Here's Mark Hulbert writing at MarketWatch:

I have some important investment advice for you in the event your favorite athlete or team loses in the Olympics:

Take a break.

Don't even consider making changes to your portfolio for at least a day or two.

Why?

Because, if you're like most investors, you are prone to act irrationally when your favorite sports team loses.

I arrived at this conclusion not by watching sports enthusiasts at the local bar. Instead, it was reached by a rigorous academic study that appeared in the August 2007 issue of the prestigious Journal of Finance. The study, "Sports Sentiment and Stock Returns," was conducted by Alex Edmans, an assistant professor of finance at the Wharton School of the University of Pennsylvania; Diego Garcia, an assistant professor of finance at the University of North Carolina (Chapel Hill); and Oyvind Norli, an associate professor of financial economics at the Norwegian School of Management.

To isolate the role that mood plays in investing, the researchers analyzed the returns of a given country's stock markets immediately following losses of its national teams in international competition. They focused primarily on soccer matches in the World Cup, but they also studied cricket, rugby and basketball matches as well.

The researchers found, on average, that if a country's team loses in the World Cup elimination stage, its stock market the next day produces a return that is 38 basis points lower than normal. To be sure, 38 basis points might not seem that big of a deal, but from a statistical point of view it is huge. It is equivalent to an annualized loss of more than 60%.