confluence model
a controversial theory that intelligence of siblings is correlated with family size. According to this model, average intelligence generally declines as the number of children in a family increases. Intelligence is also held to decline with birth order. The one exception is an only child, whose intelligence suffers because he or she does not have an older sibling to serve as a teacher. However, many variables (e.g., spacing of children) could affect and reverse such generalizations. [proposed in 1975 by Robert B. Zajonc and Greg Markus]