We examined whether the T102C polymorphism of the serotonin receptor 2A gene (HTR2A) moderated the influence of childhood or adolescence parental socioeconomic status (SES) on adulthood temperament trait harm avoidance (HA) in a population-based sample of 1246 healthy Finnish men and women, who were 24-39 years of age in the last follow-up phase. High parental SES predicted low adulthood HA. In addition, the C allele of the T102C polymorphism was associated with high HA in one of the two test settings, and with the mean of the two measurements. Most importantly, we found that the T102C polymorphism moderated the influence of parental SES, such that high parental SES predicted low adulthood HA in subjects with the T/T or T/C genotypes, while this was not true for those carrying the C/C genotype. The role of the T102C polymorphism was most pronounced among those with high parental SES. We conclude that the T102C polymorphism of the HTR2A gene may be involved in the development of temperament by moderating the influence of environmental conditions.