In his latest Forbes column, Ken Fisher says that fund investors who try to time the market play a dangerous game.

“Plenty of funds have fine long-term returns despite being tax inefficient and generally costly,” Fisher writes. “But a dirty secret is this: Average, no-load fund investors do much worse than the funds — or the market. How? Why? Because the commission-free funds are so convenient that folks trade much too often, making their moves at the wrong times.”

Fisher cites the research of Dalbar, Inc., which found that in the 20 years ending last December, the S&P 500 gained an average of 8.2% per year. In the same span, the average investor in U.S. equity funds gained just 3.2%. “Why the huge spread?” Fisher asks. “Timing. People tend to pile into hot funds after a bull market is well under way and to exit in a panic at the bottom of a bear market.”

Because most investors have bad timing, Fisher says commission-free no-load funds can be particularly dangerous. And he also cites a study that shows men trade more often than women — and perform worse than female investors. To read his full column, which includes stock picks from several countries around the world, click here.