Don’t do it.

Don’t touch your investments.

That’s the simple, yet hard-to-practice, lesson of a new of study of investor behavior by Morningstar, the financial services company. It found that over the last decade, most people hurt themselves by trading. Once they put their money into stock and bond funds, they would have been much better off if they had just left their money alone.

In fact, Morningstar found that, on average, the actual returns of fund investors were significantly less than the posted market returns, a discrepancy explained by poor trading decisions — buying when the market was high and selling when prices were low.

Over extended periods — say, 30 years — this drag on returns produces chilling results: a reduction in the money in an average investor’s portfolio of more than 18 percent, according to Morningstar calculations performed at my request.

“Investors hurt themselves when they are prone to inopportune trading,” Jeffrey Ptak, managing director of Morningstar Research Services, said in a phone conversation. Finding ways of resisting the temptation to buy and sell is critically important.

This isn’t easy, for several reasons.

First, much of the financial services industry is dedicated to incessantly selling new products and services, including advice, intended to keep you buying and selling the latest new thing. That kind of behavior generates handsome profits for asset managers and advice-givers, but not necessarily for you.

Second, the swings of the stock and bond markets provide plenty of motivation for fleeing to the sidelines — or, when the markets are rising, for jumping into hot investment prospects with all the money you can muster. This may work out well for some people, but for most of us, the markets are too complex to be outsmarted. Historically, it has been better to buy and hold.

Third, shifts in domestic policy and politics may frighten you so much that you are no longer comfortable participating fully in financial markets. Undeniably, the Trump administration is making big changes. It has imposed the highest tariff rates since the 1930s, has detained more immigrants than at any time in the last 20 years, has denigrated the government’s own statistical agencies and is swelling the country’s debt load, just as a partial list. It may be that the administration is fundamentally altering the U.S. economy and markets. In that case, the traditional approaches of buying and holding stock and bond funds may no longer apply.

I find these last arguments difficult to refute. If you are alarmed by them, consider making a sober alteration in your asset allocation — reducing risk as best you can, and making sure you’re diversified internationally. But I’d think carefully before abandoning long-term investing in stocks and bonds.

Be mindful that merely by intervening in your own portfolio, you are taking risks. It’s possible, for example, that the current problems will dim in importance when seen from a vantage of a decade or more from now, and that other developments, like artificial intelligence or something we can’t even identify yet, will propel the economy more rapidly than most people expect.

The lessons of the markets, and of the latest study, suggest that a humble approach may be wise. It’s hard to beat the markets.

Findings

Most people in the United States use mutual funds and their close cousins, exchange-traded funds (E.T.F.s), to invest. (Mutual funds are older and have more total assets in them than exchange-traded funds, but E.T.F.s are growing more rapidly.)

The Morningstar study, “Mind the Gap 2025,” included both types of funds in its research. It restricted its scope to funds established by Dec. 31, 2014, and followed those that survived through the next 10 years. Many other studies have demonstrated that most funds can’t outperform the market.

This study, the latest in a series done by Morningstar, aimed for something different and, arguably, more important. It tried to show how fund investors fared in the real world, not how their funds performed in the abstract. And its findings were clear: “The more investors traded, the less they made,” the study said.

The study used what is known as asset-weighted returns, meaning if you had $6 in one fund and $3 in another, the first fund received twice the weight of the second. If you moved $1 from one fund to the other, their weights would change, and so would the performance of your portfolio. This method enabled the researchers to compare investors’ returns with how funds performed.

The good news was that the average fund, including both stocks and bonds, did quite well, returning 8.2 percent annualized, including dividends, over the decade through December. That’s what investors would have gotten if they had bought funds and held them. But that’s not what people actually did.

Because of human decisions to buy and sell at inopportune moments, the average “investor return” was only 7 percent annualized — a gap of 1.2 percent each year. That may not seem like a large number, but it compounded every year, amounting to significant losses over extended periods. In fact, it worked out to a gap of about 15 percent over 10 years, the study found.

Over 30 years, additional Morningstar calculations found, the gap amounted to 18.4 percent. Let’s translate that into concrete numbers. Suppose you had put $30,000 into stock and bond funds on Aug. 1, 1995 — with $10,000 going into U.S. stocks, $10,000 into foreign stocks and $10,000 into U.S. bonds. On July 31, 2025, the $30,000 in those funds, if left alone, would have swelled to $279,483, on average. But because they traded, investors received only $228,079, a gap of $51,404. Over even longer periods, the average investor shortfall would have been even larger — perhaps enough to make the difference between comfort and austerity in retirement.

One positive note centered on target-date funds, which are usually held in 401(k) and other retirement accounts. They do much of the work for you, typically allocating more stock in the fund when you are starting out and ratcheting up safer bond holdings as you approach retirement. Because of this gradual, automatic rebalancing, target-date funds don’t require much human intervention — and, as a result, Morningstar found, people using them tend to stick with their investments and reap more rewards.

“That’s a major takeaway of the study,” Mr. Ptak said. “Target-date funds and other allocation funds are doing their job.”

On the other hand, investors in so-called sector funds, focused on technology or military stocks or anything else, traded much more frequently and had the biggest return gaps.

Guidelines

Make use of these findings if you’re just getting started. Go ahead and invest, by all means, by buying publicly traded stocks and bonds, preferably through cheap, diversified index funds that will give you a piece of the entire global marketplace. Automate the investment process so that you don’t need — and aren’t tempted — to interfere with it once you’re heading in the right direction.

Even veteran investors will benefit with a hands-off approach, the study suggests. Cultivate ways of avoiding being swept up in the spirit of the moment — whether it be “irrational exuberance” driving up share prices, or a spreading sense of panic and dread about the future of the country and the world.

Over long periods, global stocks have tended to produce handsome profits, investment-grade bonds have been steady income producers, and people who have been able to stick with broadly diversified holdings of public securities over decades have generally prospered.

So-called alternative investments like private equity funds and cryptocurrency are much riskier. Even if it is possible to hold them directly in retirement accounts one day — as President Trump would like — I’d proceed cautiously. Without a solid long-term track record, these options would be risky for investors — though probably quite lucrative for their promoters. Even if they are ultimately approved for inclusion in workplace accounts, I’d hope that they would be excluded from default target-date funds, and I’d do whatever I could to avoid holding them.

Reaching for better returns may hurt you in the end. Keep your investing clean and simple, avoid risks you can’t afford to take and try to stash enough money in safe places to pay the bills. If you’ve done all that, remind yourself: Leave your investments alone.