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The S&P 500 is the best reference index for US equities, but I'd be wary about using its history as a base case for retirement planning. The 60 years that its existed in its present form have seen the US economy grow consistently in a way that's almost unrivaled in human history. There are lots of good arguments for why the US will not grow as fast over the next 60 years, but for me the best one is just regression to the mean. Probably the US economy will grow at a rate closer to the lower rates common in our countries throughout human history, and over a long time frame the S&P 500 cannot grow much faster than the really economy.