In the struggle between society and family, the exponential mathematics of kinship ordinarily works to the advantage of society. As time passes or groups get larger, family trees intertwine, dynasties dissipate, and nepotistic emotions get diluted. But families can defend themselves with a potent tactic: they can graft the twig tips of the family tree together by cousin marriage. If you force your daughter to marry her first cousin, then your son-in-law is your nephew, her father-inlaw is your brother, your parents’ estate will be worth twice as much per grandchild, and the couple will never have to bicker about which side of the family to visit on holidays. For these reasons, clans and dynasties in many cultures encourage first-or second-cousin marriage, tolerating the slightly elevated risk of genetic disease. Not only does cousin marriage amplify the average degree of relatedness among members of the clan, but it enmeshes them in a network of triangular relationships, with kinsmen valuing each other because of their many mutual kin as well as their own relatedness. As a result, the extended family, clan, or tribe can emerge as a powerfully cohesive bloc–and one with little common cause with other families, clans, or tribes in the larger polity that comprises them. The anthropologist Nancy Thornhill has shown that the prohibitions against incestuous marriages in most societies are not public-health measures aimed at reducing birth defects but the society’s way of fighting back against extended families.
In January 2003, during the buildup to the war in Iraq, the journalist and blogger Steven Sailer published an article in The American Conservative in which he warned readers about a feature of that country that had been ignored in the ongoing debate. As in many traditional Middle Eastern societies, Iraqis tend to marry their cousins. About half of all marriages are consanguineous (including that of Saddam Hussein, who filled many government positions with his relatives from Tikrit). The connection between Iraqis’ strong family ties and their tribalism, corruption, and lack of commitment to an overarching nation had long been noted by those familiar with the country. In 1931, King Faisal described his subjects as ”devoid of any patriotic idea ... connected by no common tie, giving ear to evil; prone to anarchy, and perpetually ready to rise against any government whatsoever.” Sailer presciently suggested that Iraqi family structure and its mismatch with the sensibilities of civil society would frustrate any attempt at democratic nation-building. [More]
Overall, Pinker does an excellent job of synthesizing what I've been writing for years, with one lacuna, which I'll explain at another time