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.[179] In terms of mortality, a 1994 study found a mean excess pre-reproductive mortality rate of 4.4%,[180] 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.[181] 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.
