According to historical records, the average annual return for the S&P 500 since its inception in 1928 through 2016 is approximately 10%. However, that number can be very misleading. If an investor thinks that translates to just putting money in the S&P 500 Index and watching it double about every 10 years, he is likely in for a rather big disappointment. Accurate calculations of average returns, taking all significant factors into account, can be challenging.