The family dynamics of intellectual development

Birth order effects on intellectual performance show both positive and nega tive results. The confluence model reconciles these conflicting data by pro ving that these effects interact with the age of participants at testing, s uch that young children should show negative or no effects, whereas older i ndividuals (past age 11 +/-2 years) should show positive effects. Birth ord er studies strongly support this prediction. Some writers have claimed the apparent relation between birth order and intelligence is an artifact creat ed by applying a cross-sectional analysis to data that should have been ana lyzed by comparing siblings within families. However, if siblings within th e same family are compared all at the same time, their ages are necessarily different. As a result, birth order effects are confounded with age effect s. Moreover, within-family data conceal patterns of aggregate effects that cross-sectional data reveal.