Paternal Cousin Beats Maternal Cousin as a Spouse

Marriage between close relatives—or, more precisely, the inbreeding among children born as a result of such unions—is harmful. This is because offspring are more likely than usual to inherit the same gene variant from both parents.

In such cases, recessive genes become expressed in the children's phenotype, causing recessively inherited diseases to manifest. As a result, children born to related parents face a higher risk of congenital structural abnormalities, developmental disabilities, deafness and blindness, as well as infant and child mortality. In addition, inbreeding reduces the cognitive ability of offspring born from such relationships, which has a negative effect on their success in life.

If marriages between relatives occur within the same family or community over several consecutive generations, genetic diversity decreases and the genetic risks increase further. Despite these disadvantages, marriage between cousins is common in many cultures.

More than 10 percent of the world's population enters into such marriages—this has also become increasingly common in Finland and other Western countries in recent years as a result of increased humanitarian immigration. For this reason, it has been suspected that such marriages may be associated with some benefit that has caused this—in itself highly harmful—cultural practice to become established in some cultures.

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One such benefit was identified in a study published by French researcher Olympia L. K. Campbell. It concerned the effect of kinship relationships on conflicts within the household, i.e. domestic violence.

The study was based on two competing hypotheses. According to the first, blood kinship would align spouses' interests and increase monitoring by relatives, thereby reducing men's incentives and opportunities to use coercion. According to the second hypothesis, close kinship networks would place family unity above women's autonomy, suppressing help-seeking and making it easier to control women.

Campbell compared these hypotheses using data collected from approximately 36,000–46,000 women in Egypt, Jordan, Pakistan, and Turkey. In this dataset, marriage to a first cousin was associated with a 2.5-percentage-point lower probability of physical intimate-partner violence compared with marriages in which the spouses were not blood relatives.

For a woman, this corresponds to an approximately 11 percent lower relative risk of violence. In addition, the researcher found a similar reduction in coercive controlling behavior toward wives.

Interestingly, marriage to a paternal cousin reduced the risk of physical violence more than marriage to a maternal cousin. Thus, it appears that the protective effect stems from the authority of the man's relatives over men, rather than from the spouse's kinship itself. 

Previous thoughts on the same topic:
Blame the Philanthropist Or Understand the Facts
Society, Genetics, and the Impact of Consanguineous Marriages: At the Intersection of Education and Heredity
Cousin is not just a delicacy, but also a hindrance to social success