The government simultaneously assumes that 1) stocks will rise at the same rate they have in the past 75 years, while 2) assuming that economic growth will average a paltry 1.9 percent over the next 75 years, as Paul Krugman and many others have noted. GDP has risen an average of 3.4 percent over the past 75 years. If the economy only grows 1.9 percent in the next 75 years, it'll be extremely difficult for stocks to return 6.5 percent.

The assumption of 6.5 percent growth also replaces forecasting with analogy. Actuaries and stock market historians like Jeremy Siegel tell us how they think the market will perform in the future based on its past performance. What if the 20^th century—the American century—proves to be an anomalous one for American equities? It's hard to imagine how the American economy could have a better 75 years than the last 75. As every advertisement for every financial product states, past performance is no guide to future performance.