Birth order: Reconciling conflicting effects.

Secular trends in test scores are accurately predicted by trends in aggregate birth orders. The trend data contradict individual-difference analyses that show birth order as a poor predictor of individual test scores. This article demonstrates why the 2 formulations of the problem—the individually distributed birth order analysis and aggregate-pattern analysis—generate different results. A meaningful interpretation is given by the confluence model, a theory specifying the process whereby the intellectual environment modulates intellectual development. The authors introduce the concept of collective potentiation that specifies collective side effects of birth order. In contrast to genetic theories, the confluence model quantifies the differential environmental contributions to intellectual development of successive siblings, and it offers several well-confirmed derivations that genetic and other developmental theories cannot explain. (PsycINFO Database Record (c) 2010 APA, all rights reserved)