It's a shame so few of us in coming days will spend more than a moment recalling the Sept. 11, 2001 terrorist attacks, which occurred 18 years ago this week.

That's because there is no shortage of lessons we can learn from it -- not just on the emotional and geopolitical level, but in the investment arena as well. Yet, not if we actively forget about it.

I'm not faulting the more than 100 million U.S. residents who either hadn't even been born on that fateful day, or who were too young to be aware of what was happening.

But the rest of us have no excuse. And it's inevitable that, someday, there will be another crisis of the magnitude of the 9/11 terrorist attacks.

Perhaps the most important investment lesson to draw from the 9/11 tragedy is that we should not sell into a panic. The stock market plunged in the wake of that day's attacks, and at its low 10 days later, the Dow Jones Industrial Average was nearly 15% lower than where it closed on Sept. 10. Yet the market quickly began to claw its way back, and in less than two months -- by Nov. 9, in fact -- the Dow was higher than where it was the day before the attacks.

That's a remarkably quick recovery from one of the most traumatic events in U.S. history. But it is consistent with how the markets fared following prior geopolitical crises, according to analysis conducted by Ned Davis Research. The firm studied what it considered to be the 28 worst crises over the six decades prior to the 9/11 attacks, and found that, on average within six months, the Dow was higher than where it stood the day before those crises erupted.

Perhaps the most remarkable illustration of this recovery phenomenon, though, is how the airline industry fared after the 9/11 attacks. That industry was perhaps the most impacted of any, with many people swearing off ever flying again. Many individual airlines were forced to rely on government bailouts or seek bankruptcy protection.

Nevertheless, as you can see from the accompanying chart, the airline industry (as measured by the NYSE Arca Airline Index) by March 2002 was essentially back to where it stood on the day before the terrorist attacks. That's in just six months.

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To be sure, the markets don't always recover so quickly and impressively from a crisis. In nine of the 28 crises in the Ned Davis study, for example, the market in six months' time had failed to climb back to where it stood prior to the crisis.

But even if a quick recovery is not guaranteed, you can be assured that selling into a market panic will secure you some of the worst possible sale prices. The best advice is that, if you didn't get out before the crisis hit the market, you should simply sit on your hands and wait.

I acknowledge that it is unseemly to even be discussing your portfolio net worth in the context of a tragedy that led to immeasurable human loss and suffering. But think about it this way: You help no one during any crisis by thinking about your portfolio. And the message of history is that you don't have to.