The effect of the month of birth on academic achievement: heterogeneity by social origin and gender

Students born in the days before the school-entry cutoff date are one year younger at school entry than their counterparts born in the days following the cutoff, which causes an achievement gap between the youngest and oldest students within each cohort. I assess this so-called relative age effect in the Spanish context, where the school-entry rule is strictly enforced. By means of a sharp Regression Discontinuity Design, I report a remarkable effect in fourth grade that is still visible in eighth grade. The heterogeneity of the effect by social origin and gender is explored. Following the compensatory advantage model, I reason that the relative age effect should be smaller among high-SES students when assessing academic outcomes that might endanger the probability of attaining higher education (such as repetition), but not for low-stakes external examinations. I confirm this expectation. As for gender, I argue that the relative age effect might be lower among girls either because younger girls are not ever affected by their birth month or because they are better able to catch up with their older peers than boys. I provide evidence supporting this second hypothesis.