Body weight in adolescence and adulthood may result from the interplay between individuals' genetic characteristics and the social context in which they grow up, such as family socioeconomic status (SES). However, evidence on the interaction between genetic propensity for high body mass index (BMI) and family SES remains inconclusive. This study investigates whether genetic associations with BMI differ among individuals from high-SES versus low-SES families and whether these associations vary across the life course. Using data from the National Longitudinal Study of Adolescent to Adult Health (Add Health), I assess the association between a polygenic index for BMI (BMI PGI) and BMI at four life stages: adolescence (≃ 16), early adulthood (≃ 22), adulthood (≃ 28), and later adulthood (≃ 37). Results show that the BMI PGI is more predictive of BMI among individuals from low-SES families than from high-SES families. The interaction weakens over the life course but persists among individuals with high BMI levels. These findings suggest that high-SES families may buffer genetic propensities for overweight and obesity, whereas low-SES environments may exacerbate them, increasing the risk of elevated BMI. The results underscore the importance of family socioeconomic background in shaping BMI outcomes and highlight how environmental interventions, such as promoting healthy eating and physical activity, may help reduce social inequalities in health.