Turkey has a high rate of consanguineous marriage (21·1 %), indicating strong preference for this traditional form of marital union. Social and cultural factors are especially important in marriages between first and second cousins. Fertility is high, the closed birth interval is long, and the sterility rate is low among these couples. Post‐neonatal, infant and under‐5 mortalities are high in first cousin unions by comparison with non‐consanguineous marriages. According to the results of the study, first cousin marriage is a significant determinant underlying the high total fertility and infant mortality rates in Turkey.