Some economists compare schooling and earnings among identical twins to estimate the causal effect of schooling on earnings.

Some economists compare schooling and earnings among identical twins to estimate the causal effect of schooling on earnings.

Why can’t we just compare the average wage of high school and college graduates to arrive at a measure of the return to schooling?  How does a comparison of twins improve our estimate of the return to schooling? (2 points)

Are there reasons why using twins might give us an inaccurate measure of the return to schooling? (1 point)