HT Steve Sailer for linking to Alan Bittles and Michael Black’s consang.net The site offers maps and tables of consanguinity/endogamy data from around the globe as well as a research summary by Alan Bittles from which the following quotes are excerpted.
Sociodemographic aspects of consanguinity
The reasons most commonly given for the popularity of consanguineous marriage can be summarized as: a strong family tradition of consanguineous unions; the maintenance of family structure and property, and the strengthening of family ties; financial advantages relating to dowry or bridewealth payments; the ease of marital arrangements and a closer relationship between the wife and her in-laws; and greater marriage stability and durability (Bittles 1994; Hussain 1999).
Although the usual account of consanguineous marriage is broadly negative and associated with low status—so much that “inbred” is usually understood as a term of derision—Bittles also notes that:
In some populations a high prevalence of marital unions between close relatives has however been reported among land-owning families, and in traditional ruling groups and the highest socioeconomic strata (Bittles 1994, 1995a).
Consanguinity and reproductive behaviour
As we have already seen in the Iceland study the price for outbreeding/exogamy may be reduced fertility. Bittles doesn’t disagree:
In general, higher total fertility rates are reported for consanguineous marriages (Bittles 1995b).
Consanguinity, morbidity, and mortality
The dark side. Bittles reports robust associations between first-cousin levels of consanguinity and increased child mortality, increased risk of rare genetic deseases and preliminary indications of increased risk of adult disorders such as cancer and cardiovascular disease.
Irrespective of prevailing legislation, a future decline in the prevalence of consanguineous unions can be predicted, accompanying the expected reduction in family sizes. It seems probable that this decline will not be uniform in effect across populations but will be mainly observed in urbanized populations and among couples who share higher educational standards and later ages at marriage.