You're entering a train car, a restaurant, a local store. As you step inside, you scan the people there. You don't know any of them, yet in seconds you register impressions of them all. He looks friendly, she appears evasive, that teenager seems threatening. Even as you're assessing the factual cues of their bodies--gender, skin color, height, age--you already seem to know whom you perceive as likable and whom you should avoid.

The fact that our brains can reach such rapid conclusions is astounding. It is also troubling: despite the paltry information available to the brain in those initial moments, our first impressions can color our continued perception of an individual, regardless of whether his or her later words and actions contradict our hasty preliminary view.

To make such social perceptions, we rely on patterns and stereotypes that we have learned throughout our lives. For example, when we see a man driving a lavish car, what impression do we have of him: a rich show-off or a self-made achiever? If we see a teenage girl struggling to handle a crying baby, do we see an ill-prepared mother or a babysitter? We pull out dozens of labels from our heads and apply them to other people.

Our social perception is constantly active, too. We can't turn it off. And we place it in high regard; when we are making decisions, it is often the factor that tips the scales, although we may not even be aware that it is affecting us. Social perception strongly determines everything--from whom we fall in love with to whom we trust to sell us insurance. In every case, how well we like the person plays a major role. But why do we find certain people instantly likable while we mistrust others?

Beauty Favored
The process by which we "decide" whom to like is less open than we would prefer to think. We tend to follow some persistent prejudices. Twenty years ago, for example, University of Massachusetts Boston psychologist S. Michael Kalick demonstrated that we generally favor faces, body shapes and clothing styles that are similar to our own. And although it sounds shallow, we are significantly influenced by beauty. When assessing members of the opposite sex, at least, our hearts warm more readily to people who have been blessed with flawless skin, flowing hair, straight teeth, and a well-proportioned and slender figure. Evolutionary psychologists think we are attracted to these characteristics in part because they send positive signals to our primal brain circuits responsible for choosing a mate: "I'm healthy. I have strong genetic traits!"

Ironically, when our brains do take the time to think critically, excessive perfection can elicit mistrust or inferiority or jealousy. Often in court proceedings, very attractive defendants are given harsher sentences if it appears they used beauty as a means to an end. Nevertheless, studies of different social situations agree that our brains automatically react positively to attractive people.

Emotion over Logic
Of course, we have all had bad experiences with attractive people. These encounters reveal a major weakness in social perception: that preconceived notions can lead us to poor decisions. We are seldom aware of these prejudices, however, which gives them power over us. They are persistent and hard to overturn. Tania Singer and Joel S. Winston of the University College London's Institute of Neurology reached that conclusion when they showed test subjects portraits of various people. Some of the faces elicited immediate alarms in the amygdala--a structure near the brain's center that is considered the seat of emotion--indicating that the individuals pictured "did not inspire trust." Yet when the subjects were told later about the good qualities of the people they had seen, few indicated that the information changed their initial impression.

Psychologists have been researching social perception for decades, but it is only recently that brain imaging and electrical sensing techniques have begun to elucidate its biological roots. "Social neuroscience" is still a young discipline, but discoveries are helping experts decipher what makes us judge a stranger as friend or foe.

For example, visual signals from the eye's optic nerve travel to two brain regions: the forebrain, where conscious thought occurs, and the amygdala. Both regions evaluate what we see, but in completely different ways. The amygdala first makes a determination of friend or foe--within milliseconds, automatically and independently. The forebrain comes into play only after the amygdala has made its determination, and it is influenced by that assessment as it consciously categorizes and assesses the visual information.

The effect of this dual processing was tested by neuropsychologist William A. Cunningham, now at the University of Toronto, and his colleagues. He placed each of 15 subjects in a magnetic resonance imaging machine. To each of them, he read aloud the names of famous people, such as Bill Cosby, Yasir Arafat and Mahatma Gandhi. The subjects were instructed to respond to a neutral question ("Is he alive or dead?") and an emotionally driven question ("Is he a good or bad person?"). The images showed that the subjects answered the first question with ease. In the second case, Cunningham observed a considerable increase in the amygdala's activity, especially in connection with names that carried negative connotations, such as Adolf Hitler. Yet the forebrain showed approximately the same level of activity as it had during the neutral question, irrespective of whether a name elicited a positive or negative image. In essence, the amygdala cast the deciding vote on whether to declare someone good or evil. Emotional assessment outranked cognitive assessment.

Among its other duties, the amygdala functions as a danger detector, activated by potential threat. Its rapid response can instruct us whether to react with fight or flight. In pioneering work in the 1990s, Joseph LeDoux of New York University showed that angry human faces elicit stronger responses in the amygdala than known threats themselves, such as snakes. Recently Ahmad R. Hariri of the National Institute of Mental Health imaged the brains of 28 subjects while they viewed photographs of faces with angry or fearful expressions. Hariri also showed them pictures of snakes, sharks and guns. Both sets of pictures elicited a significant response in the amygdala, but its reaction to threatening faces was stronger.

The Social Brain
The more active the amygdala becomes, the more intense our emotional upheaval and the more our capacity to reason decreases. Decisions are made intuitively rather than as a result of rational assessment.

Women tend to be better than men in judging the character of others, as well as expressing empathy for them. The reason, says British psychologist Simon Baron-Cohen of the University of Cambridge, is that from childhood on, girls are instructed to be sensitive to the feelings of others and to offer consolation when warranted. This type of socialization helps to hone senses, intuition and observational skills.

Women are also more likely to make decisions based on "gut feeling"--or better yet, on "amygdala feeling." And finally, women draw on their language centers more than men. As a result, women are often better at verbalizing and therefore have an easier time in gaining emotional access to other people.

Baron-Cohen and a growing number of neuroscientists contend that the left brain contains actual "social brain regions" that enable the brain to accurately perceive other people. According to Baron-Cohen, the left brain develops much faster in female fetuses and babies, as well as in young girls, establishing a lifelong advantage in language and social intelligence.

Compared with men, women are also thought to use the sense of smell more when deciding on a person's likability. In 2002 experimental psychologist Pamela Dalton of the Monell Chemical Senses Center in Philadelphia proved that females have a much more discerning nose than males. Once again, the amygdala plays the deciding role in assessing olfactory information. Scent is therefore closely connected to emotional reactions. Psychologist Noam Sobel of the University of California at Berkeley found that the amygdala reacts strongly not just to foul odors but also to pleasant smells. Sobel thereby supports Cunningham's claim that this structure could be responsible for spontaneous feelings of attraction and for choosing a mate, as well as for fear and antipathy.

Follow Your Nose
Regardless of how important smell, language and the social brain are, the lesson is that we establish important reactions and make many important decisions based not on precise thinking but on feelings of attraction or rejection.

Nature has developed a system for quickly figuring out whether a stranger is friendly or threatening. This system operates without the intervention of the intellect. The downside is that we cannot escape its function. The amygdala and the social brain manipulate us whether we want them to or not. Of course, our forebrain and conscious reasoning have input and can veto assessments. But when it comes to emotional questions, perhaps we modern, thinking people should put more trust in our sniffers, which have been perfected over thousands of years.