If you want a tidy snapshot of what it’s like investing in the stock market, check out how things have progressed so far in 2025. The S&P 500 is up almost 13% for the year and recently hit an all-time high — just a few months after a steep sell-off had many experts warning of a coming bear market.

The lesson is that you can never tell how the stock market will react in the short term, but you can usually depend on it to keep moving higher over the long term. So why do so many investors still end up losing money?

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The short answer is that they keep making the same mistakes over and over. GOBankingRates asked ChatGPT what the biggest mistake is, and here’s what it had to say.

According to ChatGPT, the worst mistake investors make is letting emotions drive their decisions — especially fear and greed. That answer aligns with what many financial experts say.

Here’s how ChatGPT broke down two of the most damaging decisions caused by emotion-based investing:

• Fear and panic-selling: During market downturns, investors often panic and sell at the bottom, which means many lock in losses. They also miss the inevitable recovery if fear keeps them out of the market for too long.

• Greed and chasing hype: Chasing hot stocks or trends without understanding a company’s fundamentals leads to buying high and selling low — another surefire way to lose money. As an example, ChatGPT cited meme stocks, crypto bubbles and speculative tech stocks with no earnings.

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To keep emotion out of your investment decisions, ChatGPT recommended adopting a strategy that helps you avoid acting impulsively when the stock market teeters back and forth.

Here are some steps you can take:

1. Define clear goals. Before investing any money, first determine why you’re investing (e.g., retirement, house down payment, kids’ education). Next, determine the time horizon for when you need the money, and then gauge your risk tolerance so you’ll have an idea of how much volatility you can stomach.

2. Create a rules-based plan. This plan should include your desired asset allocation in terms of stocks, bonds and other investments. It should also include how much money you want to put into different economic sectors, geographies and asset classes. Finally, decide how often you want to rebalance your portfolio, and how you plan to act during bull and bear markets.

3. Use tools to “automate discipline.” Another good idea is to decide when you want to make automatic contributions to your investment accounts. For a simple, self-balancing portfolio, consider target-date funds or exchange-traded funds. No matter which tools you use, make a commitment not to check your portfolio daily.