Objective. To evaluate the relationship between states' expansion of Medicaid and county-level food insecurity, testing for the moderating effects of social multipliers. Methods. We estimated the effect of county/state Medicaid characteristics (baseline enrollment, enrollment changes, and expansion timing) on county-level food insecurity during two expansion periods (2009–2012; 2012–2014) using a pre-post design among expansion states to control for political culture. Results. Increased county Medicaid enrollment in early expansion states was associated with lower food insecurity in 2012 (b=–0.10%, p=.02). The impact of early expansion persisted into 2014 (b=–0.06%, p=.01), suggesting a social learning effect. Focusing on a single state (California) to control more fully for socio-political norms revealed larger social multiplier effects. Conclusions. Medicaid expansion was associated with reduced food insecurity, the most pronounced associations evident in counties with the largest Medicaid expansions. Cross-county variation may reflect both social learning and social norms, though evidence for the latter is inconsistent.