WSJ: 'Mean Reversion' Suggests Big Stock Gains Won't Continue

By SIMON CONSTABLE

Investors tend to be either overly optimistic or overly pessimistic based on recent experience. They often think recent good or bad performance will continue indefinitely.

In simple terms, years of subpar stock returns will be followed by better returns to bring overall performance back to the long-term norm. It works vice versa, also.But investment returns over time are more likely to exhibit what economists call "mean reversion." That's the idea that over long periods the annual returns of various assets will swing back toward their long-term average—or back to the mean. See original story here.