To evaluate the impact of family income on child outcomes, a researcher proposes to use maternal (mother's) education as an instrumental variable for family income. A critic argues that this instrumental variable isn't exogenous, claiming that
A. Maternal education is not strongly related to family income.
B. Maternal education is not strongly related to child outcomes.
C. Family income may affect child outcomes for many reasons.
D. Maternal education may affect child outcomes through channels other than family income.
Answer: D. Maternal education may affect child outcomes through channels other than family income.