The authors tested the evolutionary genetic hypothesis that the functional form of an asymmetrically risky Gene x Environment interaction will differ as a function of age-related antagonistic pleiotropy (i.e., show opposite effects in young vs. old individuals). Previous studies have identified a polymorphism in the human "IL6" promoter (rs1800795; "IL6" -174 G/C) that interacts with adverse socioenvironmental conditions to promote chronic inflammation in older adults (elevated C-reactive protein). This study identifies a protective effect of the same polymorphism in 17- to 19-year-old adolescents confronting socioeconomic adversity. Over 60% of the environmental risk contribution to the "IL6" x Socioeconomic Status interaction could be accounted for by interpersonal stress and adult role burden. Thus, the "IL6" -174G allele does not represent an undifferentiated risk factor but instead sensitizes inflammatory biology to socioenvironmental conditions, conferring either genetic vulnerability or resilience depending on the developmental "somatic environment" that interacts with social conditions to influence gene expression. (Contains 4 tables.)