At The Motley Fool, we've argued long and hard that you can't time the market -- at least not consistently. But that doesn't mean there aren't plenty of theories out there to the contrary.

One "investing" theory that does seem to work, at least often enough to have gained credence, is the idea that you should "sell in May and go away." But as I peruse the stock charts today, the wisdom of following that advice seems less and less apparent. As I type this article, the S&P 500 is almost 5% higher than it was back on May 1. The Dow Jones Industrial average is more than 7% higher. The Naz is up almost 9%.

Exchange-traded funds that aim to track these three indices are doing even better, thanks to dividends. Spyders (AMEX:SPY) are up 5.6% over the last five months. Diamonds (AMEX:DIA) gleam 8.1% brighter. PowerShares QQQ (NASDAQ:QQQQ) have powered 14.5% higher.

And of course, certain individual stocks have done even better. ExxonMobil (NYSE:XOM) has gained 15% over this period of so-called summer snoozing. Cisco (NASDAQ:CSCO) is up 22%. (NASDAQ:AMZN) rose 50%. And small-cap dirt-pounder Dawson Geophysical (NASDAQ:DWSN) has racked up 54% gains.

It's good times all around, but not for investors who "sold in May."

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This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.