Small birth size predicts various psychiatric outcomes, including depression. While biologically based temperamental traits may constitute a vulnerability factor for depression, the extent to which birth size predicts these traits in adulthood is not known. We studied, in 1369 women and men identified from a cohort born in 1934-44 in Helsinki, Finland, whether birth size predicts the temperamental traits measured with Cloninger's Tridimensional Personality Questionnaire at an average age of 63 years. Moreover, we examined whether socio-economic status (SES) in childhood modified the associations. Data on birth size were obtained from birth records, and SES in childhood was obtained from school records. Weight and length at birth showed curvilinear, reverse J-shaped effects on harm avoidance (HA), such that the highest HA scores were most characteristic of those born small. Furthermore, high HA was confined to those belonging to a low SES group in childhood regardless of birth size, and to those belonging to the high SES group in childhood if their birth size was small. The associations were independent of several confounders. Since small birth size as well as high HA in adulthood may associate with subsequent depression, our findings might shed light on understanding the early neurodevelopmental processes that predispose to depression through vulnerability characteristics.