The social security program has grown more than a thousand fold since its inception. Even fnter the increased number of retired persons and inflation are accounted for, average real social security benefit levels more than tripled from the late 1940s to the early 1980s. What accounts for this growth? This paper finds that increased retirement benefit levels are generated by changes in the constraints faced by the median voter rather than by changes in the political power of special interest groups. The results suggest that the median voter has a finite planning horizon.