Wait 'til the sun shines to sell your stock

A cold day in hell -- or at least on Wall Street -- is the best time to buy stock. At least, that's what studies spanning 16 years and 26 different stock markets have found.

According to the Washington Post, "If you consistently bought stocks when the sky was gray and overcast and consistently sold stocks when the weather was bright and sunny, and you did this over a period of 16 years across 26 stock markets around the world, you would ... well, let's just say you'd be lounging on a sunny beach right now."

Of course, past performance is no guarantee of future results. And the transaction costs of "Buy cloudy, sell sunny" are so high that I don't think investors should swap out of their index funds just yet. But the studies are interesting in that they raise questions about the efficient market hypothesis and the idea of homo econimicus.

What rational investor would sell his stock because it's cloudy? And yet it seems that, at least subconsciously, people do exactly that.

I'd be interested in what efficient market disciples like Burton Malkiel have to say about these studies.