Historical Returns for S&P 500 from 1988-1998
From this graph above, it can be seen that in years from 1989-98 the historical returns for S&P 500 averaged to be 20.056%. Of course this is only a simple average return and not the expected return. Expected return for S&P 500 would have been different based on the probability of each of the state of economy occurring. Still, this simple average return over the specified period above gives a pretty good estimate of the returns.