<prompt> Human: Everyone says invest in Vanguard index funds for an annualized return of 7-10 over several decades. The evidence cited is the performance of the stock market over the past 50 years or so. But isnt this using past performance to guarantee future results? Instead of betting on the performance of a single company, it is betting on the overall performance of the American economy. If the American economy no longer grows at the rate it did over the past 50 years, then there is no reason to expect returns to be as high. Ive seen it mentioned that some foreign stock indices have remained stagnant over many decades - whats to say the US doesnt go the same way?  <response>: You dont. 