The aim of the present study was to test the hypothesis that at the end of their second surge in testosterone production, male infants will on a group level have a slightly less pronounced tendency than female infants to exhibit a social interest and orientation towards people. The study is situated within the discipline of Evolutionary Developmental Psychology (EDP) as the hypothesis was derived by applying principles from modern evolutionary biology to understand the adaptive function of an interest and orientation towards other people in the ontogenetic process of development. Social interest and orientation was measured by 3 items on the 6-month questionnaire of the Ages & Stages Questionnaires: Social-Emotional (ASQ: SE). The participants in the study were the parents of 317 infants aged 5 – 8 months, recruited at child health clinics. The results showed that more male than female infants received responses indicating that their social interest and orientation was not strongly pronounced, indicating a support for the hypothesis. However, this sex difference was not statistically significant. On the other hand, a statistically significant sex difference was found among the participating parents in that fathers tended to rate their infant’s pro-social behaviour as less pronounced than mothers did. Additionally, the mothers’ marital status was found to be significantly correlated with their rating of the infants’ social interest and orientation. The results of the present study are interpreted to coincide with evolutionary reasoning on the matter of parental investment and as testifying for the need for an evolutionary informed approach when choosing methods and variables when studying development.
Author Keywords: Evolutionary developmental psychology; testosterone; infancy; sex difference; parental investment; parental report.