It has been said stock market returns revert to the mean

What does that mean? Reversion to the mean is a statistical phenomenon. It’s the idea the further something is from the mean – or average – the more likely it is the next thing that comes along will be closer to the mean. For example, if a golfer normally plays to par but plays three over par, it’s likely that player will finish better in the next game (unless, the player’s in a slump, but that is a different topic). 

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