Single women invest less in risky assets than do single men. This paper analyzes the determinants of the “gender investment gap” based on a structural life-cycle framework. The model can rationalize the gender investment gap without gender heterogeneity in preferences. Rather, lower deterministic income and larger household sizes shift the composition of single women toward poorer households that invest less risky (composition effect). Additionally, future outcomes of both variables (which cannot easily be controlled for in regressions) make single women more vulnerable to financial shocks and decrease their optimal equity share even conditional on state variables (policy effect).