Intellectual development in family constellations with adopted and natural children: a test of the Zajonc and Markus model.

A confluence model was developed by Zajonc and Markus to account for the effects of birth order, child spacing, and family size on intellectual development. the fit of the model was tested on a sample of families with biological and adopted children. Although the model fits population data very well, it fails to account for intellectual patterns within families for whom both parent and child IQ data are available. The discrepancy is discussed in terms of sources of variance within and between families that are undetected in average values for a population.