Understanding whether inbreeding accounts for early mortality is a long-standing concern in demographic research. Analyzing Bedouin villages in Bekaa, Lebanon, in which the marriage rate among first cousins is more than twice the national average, a new study finds that the greatest single determinant of infant mortality is not closely related parents – though this does present a significant risk – but short birth intervals.

The Bekka Bedouin are Sunni Muslims. Traditionally nomadic, migrating with herds of sheep and goats to pastures in the Syrian desert, they have recently become more sedentary, though a continuing preference for kin as marriage partners – particularly ibn ‘amm (father’s brother’s son) or bint ‘amm (father’s brother’s daughter) – remains “a salient feature of Bedouin matrimonial life,” writes Suzanne E. Joseph (University of Massachusetts, Dartmouth).

About 47 percent of Bedouin marriages are between first cousins. Thirty-three percent are patrilateral, that is, between the children of brothers.

Analyzing a sample of 1,399 Bedouin children, Joseph examined the mortality rate for infants (< 12 months) and children under the age of five (12-59 months). She found that infants born to first cousins have more than double the odds of dying as infants born to non first-cousins.

As Joseph explains, the preference for choosing relatives as marriage partners may be a default marital strategy in situations where geographic isolation restricts the size of the mating pool, such as in nomadic societies. In sedentary societies, reproductive isolation may have an economic angle, preventing the fragmentation of property or facilitating unions among the poor by allowing them to circumvent dowry payments.

“While there is a heightened risk of infant mortality associated with consanguinity, even after controlling for socioeconomic and demographic factors, there are also substantial social, economic, and emotional benefits to marrying kin,” Joseph writes. “Women in particular are able to draw upon the support of their family members after marriage, which enhances their position in the domestic unit.”

Among populations with a high level of familial endogamy, there may also be a relatively high risk of recessive disorders which develop in childhood. However, the children of Bedouin first-cousin parents were not significantly more likely to die in childhood, Joseph found.

Indeed, the most statistically significant factor in a Bedouin child’s survival – whether the child of first cousins or not – is birth interval, Joseph reveals. For every additional month that passes before the birth of the next child, the odds of infant death decrease by 3.7 percent.

Joseph, Suzanne E., “Consanguineous Marriage and Early Mortality in a Reproductive Isolate.” Current Anthropology 48:5.