Although still prevalent in many human societies, the practice of cousin marriage has precipitously declined in populations undergoing rapid demographic and socioeconomic change. However, it is still unclear whether changes in the structure of the marriage pool or changes in the fitness-relevant consequences of cousin marriage more strongly influence the frequency of cousin marriage. Here, we use genealogical data collected by the Tsimane Health and Life History Project to show that there is a small but measurable decline in the frequency of first cross-cousin marriage since the mid-twentieth century. Such changes are linked to concomitant changes in the pool of potential spouses in recent decades. We find only very modest differences in fitness-relevant demographic measures between first cousin and non-cousin marriages. These differences have been diminishing as the Tsimane have become more market integrated. The factors that influence preferences for cousin marriage appear to be less prevalent now than in the past, but cultural inertia might slow the pace of change in marriage norms. Overall, our findings suggest that cultural changes in marriage practices reflect underlying societal changes that shape the pool of potential spouses.

Pakistan has a high burden of hereditary and congenital anomalies and their incidence rate almost doubles against the background of parental consanguinity. Consanguineous unions (CU) are customary in Pakistan and deeply rooted socio-cultural norms favour CU. This study aimed to elucidate the determinants and temporal change in CU in four northwestern populations of Pakistan. In a cross-sectional study, data on marital union types, bio-demographic factors, and paternal consanguinity were collected from 6,323 ever-married individuals in four districts of northwest Pakistan: Haripur, Muzaffarabad, Mansehra, and Shangla. We used descriptive statistics and multivariable logistic regression analysis. The CU were calculated to be 55%, and inbreeding coefficient F (ICF) was estimated to be 0.029. Eight factors, including district, rural origin, age of husband, occupational group of husband, literacy of husband, parental consanguinity, exchange marriage, and extended family type, were found to be significant predictors of consanguinity in the multivariable logistic regression analysis. The rate of consanguinity decreased significantly in the younger age categories of individuals. The rate of CU was seen to be declining over time and in marriages that started ‘before 1980’ and ‘after 2010’, respectively, and there was a decline in ICF from 0.030 to 0.027. These analyses also showed that the literacy rate improved, the average age at marriage increased, and the frequency of exchange marriages decreased over time. This study employs a sizable first-hand dataset to demonstrate a lowering CU rate in northwest Pakistan. It is anticipated that the burden of inherited and congenital anomalies may likely to diminish in the study populations along with the fall in ICF.

Cousin marriage, a spousal union between close kin, occurs at high frequencies in many parts of the world. The rates of cousin marriage in humans are concordant with empirical studies that challenge the traditionally held view that reproduction with kin is generally avoided in animals. Similarly, some theoretical models in animal behaviour show that inbreeding avoidance is more constrained than previously thought. Such studies highlight the importance of quantifying the costs and benefits of reproduction among close kin over the whole life-course. Here, we use genealogical data from two human populations with high frequencies of cousin marriage (the Dogon from Mali, and the Ancien Régime nobility from Europe) to estimate these potential costs and benefits. We compare age-specific fertility and survival curves, as well as the projected growth rates, of subpopulations of each marriage type. Fitness costs of cousin marriage are present in terms of reduced child survival (in both populations), while benefits exist as increased fertility for men (in the Dogon) and for women (in the Ancien Régime nobility). We also find some differences in the projected growth rates of lineages as a function of marriage type. Finally, we discuss the trade-offs that might shape marriage decisions in different ecological conditions.