With just a few hours left in the trading day, the S&P 500 is on track to deliver a hefty gain of over 20% to investors for 2017 and the ninth straight year of gains on a total return basis. In the S&P 500’s history, there has only been one other period where the S&P 500 was in the black for nine straight years, and that was from 1991 to 1999.A big difference between that streak and now, though, is the magnitude of the gains. During the 1990s streak, the S&P 500’s total return was 450% compared to a relatively meager gain of 261% in the current period.If the S&P 500 does make further gains next year, it will be the first ten-year winning streak for the index ever. With such a big gain this year, though, can investors really expect to see gains in the year ahead?

Looking back at the S&P 500’s historical total returns, there have been 32 prior years where the index was up 20% on a total return basis. What’s stunning about this number is the frequency of these types of big years.In fact, there have been 33% more years where the S&P 500 was up 20% or more than years where it was down! The table below lists every year since 1928 where the S&P 500 was up over 20% on a total return basis along with how the index performed in the following year.In the year that followed these ‘big’ years, the index saw an average gain of 10.46% (median: 12.80%) with positive returns just over two-thirds of the time. What’s pretty surprising about these prior returns is that they are only modestly below the average (11.85%) and median (14.30%) total returns of the S&P 500 following years where the index was not up 20% or more. So if it’s a reversion to the mean you are expecting following this year’s big gain, you may be disappointed.In fact, there have been five periods where a 20%+ year was followed by another 20%+ year, and in one of those five periods, the 20%+ year was followed by another four 20%+ years!