In an act of reverse symmetry, the bull market of 1982-2000, the most bountiful period for stocks ever, was succeeded by the decade that we call the Terrible 2000s, the worst decade for stocks in modern times, even worse than the Great Depression years of the 1930s.

In the two-decade-long bull market, the S&P 500 Index soared a cumulative 2,033%, or an annualized total return of about 18%. Then came the Terrible 2000s, during which the S&P 500 Index declined a cumulative 9%, or an annualized 0.9% loss.

The market’s behavior in the past three decades is a classic example of what statisticians call reversion to the mean -- the tendency for performance over time to gravitate to a middle point, in which periods of above-average performance are followed by periods of below-average performance, and vice versa. The long-term average annual return of stocks is about 10%, so the bull-market return of 1982-2000 was a full eight percentage points above the long-term average. The market then rather tidily reverted to the mean, with its annualized performance in the 2000s more than 10 points below average.

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