This paper compares results from 11 longitudinal studies to determine whether preschool education improves the IQ of low income children when their family situations are taken into account and if so, how long the effects last. Taken together, the 11 studies represent a sample of 1,645 children -- of whom 61% are male, 87% black, and 75% former participants in experimental preschools. Many analyses of this paper include only portions of the total sample, since not all projects collected information pertinent to every analysis. The dependent variable in all analyses is Stanford-Binet IQ; the family structure variables investigated are socioeconomic status (SES), mother's education, family size, and birth order. The results indicate that there is a significant probability that lower class children who attend a preschool will have higher IQ's than those who do not, for at least 3 years after preschool. Findings also show that for lower class populations, significant correlations exist between mother's education, SES, family size, and birth order and IQ, but that the combined effect of these variables on the percent of variance explained for IQ is slight. Finally, little evidence is found that preschools affect the correlations between family variables and IQ. (Author/JMB)