In the United States consanguineous marriage (marriage between close relatives, often cousins) is frowned upon and in many states banned but it is common elsewhere in the world. Approximately 0.2% of all marriages are consanguineous in the United States but in India 26.6% marriages are consanguineous, in Saudi Arabia the figure is 38.4% and in Niger, Pakistan and Sudan a majority of marriages are consanguineous. Cousin marriage used to be more common in the West and was particularly common among royal families which gives some hints as to why it may sometimes be useful. Among families with titles or estates, cousin marriage will tend to keep the wealth intact–literally within the family–whereas wealth becomes more dilute more quickly with outside marriage. Cousin marriage may also increase cooperation within the extended family and help to fight off parasites.
A recent paper finds that consangunuity is strongly negatively correlated with democracy:
How might consanguinity affect democracy? Cousin marriages create extended families that
are much more closely related than is the case where such marriages are not practiced. To illustrate,
if a man’s daughter marries his brother’s son, the latter is then not only his nephew but also
his son-in-law, and any children born of that union are more genetically similar to the two grandfathers
than would be the case with non-consanguineous marriages. Following the principles of
kin selection (Hamilton, 1964) and genetic similarity theory (Rushton, 1989, 2005), the high
level of genetic similarity creates extended families with exceptionally close bonds. Kurtz succinctly
illustrates this idea in his description of Middle Eastern educational practices:
If, for example, a child shows a special aptitude in school, his siblings might willingly
sacrifice their personal chances for advancement simply to support his education. Yet once
that child becomes a professional, his income will help to support his siblings, while his
prestige will enhance their marriage prospects. (Kurtz, 2002, p. 37).
Such kin groupings may be extremely nepotistic and distrusting of non-family members in the
larger society. In this context, non-democratic regimes emerge as a consequence of individuals turning to reliable kinship groupings for support rather than to the state or the free market. It has
been found, for example, that societies having high levels of familism tend to have low levels of
generalized trust and civic engagement (Realo, Allik, & Greenfield, 2008), two important correlates
of democracy. Moreover, to people in closely related kin groups, individualism and the
recognition of individual rights, which are part of the cultural idiom of democracy, are perceived
as strange and counterintuitive ideological abstractions (Sailer, 2004).
By the way, cousin marriage results in an elevated risk of birth defects but on the same order as a 40 year old woman having children as opposed to a 30 year old. In other words, the risks are small relative to other accepted risks. Results do get worse when cousin marriage is prevalent over many generations.