No evidence of positive causal effects of maternal and paternal age at first birth on children's test scores at age 10 years.

Research has shown that higher maternal and paternal age is positively associated with children's education. Debate continues as to whether these relationships are causal. This is of great interest given the postponement of first births in almost all developed countries during the twentieth century. Here we use an instrumental variable approach (Mendelian randomization) using maternal and paternal polygenic indices (PGIs) for age at first birth-while conditioning on the child's PGI for age at first birth-to identify the causal effects of maternal and paternal age at first birth on children's test scores based on data from the Norwegian Mother, Father and Child Cohort study. We do not find evidence of positive causal effects of both maternal and paternal age at first birth on children's test scores at age 10 years once the children's PGI and correlations among different PGIs are controlled for. We therefore conclude that our results do not provide evidence in favour of sociological theories that predict positive causal effects of parental age on children's educational attainment.