For singles, It's double.

When Odean narrowed his focus to single people, the difference was even starker. Single men traded 67 percent more, and single women enjoyed a 2.3 percent edge in annual risk-adjusted return. The presence of a wife evidently helps rein in some of the worst male excesses.

"Psychological research has established that men are more prone to overconfidence than women," Odean notes.

Looks to me as though he's opened a window on what we might call the Testosterone Trap. It's not just in stocks that this hazard lurks. You run into it everywhere there are financial decisions to be made. John T. Reed, who publishes the newsletter Real Estate Investor's Monthly in Alamo, Calif., says men often see it when they get cold calls from securities salesmen.

As Reed tells the story in his newsletter, "Once, in a moment of weakness, I said we had some money in certificates of deposit. The salesman ridiculed my wimpiness. I forget his exact words, but the import was that 'real men' buy growth stocks, not CDs or bonds."

This is a form of intimidation that no woman has to face. Question a guy's masculinity, and the next thing you know he's margined to the hilt in tech stocks and challenging the Internal Revenue Service to a fight.

In the real estate game, Reed says, there is a standard machismo scale. Commercial real estate is more manly than single-family or two-family housing; within commercial real estate, nonresidential (say, an office building) is more manly than residential (say, an apartment complex).

The same kind of hierarchy is plain to see in stocks. Individual stocks pack a higher virility quotient than stock mutual funds; options and futures on stocks are more virile than stocks themselves.

Anything to do with the stock market is more macho than a money-market fund or a CD, which in turn are more manly than a passbook savings account or - down close to the bottom of the ladder - a Series EE savings bond.

So, fella, you think you're secure in your manhood? Well then, the next time you go to a neighborhood block party, let's see you show everybody your Christmas Club account.

The thought crossed my mind that somebody ought to write a book on the Testosterone Trap. After mulling it over, though, I don't want the assignment. What possible market could there be for such a project? No macho man reads that kind of book.

Chet Currier is a columnist for Bloomberg News.