In the 1990s, the Chicago Bulls was the best basketball team in the NBA.Michael Jordan was king. He led his team to ­­six NBA championships.But what if the coach had kept Jordan on the bench? If that were the case, the Bulls might have sputtered.

When it comes to investing, too many married couples bench their best players. Plenty of women tell me, “My husband invests our money.” A 2016 Wells Fargo survey says when it comes to investing, women aren’t as confident. That’s a shame because women, it seems, are the marquee players.

Over the past 15 years, I’ve delivered hundreds of investment talks. My message is simple. Invest in a portfolio of low-cost index funds. Ignore financial forecasts and get-rich-quicker schemes. Add money every month. Rebalance your portfolio every 12 months.

Years later, I often return to speak to the same people. They might have been building a portfolio of low-cost index funds. But temptation drew them foul. Sometimes, they speculated, pulling out of the market because they thought that stocks would crash. Other times, they added individual stocks that went south in a hurry.

“OK,” I say, “Be really honest here. Whose idea was it to mess with this portfolio?” The men usually admit that they’re to blame–or the women call them out.

Overall, single women are better at sticking to investment rules. More men, in contrast, want to slam-dunk or tweak their portfolios in the hopes of something better.

“I believe that testosterone affects men’s decisions,” I say, when giving investment talks. “Men’s overconfidence pushes them to gamble and break smart investment rules.” At this point, some of the women jab their husbands in the ribs. The men often smile and nod.

Between 2005 and 2010, Vanguard found that its female investors beat their male counterparts by about 5 percent. Women hold higher bond allocations, so that might be expected. After all, bonds beat stocks from 2005-2010.

But women beat men when stocks soar too. University researchers Brad Barber and Terrance Odean studied 35,000 household brokerage accounts between 1991 and 1997. Despite the soaring market—where higher risks can mean higher rewards—women beat men by nearly three percent per year on a risk-adjusted basis. This means that when women and men took the same amount of risk, the women beat men by 3 percent per year.

Fidelity tracked performance for 8 million of its clients in 2016.The firm found that women beat men by 0.4 percent. If Fidelity had adjusted for risk, the women might have won by more. Wells Fargo compared investment performances between 2010 and 2015. They also found that women beat men.When they adjusted for risk, women pulled even further ahead.

Men might fall behind because they trade more often and they’re much more likely to break investment rules. The Wells Fargo study says men were six times more likely to jump from 100 percent stocks to 100 percent bonds. In other words, men try to time the market more.

A 2005 Merrill Lynch investment survey also says men are more likely to mess things up. Thirty-two percent of men allocate too much money to a single investment. Just 23 percent of women make the same mistake. Merrill Lynch also says that men are twice as likely to charge into an investment without doing research.

Researchers Yan Lu and Melvyn Teo published fascinating findings for the University of Central Florida and Singapore Management University. They looked at 3,228 male hedge fund managers between January 1994 and December 2015.

But saying that they “looked at them” is a serious understatement. The researchers measured the widths of their faces. Men with wider faces tend to have higher testosterone levels. The researchers found that the hedge fund managers with narrower faces beat their higher-testosterone, wider-faced counterparts by 5.8 percent per year on a risk-adjusted basis.

The authors wrote, “In the context of the ultra-competitive and male-dominated hedge fund industry, where masculine traits such as aggression, competitiveness, and drive, are encouraged, expected, and even celebrated, our results on the underperformance of high-testosterone fund managers are indeed surprising. Investors will do well to go against conventional wisdom and eschew masculine fund managers.”

This brings me back to married couples. More often, men take the investment reigns, leaving women on the side. But instead, more women should chain their men to the bench. After all, when it comes to investing, women are the stronger sex. Male testosterone appears to be an anchor.

Andrew Hallam is a Digital Nomad. He’s the author of the bestseller, Millionaire Teacher and Millionaire Expat: How To Build Wealth Living Overseas