On the genetic bias of the quarter of birth instrument

Many studies in economics use quarter of birth as an instrument for identifying the causal effect of schooling on outcomes such as earnings and health. The key assumption in these studies is that people born in different quarters of the year do not differ systematically in their unobserved abilities. This study uses genetic data from the US Health and Retirement Study to analyze the validity of the quarter of birth instrument. We find some evidence that genetic factors influencing education are not randomly distributed over the year. However, these factors only slightly change the effect of quarter of birth on schooling.