China

Confucius described marriage as "the union of two surnames".^[23][24] In ancient China some evidence indicates that in some cases two clans had a longstanding arrangement whereby they would marry only 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 because they would have the same surname but would leave cross-cousin marriage acceptable.^[25] In the ancient system of the Erya dating from around the third century BC, the words for the two types of cross cousins were identical (甥 shēng), with father's brother's children (甥 shēng) and mother's sister's children (從母晜弟 cóngmǔ kūndì for boys and 從母姊妹 cóngmǔ zǐmèi for girls) both being distinct.^[26] However, 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 AD.^[27] 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.^[28] This usage remains today, with biǎo (表) cousins considered "outside" and paternal táng (堂) cousins being of the same house.^[29]

Anthropologist Francis Hsu described a mother's brother's daughter (MBD) as being the most preferred type of Chinese cousin marriage.^[30] Another research describes marrying a mother's sister's daughter (MSD) as being tolerated, but a father's brother's daughter (FBD, or táng relatives in Chinese) is strongly disfavored.^[31] The last form is seen as nearly incestuous and therefore prohibited, for the man and the woman in such marriage share the same surname, much resembling sibling marriage.^[31] In Chinese culture, patrilineal ties are most important in determining the closeness of a relation.^[32] In the case of the MSD marriage, no such ties exist, so 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.^[33] 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 MSD.^[31]

The following is a Chinese poem by Po Chu-yi (A.D. 772–846), in which he described an inbreeding village.^[34][35]

In Ku-feng hsien, in the district of Ch'u chou [Kiangsu]

Is a village called Chu Ch'en [the names of the two clans].

...

There are only two clans there

Which have intermarried for many generations.

...

In some periods in Chinese history, all cousin marriage was legally prohibited, as law codes dating from the Ming dynasty (1368–1644) attest. However, enforcement proved difficult and by the subsequent Qing dynasty, the former laws had been restored.^[36] During the Qing dynasty era (1636–1912), first cousin marriage was common and prevailed after the era particularly in rural regions. By the early to mid-20th century, anthropologists described cross-cousin marriage in China as "still permissible ... but ... generally obsolete" or as "permitted but not encouraged".^[36][35] Eventually, in 1981, a legal ban on first-cousin marriage was enacted by the government of the People's Republic of China due to potential health concerns.^[37]

Middle East

Cousin marriage has been allowed throughout the Middle East for all recorded history.^[38] Anthropologists have debated the significance of the practice; some view it as the defining feature of the Middle Eastern kinship system^[39] while others note that overall rates of cousin marriage have varied sharply between different Middle Eastern communities.^[40] Very little numerical evidence exists of rates of cousin marriage in the past.^[41]

Raphael Patai reports that in central Arabia, no relaxation of a man's right to the father's brother's daughter seems to have taken place in the past hundred years before his 1962 work. Here the girl is not forced to marry her male cousin, but she cannot marry another unless he gives consent.^[42] 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 male cousin. When the marriage procession progressed with the bride toward the house of the bridegroom, the male cousin rushed forward, snatched away the girl, and forced her into his own house. This was regarded by all as a lawful marriage.^[43] In Iraq, the right of the cousin also traditionally was followed ^[44] 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.^[45]

In-marriage was more 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.^[46] In Egypt, estimates from the late 19th and early 20th centuries state variously that either 80% 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 Syria-Palestina, if a girl had no paternal male cousin (father's brother's son) or he renounced his right to her, the next in line was traditionally the maternal male cousin (mother's brother's son) and then other relatives. Raphael Patai, however, reported that this custom loosened in the years preceding his 1947 study.^[43] In ancient Persia, the Achaemenid kings habitually married their cousins and nieces,^[47] while between the 1940s and 1970s, the percentage of Iranian cousin marriages increased from 34 to 44%.^[48] Cousin marriage among native Middle Eastern Jews is generally far higher than among the European Ashkenazim, who assimilated European marital practices after the diaspora.^[49]

According to anthropologist Ladislav Holý, 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 Holý, 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".^[50] 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.^[51] 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 such as attacks on her or her own unchaste behavior.^[52] 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.^[53][54] Throughout Middle Eastern history, cousin marriage has been both praised and discouraged by various writers and authorities.^[55]

A 2009 study found that many Arab countries display some of the highest rates of consanguineous marriages in the world, and that first cousin marriages which may reach 25–30% of all marriages.^[56] In Qatar, Yemen, and UAE, consanguinity rates are increasing in the current generation. Research among Arabs and worldwide has indicated that consanguinity could have an effect on some reproductive health parameters such as postnatal mortality and rates of congenital malformations.^[57]

Africa

Cousin marriage rates from most African nations outside the Middle East are unknown. An estimated 35–50% of all sub-Saharan African populations either prefer or accept cousin marriages.^[59] In Nigeria, the most populous country of Africa, the three largest ethnic groups in order of size are the Hausa, Yoruba, and Igbo.^[60] The Hausa are overwhelmingly Muslim, though followers of traditional religions do exist. Muslim Hausas practice cousin marriage preferentially, and polygyny is allowed if the husband can support multiple wives.^[61] 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 14.^[62] Divorce can be accomplished easily by either the male or the female, but females must then remarry.^[63] Even for a man, lacking a spouse is looked down upon.^[64] 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.^[65]

50% of the Yoruba people are Muslim, 40% Christian and 10% adherent of their own indigenous religious traditions.^[66] 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 this was not a general study of Yoruba, but only of highly polygynous Yoruba residing in Oka Akoko.^[67]

The Igbo people of southeastern Nigeria, who are predominantly Christian, strictly practice non-consanguineal marriages, where kinfolks and cousins are not allowed to marry or have intimacy. Consequently men and women are forbidden to marry within their recent patrilineage and matrilineage. Before the advent of Christianity through colonization, the Igbos had always frowned upon and specifically prohibited consanguineal marriages, both the parallel and cross-cousin types, which are considered incestuous and cursed. Arranged marriages, albeit in great decline, were also to consciously prevent accidental consanguineal and bad marriages, such that the impending in-laws were aware of each other's family histories. Currently, as in the old days, before courtship commences thorough enquiries are made by both families not only to ascertain character traits but to also ensure their children are not related by blood. Traditionally parents closely monitor those with whom their children are intimate to avoid them committing incest. It is customary for parents to bring their children up to know their immediate cousins and, when opportune, their distant cousins. They encourage their adult children to disclose their love interests for consanguineal screening.^[68]

In Ethiopia most of the population was 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.^[69] 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’.^[70] Though Muslims make up more than 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.^[71] In contrast to the Nigerian situation, in Ethiopia Islam cannot be identified with a particular ethnicity and is found across most of them, and conversions between religions are comparatively common.^[72] The Afar practice a form of cousin marriage called absuma, which is arranged at birth and can be forced.^[73]

Catholic Church and Europe

The number next to each box in the Table of Consanguinity indicates the degree of relationship relative to the given person according to Roman law.

Roman civil law prohibited marriages within four degrees of consanguinity.^[74] This was calculated by counting up from one prospective partner to the common ancestor, then down to the other prospective partner.^[75] Early Medieval Europe continued the late Roman ban on cousin marriage. Under the law of the Catholic Church, couples were also forbidden to marry if they were within four degrees of consanguinity.^[76] These laws would severely cripple the existing European kinship structures, replacing them with the smaller nuclear family units.^[77]

In the 9th century, however, the church raised the number of prohibited degrees to seven and changed the method by which they were calculated. Instead of the former practice of counting up to the common ancestor and then down to the proposed spouse, the new law computed consanguinity by counting only back to the common ancestor.^[78] In the Catholic Church, unknowingly marrying a closely consanguineous blood relative was grounds for a declaration of nullity. But during the 11th and 12th centuries, dispensations were granted with increasing frequency due to the thousands of persons encompassed in the prohibition at seven degrees and the hardships this posed for finding potential spouses.^[79] Eventually, the nobility became too interrelated to marry easily as the local pool of unrelated prospective spouses became smaller; increasingly, large payments to the church were required for exemptions ("dispensations"), or retrospective legitimizations of children.^[80]

In 1215, the Fourth Lateran Council reduced the number of prohibited degrees of consanguinity from seven back to four.^[81][82] After 1215, the general rule was that while fourth cousins could marry without dispensation, the need for dispensations was reduced.^[79]

For example, the marriage of Louis XIV of France and Maria Theresa of Spain was a first-cousin marriage on both sides.^[83] It began to fall out of favor in the 19th century as women became socially mobile. Only Austria, Hungary, and Spain banned cousin marriage throughout the 19th century, with dispensations being available from the government in the last two countries.^[84] First-cousin marriage in England in 1875 was estimated by George Darwin to be 3.5% for the middle classes and 4.5% for the nobility, though this had declined to under 1% during the 20th century.^[85] Queen Victoria and Prince Albert were a preeminent example.^[86][87]

The 19th-century academic debate on cousin marriage developed differently in Europe and America. The writings of Scottish deputy commissioner for lunacy Arthur Mitchell claiming that cousin marriage had injurious effects on offspring were largely contradicted by researchers such as Alan Huth and George Darwin.^[88][89] In fact, Mitchell's own data did not support his hypotheses and he later speculated that the dangers of consanguinity might be partly overcome by proper living. Later studies by George Darwin found results that resemble those estimated today. His father, Charles Darwin – who married his first cousin – had initially speculated that cousin marriage might pose serious risks, but perhaps in response to his son's work, these thoughts were omitted from a later version of the book they published. When a question about cousin marriage was eventually considered in 1871 for the census, according to George Darwin, it was rejected on the grounds that the idle curiosity of philosophers was not to be satisfied.^[90] In Southern Italy, cousin marriage was a usual tradition in regions such as Calabria and Sicily, where first-cousin marriage in the 1900s was near to 50 percent of all marriages.^[91] Cousin marriage to third cousins is allowed and considered favorably in Greece.^[92]

United States

Anthropologist Martin Ottenheimer argues that marriage prohibitions were introduced to maintain the social order, uphold religious morality, and safeguard the creation of fit offspring.^[102] Writers such as Noah Webster (1758–1843) and ministers such as Philip Milledoler (1775–1852) and Joshua McIlvaine helped lay the groundwork for such viewpoints well before 1860. This led to a gradual shift in concern from affinal unions, such as those between a man and his deceased wife's sister, to consanguineous unions. By the 1870s Lewis Henry Morgan (1818–1881) was writing about "the advantages of marriages between unrelated persons" and the necessity of avoiding "the evils of consanguine marriage", avoidance of which would "increase the vigor of the stock". To many (Morgan included), cousin marriage, and more specifically parallel-cousin marriage, was a remnant of a more primitive stage of human social organization.^[103] Morgan himself had married his cousin in 1853.^[104]

In 1846 Massachusetts Governor George N. Briggs appointed a commission to study mentally disabled people (termed ‘idiots’) in the state. This study implicated cousin marriage as responsible for idiocy. Within the next two decades, numerous reports (e.g. one from the Kentucky Deaf and Dumb Asylum) appeared with similar conclusions: that cousin marriage sometimes resulted in deafness, blindness and idiocy. Perhaps most important was the report of physician Samuel Merrifield Bemiss for the American Medical Association, which concluded cousin inbreeding does lead to the "physical and mental deprivation of the offspring". Despite being contradicted by other studies such as those of George Darwin and Alan Huth in England and Robert Newman in New York, the report's conclusions were widely accepted.^[105]

These developments led to 13 states and territories passing cousin marriage prohibitions by the 1880s. Though contemporaneous, the eugenics movement did not play much of a direct role in the bans. George Louis Arner in 1908 considered the ban a clumsy and ineffective method of eugenics, which he thought would eventually be replaced by more refined techniques. By the 1920s the number of bans had doubled.^[10] Since that time Kentucky (1943) and Texas have banned first-cousin marriage, and since 1985 Maine has mandated genetic counseling for marrying cousins to minimize the risk of any serious health defects for their children. The National Conference of Commissioners on Uniform State Laws unanimously recommended in 1970 that all such laws should be repealed, but no state has dropped its prohibition.^[8][19][106]

East Asia

In the Far 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.^[107]

Taiwan and North Korea also prohibit first-cousin marriage.^[8][108]

China has prohibited first-cousin marriage since 1981.^[109] Currently, according to the Marriage Law of the People's Republic of China, Article 7, "No marriage may be contracted under any of the following circumstances: (1) if the man and the woman are lineal relatives by blood, or collateral relatives by blood up to the third degree of kinship."^[110] This was then encompassed in the Civil Code, which takes effect in 2021, as its Article 1048.

Unlike China mainland, the two special administrative regions of China, Hong Kong^[111] and Macau,^[112] place no restrictions on marriage between cousins.

Southeast Asia

In Vietnam, Clause 3, Article 10 of the 2000 Vietnamese Law on Marriage and Family forbids marriages of people related by blood up to the third degree of kinship.^[113][114] Cousin marriage is also prohibited in the Philippines.

United States

Laws regarding first-cousin marriage in the United States

  First-cousin marriage is legal

  Allowed with requirements

  Banned with exceptions¹

  Statute bans marriage¹

  Criminal offense¹

---

¹Some US states recognize marriages performed elsewhere, especially when the spouses were not residents of the state when married.^{clarification needed}

Several states of the United States have bans on cousin marriage.^[115][116] As of February 2014, 24 U.S. states prohibit marriages between first cousins, 19 U.S. states allow marriages between first cousins, and 7 U.S. states allow only some marriages between first cousins.^[7] Six states prohibit first-cousin-once-removed marriages.^[11] Some states prohibiting cousin marriage recognize cousin marriages performed in other states, but this does not hold true in general despite occasional claims to the contrary.^[117]

Brazil

Recent 2001 data for Brazil indicate a rate of cousin marriage of 1.1%, down from 4.8% in 1957.^[121] The geographic distribution is heterogeneous: in certain regions, the rate is at typical European levels, but in other areas is much higher. Newton Freire-Maia found paternal parallel cousin marriage to be the most common type.^[122] 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,^[123] 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%,^[124] but a century later, it was merely 1.9%.^[121]

East Asia

First-cousin marriage is allowed in Japan, though the incidence has declined in recent years.^[20]

China has prohibited first-cousin marriage since 1981,^[109] although cross-cousin marriage was commonly practiced in China in the past in rural areas.^[20][125] An article in China Daily from the 1990s reported on the ban's implementation in the northeastern province of Liaoning, along with a ban on marriage of people who were physically and mentally disabled, all justified on "eugenic" grounds.^[125] Limited existing data indicate some remaining cousin marriage of types besides father's brother's daughter in many villages, with percentages usually in the lower single digits.^[121] 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.^[126]

Europe

Middle East

The Middle East has uniquely high rates of cousin marriage among the world's regions. Iraq was estimated in one study to have a rate of 33% for cousins marrying.

All Arab countries in the Persian Gulf currently require advance genetic screening for prospective married couples. Qatar was the last Persian 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.^[139] 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 Ahmad Teebi suggests consanguinity is declining in Lebanon, Jordan, Morocco, and among Palestinians, but is increasing in the United Arab Emirates.^[140]

Ahmad Teebi links the increase in cousin marriage in Qatar and other Arab states of the Persian Gulf 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."^[17]

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.^[141] One anthropologist, Ladislav Holý, argued 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 cited the Berta people of Sudan, who consider the FBD to be the closest kinswoman to a man outside of the prohibited range. If more than one relationship exists 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.^[142][143] 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, the Baggara Arabs 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. No preference is shown for marriages between matrilateral parallel cousins.

South Asia

United States

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.^[154] 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%.^[159] 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."^[160]

A bill to repeal the ban on first-cousin marriage in Minnesota was introduced by Phyllis Kahn in 2003, but it died in committee. Republican Minority Leader Marty Seifert criticized the bill in response, saying it would "turn us into a cold Arkansas".^[161] 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.^[162]

In contrast, Maryland delegates Henry B. Heller and Kumar P. Barve sponsored a bill to ban first-cousin marriages in 2000.^[163] 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".^[164]

Texas passed a ban on first-cousin marriage the same year as Amrhein and Andrews married, evidently in reaction to the presence of the polygamous Fundamentalist Church of Jesus Christ of Latter-Day Saints (FLDS). Texas Representative Harvey Hilderbran, whose district includes the main FLDS compound, authored an amendment^[165] 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".^[166] While Hilderbran stated that he would not have authored a bill solely to ban first-cousin marriage, he also said in an interview, "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."^[2] 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.^{[167][168][169][170]} The new statute made sex with an adult first cousin a more serious felony than with adult members of one's immediate family. However, this statute was amended in 2009; while sex with close adult family members (including first cousins) remains a felony, the more serious penalty now attaches to sex with an individual's direct ancestor or descendant.^[171]

The U.S. state of 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.^[172] In the other 25 states permitting at least some first-cousin marriage, double cousins are not distinguished.^[173]

States have various laws regarding marriage between cousins and other close relatives,^[174] which involve factors including whether or not the parties to the marriage are half-cousins, double cousins, infertile, over 65, or whether it is a tradition prevalent in a native or ancestry culture, adoption status, in-law, whether or not genetic counseling is required, and whether it is permitted to marry a first cousin once removed.

Russia

Hebrew Bible

Jacob encountering Rachel with her father's herds

Cousins are not included in the lists of prohibited relationships set out in the Hebrew Bible, specifically in Leviticus 18:8–18 and 20:11–21 and in Deuteronomy.^[3]

There are several examples in the Bible of cousins marrying. Isaac married Rebekah, his first cousin once removed (Genesis 24:12–15). Also, Isaac's son Jacob married Leah and Rachel, both his first cousins (Genesis 28–29). Jacob's brother Esau also married his first half-cousin Mahalath, daughter of Ishmael, Isaac's half-brother. According to many English Bible translations, the five daughters of Zelophehad married the "sons of their father's brothers" in the later period of Moses; although other translations merely say "relatives". (For example, the Catholic RSV-CE and NAB differ in Numbers 36:10–12.) The Hebrew Bible states: בְּנ֣וֹת צְלָפְחָ֑ד לִבְנֵ֥י דֹֽדֵיהֶ֖ן which translates literally as "the daughters of Zelophehad to their cousins/to their uncles' sons".^[190] 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 says 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, according to the book of Tobit, Tobias had a right to marry Sarah because he was her nearest kinsman (Tobit 7:10), though the exact degree of their cousinship is not clear.

Christianity

Islam

The Qur'an does not state that marriages between first cousins are forbidden. In Sura An-Nisa (4:22–24), Allah mentioned the women who are forbidden for marriage: to quote the Qur'an, "... 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),

O Prophet, indeed We have made lawful to you your wives to whom you have given their due compensation and those your right hand possesses from what Allah has returned to you [of captives] and the daughters of your paternal uncles and the daughters of your paternal aunts and the daughters of your maternal uncles and the daughters of your maternal aunts who emigrated with you and a believing woman if she gives herself to the Prophet [and] if the Prophet wishes to marry her, [this is] only for you, excluding the [other] believers. We certainly know what We have made obligatory upon them concerning their wives and those their right hands possess, [but this is for you] in order that there will be upon you no discomfort. And ever is Allah Forgiving and Merciful.^[201][202]

Muslims have practiced marriages between first cousins in non-prohibited countries since the time of Muhammad. In a few countries the most common type is between paternal cousins.^[201] Muhammad actually did marry two relatives.^[21] 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 the issue of adoption and not cousinship that caused controversy due to the opposition of pre-Islamic Arab norms.^[203]

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,^[204][205] while Ali married Fatimah,^[206] the daughter of his paternal first cousin Muhammad and hence his first cousin once removed.^[207]

Although marrying his cousin himself, Umar, the second Caliph, discouraged marrying within one's bloodline or close cousins recurringly over generations and advised those who had done so to marry people unrelated to them, by telling a household that did so, "You have become frail, so marry intelligent people unrelated to you."^[208]

Though many Muslims marry their cousins now, two of the Sunni Muslims madhhabs (schools, four in total) like Shafi'i (about 33.33% of Sunni Muslims, or 29% of all Muslims) and Hanbali consider it as Makruh (disliked).^[209] Imam Shafi'i, the founder of the Shafi'i madhab, went further in his condemnation of persistent generational bloodline marriages and said, "Whenever the people of a household do not allow their women to marry men outside of their line, there will be fools among their children."^[208]

Hinduism

The Hindu Marriage Act prohibits 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.^[146][210]

Hindu rules of exogamy are often taken extremely seriously, and local village councils in India administer laws against in-gotra endogamy.^[211] Social norms against such practices are quite strong as well.^[212]

In the 18th and 19th Centuries, Hindu Kurmis of Chunar and Jaunpur are known to have been influenced by their Muslim neighbors and taken up extensively the custom of cousin marriage.^[213]

In the Mahabharata, one of the two great Hindu Epics, Arjuna took as his fourth wife his cross-cousin Subhadra. Arjuna had gone into exile alone after having disturbed Yudhishthira and Draupadi in their private quarters. It was during the last part of his exile, while staying at the Dvaraka residence of his cousins, that he fell in love with Subhadra. While eating at the home of Balarama, Arjuna was struck with Subhadra's beauty and decided he would obtain her as his wife. Subhadra and Arjuna's son was the tragic hero Abhimanyu. According to Andhra Pradesh oral tradition, Abhimanyu himself married his cross-cousin Shashirekha, the daughter of Subhadra's brother Balarama.^[214][215] Cross cousin marriage is also evident from Pradyumna's (Eldest son of Krishna) marriage to Rukmi's (Brother of Rukmini) daughter. Also Krishna married his cross cousin Mitravinda (daughter of Vasudeva's sister Rajadhi who was Queen of Avanti) and Bhadra (Daughter of Vasudeva's sister Shrutakirti who was the Queen of Kekaya Kingdom.)

Other religions

Buddhism does not proscribe any specific sexual practices, only ruling out "sexual misconduct" in the Five Precepts.^[216] Zoroastrianism allows cousin marriages.^[217] Sikhism largely follows the pattern of ban on same-clan marriages; many Sikhs choose to marry their children with a partner from a different village or town, for a lower degree of consanguinity between them.

Genetics

Cousin marriages have genetic aspects that increase the chance of sharing genes for recessive traits. The percentage of consanguinity between any two individuals decreases fourfold as the most recent common ancestor recedes one generation. First cousins have four times the consanguinity of second cousins, while first cousins once removed have half that of first cousins. 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 1.1–2.0 percentage points above the average base risk for non-cousin couples of 3%, or about the same as that of any woman over age 40.^[218] In terms of mortality, a 1994 study found a mean excess pre-reproductive mortality rate of 4.4%,^[219] while another study published in 2009 suggests the rate may be closer to 3.5%.^[2] Put differently, a single 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.^[220]

Repeated consanguineous marriages within a group are more problematic. 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.^[221]

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.^[222] 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.^[223] First-cousin marriage is taboo among Amish, but they still have several rare genetic disorders. In Ohio's Geauga County, Amish make up only about 10 percent of the population but represent half the special needs cases. In the case of one debilitating seizure disorder, the worldwide total of 12 cases exclusively involves the Amish.^[224] Similar disorders have been found in the Fundamentalist Church of Jesus Christ of Latter-Day Saints, who do allow first-cousin marriage and of whom 75 to 80 percent are related to two 1830s founders.^[225][226]

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. Consanguinity seems to affect many polygenic traits such as height, body mass index, intelligence and cardiovascular profile.^{[227][228][229]} 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.^[230]

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.^[230] 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.^[231]

A BBC report discussed Pakistanis in Britain, 55% of whom marry a first cousin.^[232] 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.^[233] 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. Finally, in 2010 the Telegraph reported that cousin marriage among the British Pakistani community resulted in 700 children being born every year with genetic disabilities.^[234]

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.^[235] 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."^[236] 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.^[237]

Comprehensive genetic education and premarital genetic counseling programs can help to lessen the burden of genetic diseases in endogamous communities. Genetic education programs directed at 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.^[238] 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 government-run national health service 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 genetic counseling is mandatory to obtain a marriage license for first cousins. 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".^[239] He states that the social, cultural, and economic benefits of cousin marriage also need to be fully considered.^[240]

In Nepal, consanguineous marriage emerged as a leading cause of eye cancer in newborn children in 2017.^[241]

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 first-cousin marriages because this compensates for the observed increase in child mortality.^[242] However, there is a large increase in fertility for third and fourth cousin marriages, whose children exhibit more fitness than both unrelated individuals or second cousins.^[243] The total fertility increase may be partly explained by the lower average parental age at marriage or the age at first birth, observed in consanguineous marriages. Other factors include shorter birth intervals and a lower likelihood of outbreeding depression or using reliable contraception.^[20] 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.^[244] According to a recent paper the fertility difference is probably not due to any underlying biological effect.^[245] In Iceland, where marriages between second and third cousins were common, in part due to limited selection, studies show higher fertility rates.^[246] Earlier papers 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.^[247]