This paper investigates interactions between family size and social class with respect to intellectual achievement. One purpose of the paper is to study the limits to the applicability of the "confluence model" proposed by Zajonc and Markus; another is to investigate methods for studying interactions between variables. The study includes longitudinal data from a sample of 8,288 subjects who, at the age of 13, were given a battery of intelligence tests, standardized achievement tests, and interest inventories. Information also was gathered about social background and number of siblings. Two different multiple regression (MR) models and analysis of variance (ANOVA) were used to analyze interactions between social class and sibsize (the number of children in the family). The results indicate that for most of the outcome variables there is an interaction between social class and number of siblings, such that within the lower social classes, sibsize is more strongly negatively correlated with outcome variables than in the higher social classes. The confluence model predicts such a negative relationship between sibsize and intellectual outcome, but it does not allow for relationships of different strength within different social classes. It is suggested that socializing agencies other than the family are more important in higher social classes than in lower. (Author/LC)