Models of Environmental Effects on the Development of IQ

This paper proposes a number of models of the effects of demographic and environmental factors on IQ and its pattern of change over time. The proposed models are concerned with the determinants of an Individual’s true (but unobserved) IQ and the relationship between measured and true IQ’s. Our analyses are based on data from the school records of a panel of 1, 746 students from the Pittsburgh school system and include demographic and environmental measures as well as IQ test scores at kindergarten, fourth, sixth, and eighth grades. The results indicate that the number of parents living in the household has an important positive effect on IQ at kindergarten that persists over the period of analysis. Number of siblings (measuring both birth order and family size) has an important negative influence on cumulative changes in IQ. Females show faster development until fourth grade; the net difference between the sexes almost completely vanishes by eighth grade. However, the predominant influences on the development of IQ are the socioeconomic status of the student’s parents and peers in school, with SES of peers being the more important. We estimate that roughly half of the difference between the measured eighth grade IQ’s of the average white and black student in our sample may be attributed to differences in their peer and parental SES’s. Finally, we caution that our results may be misleading, since our sample is not from a designed experiment and thus some of the explanatory variables could be correlated with unobserved factors that affect IQ.