Season of birth has been shown to correlate with many aspects of somatic and mental disorders, development and social adaptation (so-called ‘birth-date effects’). In a sample of young Swedish men, corresponding roughly to a one-year birth cohort, the results of intelligence tests, psychologists’ ratings of psychological function, school achievement, body height, weight and self-reported health during childhood, were found to be correlated with month of birth, and – more strongly – father’s socioeconomic status. The results were more favourable for men who were born during March–May (the period of highest birth rate), and whose fathers were of higher socioeconomic status, than for those born in November and December (the period of lowest birth rate), and whose fathers were in the lower socioeconomic group. It seems reasonable to conclude, from this study and previously reported findings, that these so-called ‘birth-date effects’ are determined by varying and often interacting biological and psychosocial factors. Among these factors, the light-induced entrainment of circadian and annual rhythms in the fetus and/or infant seems to be of pivotal importance. The organization of children into one-year age classes therefore produces an unfair lack of equality of possibilities.