Cousin marriage
A cousin marriage is a marriage where the spouses are cousins (i.e. people with common grandparents or people who share other fairly recent ancestors). The practice was common in earlier times and continues to be common in some societies today, though in some jurisdictions such marriages are prohibited.[1] Worldwide, more than 10% of marriages are between first or second cousins.[2] Cousin marriage is an important topic in anthropology and alliance theory.[3]
In some cultures and communities, cousin marriages are considered ideal and are actively encouraged and expected; in others, they are seen as incestuous and are subject to social stigma and taboo. Cousin marriage was historically practiced by indigenous cultures in Australia, North America, South America, and Polynesia.[4][5][6]
In some jurisdictions, cousin marriage is legally prohibited: for example, first-cousin marriage in both Chinas, both Koreas, the Philippines, for Hindus in some jurisdictions of India, some countries in the Balkans, and 30 of the 50 U.S. states.[7][8] It is criminalized in 8 states in the US, the only jurisdictions in the world to do so. The laws of many jurisdictions set out the degree of consanguinity prohibited among sexual relations and marriage parties. Supporters of cousin marriage where it is banned may view the prohibition as discrimination,[9][10] while opponents may appeal to moral or other arguments.[11]
Opinions vary widely as to the merits of the practice. Children of first-cousin marriages have a 4-6% risk of autosomal recessive genetic disorders compared to the 3% of the children of totally unrelated parents.[12] A study indicated that between 1800 and 1965 in Iceland, more children and grandchildren were produced from marriages between third or fourth cousins (people with common great-great- or great-great-great-grandparents) than from other degrees of separation.[13]
Biological aspects[edit]
Genetics[edit]
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] 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. The 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[edit]
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]