Close inbreeding and low genetic diversity in Inner Asian human populations despite geographical exogamy

When closely related individuals mate, they produce inbred offspring, which often have lower fitness than outbred ones. Geographical exogamy, by favouring matings between distant individuals, is thought to be an inbreeding avoidancemechanism; however, no data has clearly tested this prediction. Here, we took advantage of the diversity of matrimonial systems in humans to explore the impact of geographical exogamyon genetic diversityand inbreeding. We collected ethno-demographic data for 1,344 individuals in 16 populations from two Inner Asian cultural groups with contrasting dispersal behaviours (Turko-Mongols and Indo-Iranians) and genotyped genome-wide single nucleotide polymorphisms in 503 individuals. We estimated the population exogamyrate and confirmed the expected dispersal differences: Turko-Mongols are geographically more exogamous than Indo-Iranians. Unexpectedly, across populations, exogamypatterns correlated neither with the proportion of inbred individuals nor with their genetic diversity. Even more surprisingly, among Turko-Mongols, descendantsfrom exogamous couples were significantly more inbred than descendantsfrom endogamous couples, except for large distances (>40 km). Overall, 37% of the descendantsfrom exogamous couples were closely inbred. This suggests that in Inner Asia, geographical exogamyis neither efficient in increasing genetic diversitynor in avoiding inbreeding, which might be due to kinship endogamydespite the occurrence of dispersal.