A fat tail in investing doesn’t describe an animal. It’s a statistic that indicates a rare but often important outcome. In investing it may mean the odds of a really good or really bad outcome.

Stock market investors are afraid of lots of things but they really should fear four things. One: The stock market has a terrible year. Two: After that terrible year, the stock market doesn’t recover for a long time. Third: The investor is out of the market when it goes up. Finally: they hit on one of the many ways to mess up their investments.

Looking at the last 95 years of stock market returns, the odds of number one and two are low — not zero — but low. In that time, the broad market has declined by more than 20 percent in only six years and by 10 to 20 percent an additional eight times. In 10 more years the returns have been slightly negative; disappointing years but not something to fear.

The rest of the time — 71 years/75 percent of the time — the broad stock market as measured by the CRSP database  has had positive returns and 58 percent of the years, those returns have been greater than 10 percent.

A 10 percent return is the long-term average for the last century and it’s a powerful return because at that rate portfolios double every seven years.

The overall message is that the odds of you messing up by overthinking the problem and doing something creative is one of the biggest risks. The odds of the stock market having a really terrible year and taking a long time to recover are low. And one risk you can control is the chance of the stock market having a good to great year and you not being involved.