A sample of around 2,500 adolescents in a child and adolescent psychiatry clinic in the region of Munster, Germany had their intelligence assessed. Family size (total number of siblings within a family) was significantly correlated with intelligence score categories (-0.08 and -0.19 for males and females). First borns and only children displayed higher IQs than later borns supportive of the confluence theory of Zajonc. The relationship was found only for those older children (11 or older) and not the under 11-year-olds. The relationship between birth order and intelligence was moderated by gender. (Contains 3 tables.)