gence on fertility and of household size on offspring's intelligence remain the subject of public debate. Most recently, attention has centered on the decline in Scholastic Aptitude Test scores and the possible effects of the postwar baby boom. These concerns suggest that economists would do well to introduce intelligence into the household utility maximization model used to explain differential fertility. We attempt to do this here.

According to the Easterlin hypothesis, the positive relationship between income and fertility is dependent on relative income. The hypothesis presumes that aspirations are significantly determined by family background. If income is high relative to aspirations, individuals will tend to have more children. The definition of variables as sibling differences controls family background and yields a measure of relative income. Analyzing the Kalamazoo Brothers sample in this fashion produces no evidence in support of the hypothesis.