to maintain caste differences while differentiating themselves from the exogamous Hindus of neighboring North India.

In many Middle Eastern nations a marriage to the father's brother's daughter (FBD) is considered ideal, though this type may not always actually outnumber other types. One anthropologist, Ladislav Holý, argues that it is important to distinguish between the ideal of FBD marriage and marriage as it is actually practiced, which always also includes other types of cousins and unrelated spouses. Holý cites the Berti people of the Sudan, who consider the FBD to be the closest kinswoman to a man outside of the prohibited range. If there is more than one relationship between spouses, as often results from successive generations of cousin marriage, only the patrilineal one is counted. Marriage within the lineage is preferred to marriage outside the lineage even when no exact genealogical relationship is known. Of 277 first marriages, only 84 were between couples unable to trace any genealogical relationship between them. Of those, in 64 the spouses were of the same lineage. However, of 85 marriages to a second or third wife, in 60 the spouses were of different lineages. The Marri have a very limited set of incest prohibitions that includes only lineal relatives, the sister, and aunts except the mother's brother's wife. Female members of the mother's lineage are seen as only loosely related. Finally there are the Baggara Arabs who favor MBD marriage first, followed by cross-cousin marriage if the cross cousin is a member of the same surra, a group of agnates of five or six generations depth. Next is marriage within the surra. There is no preference for marriages between matrilateral parallel cousins.

India
Attitudes in India on cousin marriage vary sharply by region and culture. For Muslims it is acceptable and legal to marry a first cousin. But for Hindus it may be illegal under the 1955 Hindu Marriage Act, though the specific situation is more complex. The Hindu Marriage Act makes cousin marriage illegal for Hindus with the exception of marriages permitted by regional custom. Practices of the small Christian minority are also location dependent: their cousin marriage rates are higher in southern states like Karnataka with high overall rates.

Cousin marriage is proscribed and seen as incest for Hindus in north India. In fact it may even be unacceptable to marry within one's village or for two siblings to marry partners from the same village. The northern kinship model prevails in the states of Rajasthan, Gujarat, Uttar Pradesh, Haryana, and Punjab. But in south India it is common for Hindu cross cousins to marry, with matrilateral cross-cousin (mother's brother's daughter) marriages being especially favored. The southern kinship model prevails in the states of Kerala, Tamil Nadu, Karnataka, and Andhra Pradesh. South Indian women have been described as having more personal autonomy than North Indian women and a kinship system leaving women closer to their families of origin and not based on patrilineal descent may explain both this and somewhat lower South Indian birth rates. In the exogamous north, a woman will not usually be in a position to help her family after marriage, since the house of her husband is likely to be far away. Female children are consequently less valued. In the South Indian system males are also likely to enter into arrangements that include males related by marriage, whereas in the north such relationships are more defined by blood ties. But the higher the caste in south India, in general the closer the position of females is to that under the northern system.

Practices in central India overall are closer to the northern model than the southern, but differences exist from each. For example, in Mumbai, studies done in 1956 showed 7.7% of Hindus married to a second cousin or closer. By contrast, in the northern city of New Delhi only 0.1% of Hindus were married to a first cousin during the 1980s. At the other extreme, studies done in the South Indian province of Karnataka, which contains Bangalore, during that period show fully one third of Hindus married to a second cousin or closer. Pre-2000 Madhya Pradesh, from which Chhattisgarh has now split, and Maharashtra, which contains Mumbai, are provinces that are intermediate in their kinship practices.

India's Muslim minority represents about 12% of its population (excluding Jammu and Kashmir) and has an overall rate of cousin marriage of 22% according to a 2000 report. Most Muslim cousin marriages were between first cousins, with the rate of first-cousin marriage being 20%. Muslim consanguinity in north India was typical, but below the overall northern statistic lies a sharply divided picture: Jammu and Kashmir is the only Indian state with a Muslim majority, with a Muslim consanguinity rate of 40%, while at the other extreme Haryana, though its population is 17% Muslim, has a Muslim consanguinity rate of only 1%. This dichotomy may be a legacy of the partition of the sub-continent into India and Pakistan, when there was substantial Muslim migration to Pakistan from the eastern parts of the former unified state of Punjab. In south India by contrast the rates are fairly constant, except for the South Indian Malabar Muslims of Kerala (9%) who claim descent from Arab traders who settled permanently in India in the 8th century. Most Indian Muslims by contrast are the result of Hindu conversions to Islam in the 16th century or later. The lowest rate for a whole Indian region was in East India (15%). Consanguinity rates were generally stable across the four decades for which data exists, though second-cousin marriage appears to have been decreasing in favor of first-cousin marriage.

United States
[[File:Cousin marriage map1.svg|thumb||| Laws regarding first-cousin marriage in the United States {{legend|#000099|First-cousin marriage}} {{legend|#0066ff|Allowed with restrictions or exceptions}} {{legend|#ff7777|Banned with exceptions1}} {{legend|#FF0000|Statute bans first-cousin marriage1}} {{legend|#990000|Criminal offense1}}

1Certain states recognize marriages performed elsewhere. ]]

The United States has the only bans on cousin marriage in the Western world. , 30 U.S. states prohibit most or all marriage between first cousins, and a bill is pending in Maryland which would prohibit most first cousins from marrying there. The US also prohibits first-cousin-once-removed marriages in six states. Some states prohibiting cousin marriage recognize cousin marriages performed in other states, but despite occasional claims to the contrary, states declaring such marriages void do not. Data on cousin marriage in the United States is sparse. It was estimated in 1960 that 0.2% of all marriages between Roman Catholics were between first or second cousins, but no more recent nationwide studies have been performed. It is unknown what proportion of that number were first cousins, which is the group facing marriage bans. To contextualize the group's size, the total proportion of interracial marriages in 1960, the last census year before the end of anti-miscegenation statutes, was 0.4%, and the proportion of black-white marriages was 0.13%. While recent studies have cast serious doubt on whether cousin marriage is as dangerous as is popularly assumed, professors Diane B. Paul and Hamish G. Spencer speculate that legal bans persist in part due to "the ease with which a handful of highly motivated activists—or even one individual—can be effective in the decentralized American system, especially when feelings do not run high on the other side of an issue."

Among supporters of repealing the laws, the Cousin Couples organization describes itself as "the world's primary resource for romantic relationships among cousins including cousin marriage." This group likens laws against cousin marriage to the anti-miscegenation laws of decades past. Their website includes information on state and international laws, world religious viewpoints, famous cousin couples and the genetic risk due to cousin marriage. It also includes a message board with several messages posted daily as of November 2009, which allows cousin couples to provide each other with emotional support, share pictures and experiences, and comment on the legal situation.

A bill to repeal the ban on first-cousin marriage in Minnesota was introduced by Phyllis Kahn in 2003, but it died in committee. By training Kahn is a biophysicist and holds a PhD from Yale. Republican Minority Leader Marty Seifert criticized the bill in response, saying it would "turn us into a cold Arkansas." According to the University of Minnesota's The Wake, Kahn was aware the bill had little chance of passing but introduced it anyway to draw attention to the issue. She reportedly got the idea after learning that cousin marriage is an acceptable form of marriage among some cultural groups that have a strong presence in Minnesota, namely the Hmong and Somali.

In contrast, Maryland delegates Henry B. Heller and Kumar P. Barve sponsored a bill to ban first-cousin marriages in 2000. (Barve later became Majority Leader.) It got further than Kahn's bill, passing the House of Delegates by 82 to 46 despite most Republicans voting no, but finally died in the state Senate. In response to the 2005 marriage of Pennsylvanian first cousins Eleanor Amrhein and Donald W. Andrews Sr. in Maryland, Heller said that he might resurrect the bill because such marriages are "like playing genetic roulette."

Texas actually did pass a ban on first-cousin marriage the same year as Amrhein and Andrews married, evidently in reaction to the presence of the polygamous FLDS. Texas Representative Harvey Hilderbran, whose district includes the main FLDS compound, authored an amendment to a child protection statute to both discourage the FLDS from settling in Texas and to "prevent Texas from succumbing to the practices of taking child brides, incest, welfare abuse and domestic violence." While Hilderbran stated that he would not have authored a bill solely to ban first-cousin marriage, he also said in an interview that "Cousins don’t get married just like siblings don’t get married. And when it happens you have a bad result. It’s just not the accepted normal thing." Some news sources then only mentioned the polygamy and child abuse provisions and ignored the cousin marriage portion of the bill, as did some more recent sources as well. The new statute makes sex with an adult first cousin a more serious felony than with adult members of one's immediate family.

Two US states are unusual in permitting cousin marriage with minor caveats. Maine allows first-cousin marriage if the couple agrees to have genetic counseling, while North Carolina allows it so long as the applicants for marriage are not rare double first cousins, meaning cousins through both parental lines. In the other 25 states permitting at least some first-cousin marriage, double cousins are not distinguished.

Britain
Britain has been having a debate in the past few years about whether to discourage cousin marriages through government public relations campaigns or ban them entirely. The debate has been prompted by a Pakistani immigrant population making up 3% of Britain's population, of whom about 55% marry a first cousin. For example, Environment Minister (now Immigration Minister) Phil Woolas said in 2008, "If you have a child with your cousin the likelihood is there'll be a genetic problem" and that such marriages were the "elephant in the room." Muslim physician Mohammad Walji has spoken out against the practice, saying that it is a "very significant" cause of infant death, and his practice has produced leaflets warning against it. But in sharp contrast, Professor Alan Bittles of the Centre for Comparative Genomics in Australia states that the risk of birth defects rises from roughly 2% in the general population to 4% for first cousins and therefore that "It would be a mistake to ban it." Researcher Aamra Darr of Britain's University of Leeds has also criticized what she called an "alarmist presentation of data" that exaggerates the risk.

There is evidence that the rate of cousin marriage has increased among British Pakistanis from rates in their parents' generation. Most British Pakistani marriages are arranged, but these can be of two types: conventionally arranged marriages where the bride and groom have little or no say, and what some British Pakistanis describe as "arranged love marriages" where the bride and groom play an important role. The latter are less frequent but their number is increasing. Among traditional arranged marriages the outcome typically depends on the balance of power between parents and the number of cousins on each side of the family; each parent may try to sway cousin marriages to their respective side. Parents usually first consider the claims of their own close kin, especially siblings, upon their offspring as spouses. Those who violate these obligations can be accused of being lalchi or greedy.

Among British Pakistanis most marriages are transnational and have the effect of bringing in another Pakistani under the constraints of British immigration control. In one small sample, 92% of transnational marriages were with either first or second cousins. A small but non-negligible proportion end in divorce because the immigrant spouse was simply using the marriage as an excuse to enter Britain. Marriages of British Pakistan women with immigrant male Pakistanis proved more problematic than with immigrant women, partly because such men do not always have the skills to support their wives. Immigrant spouses are fairly even in gender, so the motive is not simply to import income-generating males. Rather immigration is defined by kinship ties and obligations to the extended family. It seems probable that the longer a Pakistani has been in Britain, the less their likelihood of marrying a close relative. Not surprisingly, marriages to kin in Britain has also allowed some families consolidate their social position in Pakistan using the resulting income. Researcher Alison Shaw expects marriage within the biradari among British Pakistanis to continue for some time to come, especially in the landowning castes who do not wish to marry beneath them. While those who move up in status may more confidently overlook obligations to kin and marry those of equivalent class status, possibly through "arranged love marriages," the majority may not have this luxury. The rate of biradari marriages will also depend on the extent to which young adults raised in Britain continue to value meeting obligations to kin and maintaining marital ties to Pakistan.

The Netherlands
The Netherlands has also had a recent debate that has reached the level of the Prime Minister proposing a cousin marriage ban. The proposed policy is explicitly aimed at preventing "import marriages" from certain nations like Turkey and Morocco with a high rate of cousin marriage (roughly one quarter according to one study). Critics argue that such a ban would contradict Section 8 of the European Convention on Human Rights, is not based on science, and would affect more than immigrants. While some proponents argue such marriages were banned until 1970, according to Frans van Poppel of the Netherlands Interdisciplinary Demographic Institute, they are confusing cousin marriage with uncle-niece marriage.

Other regions
In the East, South Korea is especially restrictive with bans on marriage out to third cousins, with all couples having the same surname and region of origin having been prohibited from marrying until 1997. Taiwan, North Korea, and the Philippines also prohibit first-cousin marriage. It is allowed in Japan, though the incidence has declined in recent years. China has banned it since passing its 1981 Marriage Law, yet there is a conspicuous lack of data on actual cousin marriage rates there. An article in China Daily from the 1990s reported on the ban's implementation in the northeastern provience of Liaoning, along with a ban on marriage of the physically and mentally handicapped, all justified on "eugenic" grounds. Limited existing data indicates some remaining cousin marriage of types besides father's brother's daughter in many villages, with percentages usually in the lower single digits. A 2002 Time article claims that an increasing imbalance in the number of males and females is causing more cousin marriages, as "desperate" males struggle to find brides.

Recent 2001 data for Brazil indicates a rate of cousin marriage of 1.1%, down from 4.8% in 1957. The geographic distribution is heterogeneous: in certain regions the rate is at typical European levels, but in other areas is much higher. Freire-Maia found paternal parallel cousin marriage to be the most common type of cousin marriage. In his 1957 study the rate varied from 1.8% in the south to 8.4% in the northeast, where it increased moving inward from the coast, and was higher in rural regions than in urban. Consanguinity has decreased over time and particularly since the 19th century. For example, in São Paulo in the mid-19th century the rate of cousin marriage apparently was 16%, but a century later it was merely 1.9%.

Social aspects
Robin Bennett, a University of Washington researcher who led a major NSGC study on cousin marriage, has said that much hostility towards married cousins constitutes discrimination. "It's a form of discrimination that nobody talks about. People worry about not getting health insurance — but saying that someone shouldn't marry based on how they're related, when there's no known harm, to me is a form of discrimination."

A recent New York Times article by writer Sarah Kershaw documents fear by many married cousins of being treated with derision and contempt. "While many people have a story about a secret cousin crush or kiss, most Americans find the idea of cousins marrying and having children disturbing or even repulsive," notes the article. It gives the example of one mother, Mrs. Spring, whose daughter Kimberly Spring-Winters, 29, married her cousin Shane Winters, 37. She stated that when she has told people about her daughter's marriage they have been shocked, and consequently she is afraid to mention it. Living in a small Pennsylvania town, she also worries that her grandchildren will be treated as outcasts and ridiculed due to their parental status. Another cousin couple, who withheld their full names from publication, stated that their children's maternal grandparents have never met their two grandchildren because the grandparents severed contact out of disapproval for the couple's marriage.

It appears that in most societies cousin marriage is more common among those of low socioeconomic status, among the illiterate and uneducated, and in rural areas. This may be due in part to the token or significantly reduced dowries and bridewealths that exist in such marriages. But some societies also report a high prevalence among land-owning families and the ruling elite: here the relevant consideration is thought to be keeping the family estate intact over generations. There is also a lower average age at marriage for cousin marriages, the difference in one Pakistani study being 1.10 and 0.84 years for first and second cousins respectively. In Pakistan the ages of the spouses were also closer together, the age difference declining from 6.5 years for unrelated couples to 4.5 years for first cousins. A marginal increase in time to first birth, from 1.6 years generally to 1.9 years in first cousins, may occur due to the younger age at marriage of consanguineous mothers and resultant adolescent subfertility or delayed consummation.

Predictions that cousin marriage would decline during the late 20th century in areas where it is preferential appear to have been largely incorrect. One reason for this is that in many regions cousin marriage is not merely a cultural tradition but is also judged to offer significant social and economic benefits. In South Asia, rising demands for dowry payments have caused dire economic hardship and have been linked to "dowry deaths" in a number of North Indian states. Where permissible, marriage to a close relative is hence regarded as a more economically feasible choice. Second, improvements in public health have led to decreased death rates and increased family sizes, making it easier to find a relative to marry if that is the preferred choice. Increases in cousin marriage in the West may also occur as a result of immigration from Asia and Africa. In the short term some observers have concluded that the only new forces that could discourage such unions are government bans like the one China enacted in 1981. In the longer term it is thought that rates may decline due to decreased family sizes, making it more difficult to find cousins to marry.

Cousin marriage is important in several anthropological theories by prominent authors such as Claude Levi-Strauss, Sir Edward Tylor, and Henry Lewis Morgan. Levi-Strauss viewed cross-cousin marriage as a form of exogamy in the context of a unilineal descent group, meaning either matrilineal or patrilineal descent. Matrilateral cross-cousin marriage in societies with matrilineal descent meant that a male married into the family his mother's brother, building an alliance between the two families. However, marriage to a mother's sister daughter (a parallel cousin) would be endogamous, here meaning inside the same descent group, and would therefore fail to build alliances between different groups. Correspondingly, in societies like China with patrilineal descent, marriage to a father's brother's daughter would fail at alliance building. And in societies with both types of descent, where a person belongs to the group of his mother's mother and father's father but not mother's father or father's mother, only cross-cousin marriages would successfully build alliances.

Levi-Strauss postulated that cross-cousin marriage had the two consequences of setting up classes which automatically delimit the group of possible spouses and of determining a relationship that can decide whether a prospective spouse is to be desired or excluded. Whereas in other kinship systems one or another of these aspects dominates, in cross-cousin marriage they overlap and cumulate their effects. It differs from incest prohibitions in that the latter employs a series of negative relationships, saying whom one cannot marry, while cross-cousin marriage employs positive relationships, saying whom should marry. Most crucially, cross-cousin marriage is the only type of preferential union that can function normally and exclusively and still give every man and woman the chance to marry a cross-cousin. Unlike other systems such as the levirate, the sororate, or uncle-niece marriage, cross-cousin marriage is preferential because for obvious reasons these others cannot constitute the exclusive or even preponderant rule of marriage in any group. Cross-cousin marriage divides members of the same generation into two approximately equal groups, those of cross-cousins and "siblings" that include real siblings and parallel cousins. Consequently cross-cousin marriage can be a normal form of marriage in a society, but the other systems above can only be privileged forms. This makes cross-cousin marriage exceptionally important.

Cross-cousin marriage also establishes a division between prescribed and prohibited relatives who, from the viewpoint of biological proximity, are strictly interchangeable. Levi-Strauss thought that this proved that the origin of the incest prohibition is purely social and not biological. Cross-cousin marriage in effect allowed the anthropologist to control for biological degree by studying a situation where the degree of prohibited and prescribed spouses were equal. In understanding why two relatives of the same biological degree would be treated so differently, Levi-Strauss wrote, it would be possible to understand not only the principle of cross-cousin marriage but of the incest prohibition itself. For Levi-Strauss cross-cousin marriage was not either socially arbitrary or a secondary consequence of other institutions like dual organization or the practice of exogamy. Instead, the raison d'etre of cross-cousin marriage could be found within the institution itself. Of the three types of institution of exogamy rules, dual organization, and cross-cousin marriage, the last was most significant, making the analysis of this form of marriage the crucial test for any theory of marriage prohibitions.

Matrilateral cross-cousin marriage has been found by some anthropological researchers to be correlated with patripotestal jural authority, meaning rights or obligations of the father. According to some theories, in these kinship systems a man marries his matrilateral cross-cousin due to associating her with his nurturant mother. Due to this association, possibly reinforced by personal interaction with a specific cousin, he may become "fond" of her, rendering the relationship "sentimentally appropriate." Interestingly, patrilateral cross-cousin marriage is the rarest of all types of cousin marriage, and there is some question as to whether it even exists.

In contrast to Levi-Strauss who viewed the exchange of women under matrilateral cross-cousin marriage as fundamentally egalitarian, anthropologist Edmund Leach held that such systems by nature created groups of junior and senior status and were part of the political structure of society. Under Leach's model, in systems where this form of marriage segregates descent groups into wife-givers and wife-takers, the social status of the two categories also cannot be determined by a priori arguments. Groups like the Kachin exhibiting matrilateral cross-cousin marriage do not exchange women in circular structures; where such structures do exist they are unstable. Moreover, the exchanging groups are not major segments of the society, but rather local descent groups from the same or closely neighboring communities. Levi-Strauss held that women were always exchanged for some "prestation" which could either be other women or labor and material goods. Leach agreed but added that prestations could also take the form of intangible assets like "prestige" or "status" that might belong to either wife-givers or wife-takers.

Anthropologists Robert Murphy and Leonard Kasdan describe preferential parallel cousin marriage as leading to social fission, in the sense that "feud and fission are not at all dysfunctional factors but are necessary to the persistence and viability of Bedoin society." Their thesis is the converse of Fredrik Barth's, who describes the fission as leading to the cousin marriage." Per Murphy and Kasdan, the Arab system of parallel cousin marriage works against the creation of homogenous "bounded" and  "corporate" kin groups and instead creates arrangements where every person is related by blood to a wide variety of people, with the degree of relationship falling off gradually as opposed to suddenly. Instead of corporate units, Arab society is described as having "agnatic sections," a kind of repeating fractal structure in which authority is normally weak at all levels but capable of being activated at the required level in times of war. They relate this to an old Arab proverb: "Myself against my brother; my brother and I against my cousin; my cousin, my brother and I against the outsider." In such a society even the presence of a limited amount of cross-cousin marriage will not break the isolation of the kin group, for first cross cousins often end up being second parallel cousins." Instead of organizing horizontally through affinal ties, when large scale organization is necessary it is accomplished vertically, by reckoning distance from shared ancestors. This practice is said to possess advantages such as resilience and adaptability in the face of adversity.

In an essay published for The American Conservative, Steve Sailer has claimed that high rates of cousin marriage play an important role in discouraging political democracy. Sailer believes that because families practicing cousin marriage are more related to one another than otherwise, their feelings of family loyalty tend to be unusually intense, fostering nepotism.

Islam
It has been proposed that the Quranic law of inheritance through the daughter as well as the son encouraged first-cousin marriage in order to keep wealth in the family. Among Islamic societies this rule has not always been followed, but where it was, marrying a first cousin prevented familial wealth from escaping to another clan. According to the Hanafi school of legists, a man may give his daughter in marriage to his brother's son without her consent, and a young man decide for himself whether he wants or does not want to marry his bint 'amm. In Arabic a man commonly refers to his wife as bint 'ammi, daughter of my father's brother, whether or not she is actually his relation.

The Qur'an states that marriages between first cousins are allowed. In Sura An-Nisa (4:22–24), Allah mentioned the women who are forbidden for marriage: to quote the Qu'ran, "… Lawful to you are all beyond those mentioned, so that you may seek them with your wealth in honest wedlock…" In Sura Al-Ahzab (33:50), Allah mentioned to Muhammad that he may marry the daughters of his uncles and aunts from the father's side or the mother's side. It is the consensus of the jurists that this permission was not only for Muhammad, but it is also a permission for other believers. Muslims have practiced marriages between first cousins in all countries since the time of Muhammad. In many countries the most common type is between paternal cousins.

Muhammad actually did marry two relatives. One was a first cousin, Zaynab bint Jahsh, who was not only the daughter of one of his father's sisters but was also divorced from a marriage with Muhammad's adopted son, Zayd ibn Haritha. It was this last issue that caused the most controversy, with traditional Arab norms at the time being opposed, though not the Qur'an (Sura Al-Ahzab 33:37). According to Ibn Sa'd, after Zaynab's marriage to his adopted son, Muhammad went to pay Zayd a visit, but instead found a hastily clad Zaynab. Though he did not enter the house, the sight of her pleased him. Tabari embroiders the story; according to him Zaynab was only wearing a single slip, and the wind pushed away a curtain when Muhammad entered, revealing her "uncovered." In any case, thereafter Zayd no longer found her attractive and thought of proposing divorce, but Muhammad told him to keep her. Eventually, however, Zayd did divorce her.

Many of the immediate successors of Muhammad also took a cousin as one of their wives. Umar married his cousin Atikah bint Zayd ibn Amr ibn Nifayl, while Ali married Fatimah, the daughter of his paternal first cousin Muhammad and hence his first cousin once removed.

Judaism and Christianity


Cousins are not included in the lists of prohibited relatives provided in the Bible, specifically in the books of Leviticus and Deuteronomy. The Old Testament also contains several examples of married cousins. Two of the most famous are prominent in Genesis. Isaac was married to Rebekah, his first cousin once removed (Genesis 24:12–15). Also, Rachel and Leah were both cousins of Isaac's son Jacob. Jacob loved Rachel and worked seven years for her father Laban in return for permission to marry (Genesis 28–29). Both marriages were arranged over significant distances, with the eligible women nearby implied to be displeasing to the groom's parents (Genesis 24:3, 28:1). Jacob's brother Esau also married his cousin Mahalath, daughter of Ishmael. According to many English Bible translations, a fourth example is the five daughters of Zelophehad, who married the "sons of their father's brothers" in the later period of Moses, although other translations merely say "relatives." (Compare the Catholic RSV-CE and NAB in Numbers 36:10–12.) During the apportionment of Israel following the journey out of Egypt, Caleb gives his daughter Achsah to his brother's son Othniel according to the NAB (Joshua 15:17), though the Jewish Talmud argues Othniel was simply Caleb's brother (Sotah 11b). The daughters of Eleazer also married the sons of Eleazer's brother Kish in the still later time of David (1 Chronicles 23:22). King Rehoboam and his wives Maacah and Mahalath were grandchildren of David (2 Chronicles 11:20). Finally, Tobias in the book of Tobit has a right to marry his cousin Sarah because he is her nearest kinsman (Tobit 7:10), though the exact degree of their cousinship is not clear.

In Roman Catholicism, all marriages more distant than first-cousin marriages are allowed, and first-cousin marriages can be contracted with a dispensation. This was not always the case, however: the Catholic Church has gone through several phases in kinship prohibitions. At the dawn of Christianity in Roman times, marriages between first cousins were allowed. For example, Emperor Constantine, the first Christian Roman Emperor, married his children to the children of his half-brother. First and second cousin marriages were then banned at the Council of Agde in AD 506, though dispensations sometimes continued to be granted. By the 11th century, with the adoption of the so-called canon-law method of computing consanguinity, these proscriptions had been extended even to sixth cousins, including by marriage. But due to the many resulting difficulties in reckoning who was related who, they were relaxed back to third cousins at the Fourth Lateran Council in AD 1215. Pope Benedict XV reduced this to second cousins in 1917, and finally, the current law was enacted in 1983. In Catholicism, close relatives who have married unwittingly without a dispensation can receive an annulment.

There are several explanations for the rise of Catholic cousin marriage prohibitions after the fall of Rome. One explanation is increasing Germanic influence on church policy. G.E. Howard states, "During the period preceding the Teutonic invasion, speaking broadly, the church adhered to Roman law and custom; thereafter those of the Germans...were accepted." On the other hand it has also been argued that the bans were a reaction against local Germanic customs of kindred marriage. At least one Frankish King, Pepin the Short, apparently viewed close kin marriages among nobles as a threat to his power. Whatever the reasons, written justifications for such bans had been advanced by St. Augustine by the fifth century. "It is very reasonable and just," he wrote, "that one man should not himself sustain many relationships, but that various relationships should be distributed among several, and thus serve to bind together the greatest number in the same social interests." Taking a contrary view, Protestants writing after the Reformation tended to see the prohibitions and the dispensations needed to circumvent them as part of an undesirable church scheme to accrue wealth, or "lucre."

Since the 13th century the Catholic Church has measured consanguinity according to what is called, perhaps confusingly, the civil-law method. Under this method, the degree of relationship between lineal relatives (i.e., a man and his grandfather) is simply equal to the number of generations between them. However, the degree of relationship between collateral (non-lineal) relatives equals the number of links in the family tree from one person, up to the common ancestor, and then back to the other person. Thus brothers are related in the second degree, and first cousins in the fourth degree.

Protestant churches generally allow cousin marriage, in keeping with criticism of the Catholic system of dispensations by Martin Luther and John Calvin during the Reformation. This includes most of the major US denominations, such as Baptist, Pentecostal, Lutheran, Presbyterian, and Methodist. The Anglican Communion has also allowed cousin marriage since its inception during the rule of King Henry VIII. According Luther and Calvin, the Catholic bans on cousin marriage were an expression of Church rather than divine law and needed to be abolished. Protestants during the Reformation struggled to interpret the Biblical proscriptions against incest in a sensible manner, a task frustrated by facts like their omission of the daughter (but inclusion of the granddaughter) as a directly prohibited relation. John Calvin thought of the Biblical list only as illustrative and that any relationship of the same or smaller degree as any listed, namely the third degree by the civil-law method, should therefore be prohibited. The Archbishop of Canterbury reached the same conclusion soon after. But in contrast to both Protestantism and Catholicism, the Eastern Orthodox Church bars up to second cousins from marrying. The 1913 Catholic Encyclopedia refers to a theory by the Anglican bishop of Bath and Wells speculating that Mary and Joseph, the mother of Jesus and her husband, were first cousins. Jack Goody describes this theory as a "legend."

Hinduism
In Hinduism marriage within the same gotra is prohibited, where a gotra is believed to be the group of descendants of a sage who lived in the remote past. Two persons in the same gotra cannot marry even if they come from different linguistic areas. However, same-gotra marriages have been legal under Indian civil law since the Hindu Marriage Act of 1955. Additionally, marriages within certain degrees of consanguinity are considered sapinda and banned in Hinduism. Hindu lawgivers differ in the definition of sapinda: at one extreme, according to some sources marriages are prohibited within seven generations on the father's side and five on the mother's side. In contrast, other sources allow cross cousins to marry, including first cross cousins. The Hindu Marriage Act bars marriage for five generations on the father's side and three on the mother's side, but allows cross-cousin marriage where it is permitted by custom.

Hindu rules of exogamy are often taken extremely seriously. For example, in the Jhajjar district of north India, the entire family of 23-year-old Ravinder Gehlawat was told to leave their village after he married Shilpa Kadiyan, a 20-year-old girl from another village. He was found guilty of violating a custom in which the surnames Gehlawat and Kadiyan cannot marry because they are part of the same gotra. Ravinder Gehlawat later attempted suicide in order to save his family from eviction. Reporters described the situation as "a rare occasion when the collective panchayat is in conflict not only with the boy's family but also with the government." The panchayat is the local village council that served the eviction notice, which the government had protected Gehlawat's family against. It has been alleged that the government often colludes with parents and files false abduction charges to recapture daughters who have run away to enter same-gotra or other unacceptable marriages. In other cases same-gotra couples have been victims of honor killings.

Other religions
Buddhism does not proscribe any specific sexual practices, only ruling out "sexual misconduct" in the Five Precepts. Zoroastrianism allows cousin marriages, but Sikhism does not.

Genetics
Cousin marriage has genetic aspects that do not arise in the case of other marriage-related political and social issues like interracial marriage. This is because married couples possessing higher than normal consanguinity have, on average, an increased chance of sharing genes for recessive traits. Consanguinity means the amount of shared (identical) DNA, the genetic material. The percentage of consanguinity between any two individuals decreases fourfold as the most recent common ancestor recedes one generation. To cite some examples, first cousins have four times the consanguinity of second cousins, while first cousins once removed have half that of first cousins. Rare double first cousins have twice that of first cousins and are as related as half-siblings.

In April 2002, the Journal of Genetic Counseling released a report which estimated the average risk of birth defects in a child born of first cousins at 1.7–2.8% over an average base risk for non-cousin couples of 3%, or about the same as that of any woman over age 40. In terms of mortality, a 1994 study found a mean excess pre-reproductive mortality rate of 4.4%, While another study published in 2009 suggests the rate may be closer to 3.5%. Put differently, first-cousin marriage entails a similar increased risk of birth defects and mortality as a woman faces when she gives birth at age 41 rather than at 30. Critics argue that banning first-cousin marriages would make as much sense as trying to ban childbearing by older women. After repeated generations of cousin marriage the actual genetic relationship between two people is closer than the most immediate relationship would suggest. In Pakistan, where there has been cousin marriage for generations and the current rate may exceed 50%, one study estimated infant mortality at 12.7 percent for married double first cousins, 7.9 percent for first cousins, 9.2 percent for first cousins once removed/double second cousins, 6.9 percent for second cousins, and 5.1 percent among nonconsanguineous progeny. Among double first cousin progeny, 41.2 percent of prereproductive deaths were associated with the expression of detrimental recessive genes, with equivalent values of 26.0, 14.9, and 8.1 percent for first cousins, first cousins once removed/double second cousins, and second cousins respectively.

Even in the absence of preferential consanguinity, alleles that are rare in large populations can randomly increase to high frequency in small groups within a few generations due to the founder effect and accelerated genetic drift in a breeding pool of restricted size. For example because the entire Amish population is descended from only a few hundred 18th century German-Swiss settlers, the average coefficient of inbreeding between two random Amish is higher than between two non-Amish second cousins. First-cousin marriage is taboo among Amish but they still suffer from several rare genetic disorders. In Ohio's Geagua County, Amish make up only about 10 percent of the population but represent half the special needs cases. In one debilitating seizure disorder the worldwide total of 12 cases is exclusively Amish. Similar disorders have been found in the highly polygynous FLDS, who do allow first-cousin marriage and of whom 75 to 80 percent are related to two 1930s founders.

Studies into the effect of cousin marriage on polygenic traits and complex diseases of adulthood have often yielded contradictory results due to the rudimentary sampling strategies used. Both positive and negative associations have been reported for breast cancer and heart disease. Long-term studies conducted on the Dalmatian islands in the Adriatic Sea have indicated a positive association between inbreeding and a very wide range of common adulthood disorders, including hypertension, coronary heart disease, stroke, cancer, uni/bipolar depression, asthma, gout, peptic ulcer, and osteoporosis. However, these results may principally reflect village endogamy rather than consanguinity per se. Endogamy is marrying within a group and in this case the group was a village. The marital patterns of the Amish are also an example of endogamy.

The Latin American Collaborative Study of Congenital Malformation found an association between consanguinity and hydrocephalus, postaxial polydactyly, and bilateral oral and facial clefts. Another picture emerges from the large literature on congenital heart defects, which are conservatively estimated to have an incidence of 50/1,000 live births. A consistent positive association between consanguinity and disorders such as ventricular septal defect and atrial septal defect has been demonstrated, but both positive and negative associations with patent ductus arteriosus, atrioventricular septal defect, pulmonary atresia, and tetralogy of Fallot have been reported in different populations. Associations between consanguinity and Alzheimer's disease have been found in certain populations. Studies into the influence of inbreeding on anthropometric measurements at birth and in childhood have failed to reveal any major and consistent pattern, and only marginal declines were shown in the mean scores attained by consanguineous progeny in tests of intellectual capacity. In the latter case, it would appear that inbreeding mainly leads to greater variance in IQ levels, due in part to the expression of detrimental recessive genes in a small proportion of those tested.

A BBC report discussed Pakistanis in Britain, 55% of whom marry a first cousin. Given the high rate of such marriages, many children come from repeat generations of first-cousin marriages. The report states that these children are 13 times more likely than the general population to produce children with genetic disorders, and one in ten children of first-cousin marriages in Birmingham either dies in infancy or develops a serious disability. The BBC also states that Pakistani-Britons, who account for some 3% of all births in the UK, produce "just under a third" of all British children with genetic illnesses. Published studies show that mean perinatal mortality in the Pakistani community of 15.7 per thousand significantly exceeds that in the indigenous population and all other ethnic groups in Britain. Congenital anomalies account for 41 percent of all British Pakistani infant deaths. The BBC story contained an interview with Myra Ali, whose parents and grandparents were all first cousins. She has a very rare recessive genetic condition, known as Epidermolysis bullosa which will cause her to lead a life of extreme physical suffering, limited human contact and probably an early death from skin cancer. Knowing that cousin marriages increase the probability of recessive genetic conditions, she is understandably against the practice.

The increased mortality and birth defects observed among British Pakistanis may, however, have another source besides current consanguinity. This is population subdivision among different Pakistani groups. Population subdivision results from decreased gene flow among different groups in a population. Because members of Pakistani biradari have married only inside these groups for generations, offspring have higher average homozygosity even for couples with no known genetic relationship. According to a statement by the UK's Human Genetics Commission on cousin marriages, the BBC also "fails to clarify" that children born to these marriages were not found to be 13 times more likely to develop genetic disorders. Instead they are 13 times more likely to develop recessive genetic disorders. The HGC states, "Other types of genetic conditions, including chromosomal abnormalities, sex-linked conditions and autosomal dominant conditions are not influenced by cousin marriage." The HGC goes on to compare the biological risk between cousin marriage and increased maternal age, arguing that "Both represent complex cultural trends. Both however, also carry a biological risk. They key difference, GIG argue, is that cousin marriage is more common amongst a British minority population." Genetic effects from cousin marriage in Britain are more obvious than in a developing country like Pakistan because the number of confounding environmental diseases is lower. Increased focus on genetic disease in developing countries may eventually result from progress in eliminating environmental diseases there as well.

Comprehensive genetic education and premarital genetic counseling programs can help to lessen the burden of genetic diseases in endogamous communities. Genetic education programs directed as high school students have been successful in Middle Eastern countries such as Bahrain. Genetic counseling in developing countries has been hampered, however, by lack of trained staff, and couples may refuse prenatal diagnosis and selective abortion despite the endorsement of religious authorities. In Britain, the Human Genetics Commission recommends a strategy comparable with previous strategies in dealing with increased maternal age, notably as this age relates to an increased risk of Down Syndrome. All pregnant women in Britain are offered a screening test from the socialized medical system to identify those at an increased risk of having a baby with Down syndrome. The HGC states that similarly, it is appropriate to offer genetic counseling to consanguineous couples, preferably before they conceive, in order to establish the precise risk of a genetic abnormality in offspring. Under this system the offering of genetic counseling can be refused, unlike for example in the US state of Maine where it is mandatory to marry. Leading researcher Alan Bittles also concluded that though consanguinity clearly has a significant effect on childhood mortality and genetic disease in areas where it is common, it is "essential that the levels of expressed genetic defect be kept in perspective, and to realize that the outcome of consanguineous marriages is not subject to assessment solely in terms of comparative medical audit." He states that the social, cultural, and economic benefits of cousin marriage need to also be fully considered.

Fertility
Higher total fertility rates are reported for cousin marriages than average, a phenomenon noted as far back as George Darwin during the late 19th century. There is no significant difference in the number of surviving children in cousin marriages because this compensates for the observed increase in child mortality. The total fertility increase may be partly explained by the lower average parental age at marriage, and age at first birth, observed in consanguineous marriages. Other factors include shorter birth intervals and possibly a lower likelihood of using reliable contraception. There is also the possibility of more births as a compensation for increased child mortality, either via a conscious decision by parents to achieve a set family size or the cessation of lactational amenorrhea following the death of an infant. According to a recent paper the fertility difference is probably not due to any underlying biological effect. Earlier papers have claimed that increased sharing of human leukocyte antigens, as well as of deleterious recessive genes expressed during pregnancy, may lead to lower rates of conception and higher rates of miscarriage in consanguineous couples. Others now believe there is scant evidence for this unless the genes are operating very early in the pregnancy. Studies consistently show a lower rate of primary infertility in cousin marriages, usually interpreted as being due to greater immunological compatibility between spouses.

Famous cousin marriages
Famous cousin marriages in the United States include Edgar Allan Poe, Albert Einstein, Franklin Delano Roosevelt, and Jerry Lee Lewis.