Modelling the constraints on consanguineous marriage when fertility declines

Background: Consanguinity - or marriage between close blood relatives, in particular first cousins - is widely practised and even socially encouraged in many countries. However, in the face of fertility transition where the number of cousins eligible to marry declines, how might such constraints on consanguinity develop in the future? Objective: Numerous studies have stated that the practice cannot continue at present levels and in ist present form in the face of fertility transition. However, the future impact of fertility transition on availability of cousins to marry has not yet been quantified. Methods: We perform a simulation exercise using past and projected net reproduction rates (NRRs) derived from the UN. We calculate the average number of cousins of the opposite sex as a function of the average number of children, the average probability of an individual having at least one eligible paternal cousin of the opposite sex, and conclude with an examination of constraints on consanguineous marriage in selected countries under different fertility assumptions. Results: Current and projected fertility levels in Middle Eastern countries will create challenging constraints on the custom once today's birth cohorts reach marriageable age. Conclusions: Either consanguinity prevalence will diminish significantly, or the institution will be forced to adapt by becoming more coercive in the face of reduced choice or at the expense of other social preferences (such as for an older groom wedding a younger bride). Fertility decline affects prospects for social change not only through its well-known consequences for mothers but also through shaping marriage conditions for the next generation.