Ageing has opposite economic and political effects on the size of the welfare state. On one side, it tends to decrease the profitability of a welfare state that features a PAYG pension system, thus inducing individuals to prefer a smaller system; on the other side, the pivotal (median) voter becomes older (or poorer) and hence more willing to support a larger system. The overall effect is thus ambiguous.We show that specific features of the welfare system, such as its composition and the redistributive design of social security, may change the magnitude of the economic effect and thus of the overall impact of ageing on the size of the welfare state.