1. I'm selling because the price is going down
Selling Morgan Stanley (NYSE: MS) or Bear Stearns (NYSE: BSC) because you think there will be more subprime pain to come -- pain not reflected in the current price -- is perfectly sensible. These companies created a mess for themselves, and their shareholders have been feeling the pain.
But selling a stock for no other reason than that the price is declining? That's plain dumb. If you hadn't factored into your valuation Bear's subprime exposure, then sure, sell it. Fast. But if you did, and are just fidgeting at the sight of red ink, you're forgetting that you're selling part of a living, breathing business -- not trading a slip of abstract paper.
2. I'm selling because someone told me the market is going lower
If you're selling off stocks because you read in a newspaper that a recession is now inevitable, or that the subprime crisis was only going to get worse, or that a multitude of "experts" predict that a bear market is nigh, then you're overlooking the trees for the forest.
See, as a retail investor, you have no control over the direction of the economy. Get used to it and deal with it. You do have control over the companies in your portfolio. If you've bought quality companies at reasonable prices, short-term economic movements should have little effect on the long-term returns of those businesses.
3. I'm selling and buying back in when the market bottoms
In the short term, no one knows which direction the stock market is going to move, or by how much. Trying to pick the bottom of the market is an utterly futile task. In a recent article, Foolish colleague Tim Hanson cited a study that illustrates exactly how futile market-timing is:
[IESE Business School professor Javier] Estrada studied 15 major global stock markets for periods ranging from 31 to 79 years, with the full data encompassing more than 160,000 trading days. What he found is "less than 0.1% of the days considered" actually matter to long-term returns, which means that "the odds against successful market timing are staggering."
The entire article can be read here.
BEEN THERE, DONE THAT......"whats worse".......sometimes still do it (but now only in case of instance 3).