Sex Differences in Developmental Milestones During the First Year of Life

Differences were revealed in the age at which boys and girls attain developmental milestones during the first year of life and in the intervals between milestone attainments. Three of ten milestones were reached significantly earlier in boys than girls while none of the milestones appeared earier in girls than boys. Of the 45 intervals between milestones, seven were longer in boys and twelve were longer in girls. Data were recorded by the mothers of 4653 infants participating in the Copenhagen Consecutive Perinatal Cohort. Full-term gestation (38-41 weeks) and survival through the first year were the criteria for inclusion in the study. Nine potential confounding variables, including SES, birth weight and complications of pregnancy, were not responsible for the behavioral sex differences. This sexually dimorphic pattern of milestone achievement is discussed as supportive of hypothesized biologically-based sex differences in the ontogeny of social responsiveness and is consistent with sex differences ident...