The S&P 500 has already met its average return for a full year, but don't expect it to stay here

The average annualized total return for the S&P 500 index over the past 90 years is 9.8 percent. For 2017, in just under half a year, the S&P 500’s total return is 9.7 percent.

Looking at these facts side by side, it might seem the market has been twice as generous as usual so far this year, tempting a wary investor to back away from stocks or expect next to nothing more over the coming six months.

Yet equity returns come in waves, not in metered doses. The market gets on a roll, overshoots, retrenches, and sometimes—as in the 18 months ended last November—just slides sideways.

One of the market’s more intriguing and mischievous traits is that it rarely produces the long-term “average” return in a given calendar year.

Looking now only at price returns (not counting…

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