Negative associations between birth order and intelligence level have been found in numerous studies. The explanation for this relation is not clear, and several hypotheses have been suggested. One family of hypotheses suggests that the relation is due to more-favorable family interaction and stimulation of low-birth-order children, whereas others claim that the effect is caused by prenatal gestational factors. We show that intelligence quotient (IQ) score levels among nearly 250,000 military conscripts were dependent on social rank in the family and not on birth order as such, providing support for a family interaction explanation.