Even though we were taught not to “judge a book by its cover,” we all do it—especially when meeting someone for the first time. Most of us would like to believe that we revise those snap judgments later, after we've spent some time with the new person. But according to a new study, we're less open to changing our minds than we think, and our initial impressions don't fade easily from memory.

First things first, though: Our tendency to make split-second decisions about people isn’t inherently bad, says Vivian Zayas, PhD, professor of psychology at Cornell University. It’s human nature, and an evolutionary defense against those who might be dangerous or just ill-suited for us.

“Humans are very social, so we want to know when we meet someone what that person is really about,” Zayas explains. “We are wired to do this, and we’ve become experts at gathering a wealth of information from people's faces—things like gender and ethnicity, but also more subtle personality cues as well.”

Relying on these initial assumptions can be helpful in some circumstances, says Zayas, but in others it can keep us from making valuable friendships and connections. So she and her colleagues set out to determine just how strong first impressions really are.

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The researchers asked 55 study participants to evaluate whether they’d be friends with four women, based solely on headshot photographs. (Each woman smiled for one photo and made a neutral expression for a second.) The participants were also asked if they thought these women were extroverted, agreeable, emotionally stable, conscientious, and/or open to new experiences. 

Between one and six months later, the participants returned for a supposedly unrelated experiment, and met one of the women whose photos they’d pre-judged. (Only four participants remembered seeing her before, and they were later factored out of the analysis.) Each participant spent 20 minutes with this woman, during which they played a trivia game and were instructed to get to know each other as well as possible.

After the interaction, the participants were asked the same questions about the woman’s personality traits—and their responses showed a “strong consistency” with their previous thoughts, says Zayas. Those who had guessed that the woman was likeable and had appealing personality traits had generally positive impressions after meeting her. And for those who had judged the woman negatively, their opinions tended to stick, too.

"What is remarkable is that despite differences in impressions, participants were interacting with the same person,” Zayas says. The findings showed that some changes in opinion did occur. But for the most part, people's views didn't waver.

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The woman didn't know how the participants had rated her photograph, so she didn’t go into the meetings with any biases of her own. But Zayas says it’s likely that participants’ initial impressions were reflected in their behavior, and that the woman picked up on those clues.

Those who had liked the woman in the photo tended to interact with her in a friendlier way. "They're smiling a little bit more, they're leaning forward a little bit more. Their nonverbal cues are warmer," she said in a press release. "When someone is warmer, when someone is more engaged, people pick up on this. They respond in kind. And it's reinforcing: The participant likes that person more."

This concept, known as behavioral confirmation, shows how first impressions can become a self-fulfilling prophecy, she says.

This isn’t something people plan to do, of course: In a related study, the researchers found that participants overwhelmingly said they would update their opinions of people in photographs if they had the chance to meet them in person. "And people really think they would revise," she said. "But in our study, people show a lot more consistency in their judgments, and little evidence of revision."

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Participants’ assumptions about different personality traits also supported the concept of a “halo effect” based on appearance. "We see an attractive person as also socially competent, and assume their marriages are stable and their kids are better off,” she says. “We go way beyond that initial judgment and make a number of other positive attributions.”

Zayas admits that the study, published in Social Psychological and Personality Science, only gave people a short amount of time together. It's possible that they only engaged in small talk, and didn’t dig deep enough to truly alter their opinions of each other. Longer, more intimate interactions could certainly lead to different results, she says. But she also points out that in many situations, such as job interviews, a few minutes together is all people get.

The fact that we’re likely to hang onto our first impressions—especially of people who are unfamiliar to us in some way—isn’t something we should be defensive about, says Zayas. But it can help to be aware of it when it happens.

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“There are good, functional reasons for why our brains do this,” she says. “But there’s a fine line between appreciating this ability and realizing that sometimes our judgments are incorrect. If we fully embrace them, we might miss a lot of opportunities to open up and get to know people.”

Acknowledging that your first impressions have the potential to shape your interactions going forward may help you override some of your subconscious behavior. “Maybe you’re a little less guarded, more engaged, smiling more,” Zayas says. “Then the person has the opportunity to respond back, and you may be able to break the cycle.”