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KAI RYSSDAL: It's been a blissfully quiet August in Washington. The president's been spending plenty of time down in Crawford at the ranch. Congress is on vacation. Don't know if you noticed, but Wall Street's been doing pretty well since the end of July. And commentator Amity Shlaes says investors ought to take advantage.

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AMITY SHLAES: They call it the Congressional Effect: the fact that Wall Street does better in periods like August, when Congress is on holiday. And not just somewhat better.

Nearly all the gains in the Dow over a century have come when Congress was on holiday. That's what two scholars, Michael Ferguson and H.D. Witte, recently demonstrated. The S&P 500 also shows a strong correlation between Congressional breaks and gains.

The market fears Congress and so tends to keep its head down when Congress is sitting.

That's the bad news. But can we make money off it? Perhaps. In New York, a Wall Streeter has started up a hedge fund to capture the holiday gains and minimize in-session losses. He calls it the Singer Congressional Fund.

There may be other ways to cash in. You can buy futures contracts that go long on stocks while lawmakers are out of town. Small companies with only one business are more vulnerable to decisions by Congress than large businesses. So, especially when Congress is in session, invest in large firms.

Or you can put your cash in companies that claim to have learned how to out-think Congress. One of those is GE. Its new strategy is making its business as green as possible. The company is hoping to anticipate environmental laws that Congress will pass anyway.

Companies themselves can avoid Congress altogether by never coming to the US in the first place. That's what all those firms that are making initial public offerings in London or Asia are doing.

But what about the simple small investor? Congress comes back after Labor Day. Even that investor can take heart. Lawmakers have another break in October, before elections. And who knows? The Congressional Effect can give Christmas a whole new meaning.

RYSSDAL: Amity Shlaes is a columnist for Bloomberg.