As chairman of the president’s Council of Economic Advisers during the 1990s, I had to make projections about where the economy was going to be one, two, or three years ahead; frankly, I often found myself facing a cloudy crystal ball. Forecasting a few years out is hard enough; forecasting seventy-five years out is essentially impossible. Slight changes in life expectancy, immigration, wage growth, and interest rates could have large effects on the solvency of the (Social Security System.)