It was certainly not unconsidered by labor economists who have worked on such problems for the past half century (http://davidcard.berkeley.edu/papers/causal_ed

It was certainly not unconsidered by labor economists who have worked on such problems for the past half century (http://davidcard.berkeley.edu/papers/causal_educ_earnings.pdf http://davidcard.berkeley.edu/papers/causal_educ_earnings.pd...)