Background Different theories of the link between socio-economic status (SES) and mental illness have been postulated. In particular, two theories of this association, social causation and social selection, differ in the implied causal pathway. The authors employ behavior genetic modeling to consider evidence for both social selection and social causation in the relationship between income variation and internalizing disorders.Method Behavior genetic modeling was used to estimate the presence of gene-environment interaction (GxE, social causation) in the presence of gene-environment correlation (rGE, social selection). Participants were members of a sample of 719 twin pairs from the Midlife in the United States study. Four internalizing (INT) syndromes were assessed: major depression (MD); generalized anxiety disorder (GAD); panic attacks (PA); neuroticism (N). SES was measured with total family household income.Results One factor best accounted for the variance shared between MD, GAD, PA and N. The etiology of variation in INT changed from high to low levels of income, with unique environmental factors playing a larger role in INT variation at lower levels of income. Across levels of income, rGE between income and INT was modest (low income ra=0.39 to high income ra=0.54), implying a selection process operating through genetic effects linking lower income with INT psychopathology.Conclusions Findings support social causation by suggesting that low income contributes significantly to environmental variation in INT. Modest support was found for social selection, but should be extended using longitudinal designs. Effective interventions for internalizing psychopathology may differ depending on income.