Below I will present the returns for S&p 500 from years 1999-2009.
From this graph above it can be seen that the average historical return for S&P 500 index has been 3.01%. This is way worse than the returns we averaged on years from 1988-1998 in which it was around 19%. These abnormally low or negative returns in some of the past years are due to economic downturns. The economy was at its all time low in 2008 since the great depression while the early millenium years also experienced downfall in the economy. So keeping in mind these two past recessions is a good idea when lookign at the returns.