Average body weight differences between urban and rural areas have been reported in many countries, but it is unknown whether these are due to effects of social selection or social causation. We examined whether adolescent body mass index (BMI) predicted selective urban/rural migration over a 21-year period and whether urban/rural living over the same period predicted differences in BMI increase from adolescence to adulthood in Finland. The participants were from the prospective, population-based Cardiovascular Risk in Young Finns study (n=1787) aged 12-18 years at baseline and 33-39 years at the final follow-up, with data collected at six follow-up phases. Supporting social selection, heavier adolescents were less likely to migrate to urban areas as adults: in obese adolescents the likelihood of living in an urban area at 33-39 years age was one third of that in normal weight adolescents. Supporting social causation, rural residence over the study period predicted a greater increase in BMI from adolescence to adulthood than urban residence did. These associations were independent of parental socioeconomic status and BMI, and of participants' own educational level, occupational class, marital status, and parenthood status. Together the findings suggest that the higher body weight of people living in rural areas of Finland may be due to both social selection and social causation mechanisms, i.e. heavier people tend to migrate to more rural areas where people tend to get heavier.