It's a phenomenon called "memory conformity."
Researchers tested this by having some people watch a video and then answer questions about the plot. Then they were shown incorrect answers from some fictional participants.
Nearly 68 percent changed their correct answers.
And then, even when they were told it was a trick, 40 percent still stuck with those wrong answers when they were retested.
Brain scans captured the process.
The study in Science magazine concludes human memory is highly susceptible to social influences.