After first controlling for the effects of age, race, gender, union status, and location on earnings, suppose a researcher estimates the rate of return to an additional year of schooling to be 10%....

Extracted text: After first controlling for the effects of age, race, gender, union status, and location on earnings, suppose a researcher estimates the rate of return to an additional year of schooling to be 10%. The researcher did not (could not?) control for unobserved differences in cognitive ability. Explain why the failure to control for cognitive ability among individuals in the sample creates a potential problem for the credibility of the researcher's estimate of the rate of return to schooling?