On the Validity of Season of Birth as an Instrument in Wage Equations: A Comment on Angrist & Krueger's "Does Compulsory School Attendance Affect Scho

In an important and provocative paper, `Does Compulsory School Attendance Affect Schooling and Earnings?', Angrist and Krueger use quarter of birth as an instrument for educational attainment in wage equations. To support a causal interpretation of their estimates, they argue that compulsory school attendance laws alone account for the association between quarter of birth and earnings. In this note we present evidence that the association between quarter of birth and earnings is too strong to be fully explained by compulsory school attendance laws in the samples studied by Angrist and Krueger. Moreover, while the association between quarter of birth and educational attainment was weaker for more recent cohorts, we found no evidence that the strength of the relationship between quarter of birth and earnings was also weaker in those cohorts. In addition, we present evidence that suggests the association between quarter of birth and earnings or other labor market outcomes existed for cohorts that were not bound by compulsory school attendance laws. Our results call into question the validity of any causal inferences based on Angrist and Krueger's estimates regarding the effect of education on earnings.