Where do you think you were on September 11, 2001? Turns out there’s a good chance you’re wrong, according to a new study in the Journal of Experimental Psychology.

In the days following the 9/11 attacks, researchers from more than a dozen universities asked 2,100 Americans across the country about their personal 9/11 experience—questions like where they were, who they were with and how they responded. Forty percent of people in the study changed their stories and gave fundamentally different answers when the researchers followed up at 1-year, 3-year and 10-year intervals.

Get The Brief. Sign up to receive the top stories you need to know right now. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

“Human memory is not like a computer,” says study author William Hirst, PhD, a professor of psychology at the New School for Social Research. “Human memory is extremely fallible.”

The tendency to misremember is likely the result of a “time-splice error,” Hirst explains. In other words, people remembered facts about their 9/11 experience, but they forgot how pieces fit together. In the survey, one man remembered being on the street when he heard news of the attack but was actually in his office. The man probably spent time in both places at some point that day, but his memory of the truth blurred with time, Hirst says.

Once people have come up with an inaccurate but coherent narrative, they often stick with it, the study finds. While many people’s memory of their experiences changed in the year following 9/11, they tended to continue telling the same false memory in the decade that followed.

“You begin to weave a very coherent story,” says Hirst. “And when you have a structured, coherent story, it’s retained for a very long period of time.”

In contrast to poor memory of personal details, Americans recalled the actual events of 9/11 remarkably well. Researchers found that the people surveyed recalled event information about 80% accurately at all time intervals after the initial survey. Some inaccurate memories tended to be corrected over time, likely a consequence of frequent media reports on the topic, Hirst says.

The study may also help explain why suspended NBC anchor Brian Williams retold a false narrative about his time in Iraq, says Hirst. Williams’ evolving story is consistent with the patterns in the study, he says.

There’s no way to tell for certain whether your 9/11 memories are accurate, Hirst says. “We don’t have a memory that allows you to check things so easily,” he says. “In fact, one could argue that the whole notion of accurate memory is some invention of technology.”

Read next: We’re Not Doing Enough To End Hate Among Our Children

Listen to the most important stories of the day.

Write to Justin Worland at justin.worland@time.com.