Consanguinity in the contemporary world.

The rarity of consanguineous marriage in much of the industrialized world, and the current restriction or outright prohibition of first-cousin unions in a majority of US states and in East Asian countries such as China, North and South Korea and the Philippines, has led many commentators to overlook the fact that the Out-of-Africa migration some 60,000–70,000 years ago had been undertaken by quite small numbers of individuals. Current estimates of the numbers of migrants generally agree on a maximum of approximately 10,000 breeding individuals [6–9] , but with some simulations suggesting as few as 700 or even 450 emigrating adults [10, 11] . There is some controversy as to whether the migration from Africa was accompanied by one or more bottlenecks [12–14] and the possibility of limited back-migration to North Africa [15] , but population expansions over ensuing generations have led to the fact that some 6.0 billion of the current global population of 7.3 billion are Out-of-Africa descendants [16] . It seems inevitable that reproduction would have involved closely related individuals during the early days of these population expansions, when human groups remained small in number, largely nomadic, and geographically scattered, and this pattern of partner choice perIn medical genetics literature, a consanguineous marriage is usually defined as the union of a couple related as second cousins or closer ( F = 0.0156) [1] , although in some subpopulation isolates, for example in Finland, it has been demonstrated that more distant genetic relationships may also be important in the expression of rare recessive disorders [2] . Until the middle of the 19th century, close-kin marriage was commonplace and widely accepted in Western countries, albeit with general disapproval from the Orthodox Church and diocesan dispensation required for cousin marriages conducted within the Roman Catholic Church [3] . A significant change in the public attitude occurred thereafter, with a gradual decline in the popularity of consanguineous unions in Western Europe, where affine relationships had previously been regarded with suspicion, in particular between a man and his deceased wife’s sister in Victorian England [4] . At the same time, in the USA, an increasing number of states introduced a legislation banning first-cousin marriages [5] . These trends have continued so that, with the exception of recent migrant communities, close-kin unions now account for <1% of marriages in Western countries (www.consang.net; fig. 1 ). Published online: July 29, 2014