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We all know how important it is to make a good . At the same time, we know we shouldn’t judge a book by its cover, because looks can be . So how do we make sense of these two contradictory pieces of advice?

Here’s the conundrum: We know other people will make snap judgments about us. But we tell ourselves we shouldn’t make snap judgments about them. The “shouldn’t” in the previous sentence is key, since moral injunctions are always about dissuading us from following our natural inclinations.

Psychologist Leslie Zebrowitz of Brandeis University studies the facial information we use to judge other people. Although first impressions are notoriously prone to error, we just can’t stop ourselves from making them — and it only takes a tenth of a second to form a judgment about another person’s character, even from a still photograph.

We’re also very consistent in the judgments we make about others. When researchers ask participants to rate faces in photos on traits such as dominance and warmth, most will judge the persons in the pictures the same way — and these results hold across cultures, suggesting that the processes we use for creating first impressions are innate. There’s even evidence that infants and young children judge faces similarly to adults, strengthening the case for an evolved mechanism.

But if we do have an innate mechanism for making snap judgments of character from people’s faces, just how accurate is it? After all, natural selection should have weeded out this behavior if it wasn’t beneficial. When we compare people’s snap judgments of character with objective forms of measurement, such as personality tests, we find the accuracy of our first impressions is better than chance. In other words, we often judge correctly, but there are still plenty of times when we’re completely wrong.

As Zebrowitz points out, incorrect first impressions can have significant social consequences. People with the “right” kind of face are judged as more likable, knowledgeable, and capable. However, those with the “wrong” kind of face are deemed unapproachable, incompetent, and untrustworthy. So your face alone can have a big impact on your , your success, and even legal decisions.

In her research, Zebrowitz has identified four facial cues that people use to judge the characteristics of other people:

The first facial cue is "babyfaceness" — that is, having a baby-like face. Plenty of evidence shows we have an innate tendency to find baby faces appealing. Savvy public speakers know a baby picture or two in their PowerPoint slides will elicit “oohs” and “aahs” from their audience. Likewise, videos of babies and kittens (which also have baby faces) are among the most popular on YouTube.

It makes evolutionary sense to have an inborn soft spot for babies, who need to be protected and cared for. But we overgeneralize this predilection to adults whose faces are baby-like. Specifically, baby faces have big eyes, large foreheads, short chins, and rounded heads, among other characteristics. Research shows that men tend to prefer women with baby-face qualities as sexual partners, presumably because these features signal youth and fertility.

We tend to treat baby-faced adults kindly, but we also assume them to be weak and needing care. For example, the elderly often take on a baby-face appearance as their hairlines recede, and their faces round out. We tend to treat senior citizens as babies, whether they need special care or not. In short, if you have a baby face, people will tend to treat you kindly, but they may not think you’re very capable.

The second facial cue is familiarity; we tend to judge people based on their facial similarity to other people we know. So, if you meet someone who looks like your grumpy Uncle John, you’ll assume he’s grumpy, too. And if you see someone who resembles your friendly Aunt Joan, you’ll think she must be friendly as well. In part, this stems from the fact that our innate facial-processing mechanisms lead us to believe in a correlation between facial features and personality characteristics.

However, most people we judge as familiar don’t remind us of anyone in particular. Rather, it’s just a vague sense that they look like the other people we hang around with — specifically, members of the same family, race, and . In this case, the familiarity effect kicks in. That is, we like what we’re familiar with. My family members all look sort of like me, and I like them more than your family members (even though they’re probably quite nice, too).

The third facial cue is fitness. There’s general agreement about what makes a face attractive. Underlying these judgments of are facial features that are arranged in symmetry and proper proportion. The more faces diverge from the symmetrical and properly proportioned, the more they’re judged to be unattractive. Facial symmetry and proportion are signals of genetic diversity (inbreeding increases the likelihood of genetic diseases) and a strong immune system capable of fighting off disease.

Healthy people look attractive. We also assume they’re likable, intelligent, and capable. We think they’re good people to have as friends. Unhealthy people are unattractive, and we feel they need to be treated with caution. Whatever diseases they have may be infectious, so it’s best to stay away. Thus goes the evolutionary logic that drives our intuitions about other people.

The final cue is emotional resemblance. We’re very good at reading the emotional expressions of other people. However, some people have facial features that resemble emotional expressions. For example, people with lower eyebrows may just look angry, even when they’re not. Likewise, those whose mouths are turned upward at the corners appear to be happy no matter how they feel.

The intuitions that guide our social interactions are largely innate, but that doesn’t mean we’re powerless to overcome them. If we understand how our intuitions work, we can avoid pitfalls and guide them to our advantage. This means skillfully manipulating our own facial features to make the right first impression in others, while guarding ourselves against intuitive judgments that may lead us astray.