Biological aspects
[edit] 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.[145] In terms of mortality, a 1994 study found a mean excess pre-reproductive mortality rate of 4.4%,[146] while another study published in 2009 suggests the rate may be closer to 3.5%.[2] 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.[147] Critics argue that banning first-cousin marriages would make as much sense as trying to ban childbearing by older women. It should be noted that 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%, it was estimated that infant mortality was 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.[148]

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, because of the founder effect and accelerated genetic drift in a breeding pool of restricted size.[149] Consider the case of the Amish: because the entire 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.[150] First-cousin marriage is taboo, but despite this, the Amish suffer from several rare genetic disorders. In Ohio’s Geagua County, the 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.[151] 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.[152][153]

Traits with a polygenic multifactorial pattern of inheritance appear to be little affected by consanguinity. 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.[154]

A BBC report[155] 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.[156]

According to a statement by the UK’s Human Genetics Commission on cousin marriages, the BBC “fails to clarify” that children born to these marriages are not 13 times more likely to develop genetic disorders. Rather, 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.”[157] Genetic effects from cousin marriage in a developed country like Britain are also more obvious because the number of confounding environmental diseases is lower than in a developing country like Pakistan. Increased focus on genetic disease in developing countries may eventually result from progress in eliminating environmental diseases there as well.[158]

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 Human Genetics Commission recommends comparing the strategy taken in dealing with cousin marriage to previous strategies on 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 couples whose relationship is consanguineous, preferably before they conceive, in order to establish the precise risk of a genetic abnormality in their children.” 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.”[159] Social, cultural, and economic benefits of cousin marriage need to also be fully considered.[160]