Recent evidence based on US data suggests that the quarter or month of birth (QOB or MOB) may be endogenous, since family characteristics can explain up to 50% of the effects of QOB on the education outcomes and earnings of adult males. In this study, based on a sample of one million Taiwanese siblings, we examine university admission at age 18 as our outcome variable and find that at school entry, the oldest (September born) children are 31–38% more likely to be admitted into university at age 18 than the youngest (August born) children, indicating strong seasonality in university admission. The inclusion of controls for family background is found to explain only a small portion of these effects, particularly for males. Given that such results are at odds with the recent US evidence, we revisit the US Census data and find that when racial differences are properly controlled for in the estimation, even a rich set of family characteristics is capable of explaining only a minor proportion of the QOB effects. Furthermore, using data from the US and Indonesia, we find that seasonal temperature variation is unlikely to be an important contributor to the US‐Taiwan disparity. Our findings imply that the validity of using QOB or MOB as an instrumental variable may be dependent on the population being studied and the sample selected.