So is trading stocks and bonds so harmful that taxing it belongs in the same category? One of the best cases for considering it this way comes from two finance professors, Brad M. Barber and Terrance Odean, who published an account in 2000 that examined the trading records of 66,465 households. The 20 percent who traded the most earned an average net annual return that was 7.2 percentage points less than that of the least active investors.

This week, I asked Mr. Odean, who is a professor at the Haas School of Business at the University of California, Berkeley, for more recent data, and he said he was trying hard to get some. He recalled that the last senior brokerage executive he requested it from said that he respected the professor’s work but did not see what was in it for his company. You would probably say the same thing if it was your responsibility to convince people that trading all the time was the way to riches, or at least a comfortable retirement.

Why do people hurt themselves by trading in this manner? Is it like gambling, with its danger of addiction akin to cigarettes? Maybe a little. A different study Mr. Odean worked on showed that when Taiwan introduced a national lottery, trading on the stock exchange fell 25 percent. So at least people there were able to substitute something else for their wagering activity.

Mr. Odean chalks up much of the trading to overconfidence. Most of us think we’re better than the average investor, and men trade more than women. The Internet gives us the illusion of feeling well informed, so we conveniently forget that somebody else read everything we found before we did. Then, we compound the sins by failing to diversify. After all, we’ve found the best investments, so why spread our risks among thousands of companies in a bunch of index mutual funds?

Would a tax make a difference in the amount of trading? Mr. Odean says he thinks it is possible, though he noted that the Taiwanese seem to trade plenty, even though they have a financial transaction tax.