The man who walks out of his house with a mind devoid of stereotypes is like the man who goes to the Antarctic without having inquired about the weather. But there is no such man: for even to know that the Antarctic exists is to know that it tends to be cold there. Our minds are necessarily full of stereotypes and we could not negotiate the world without them.

George Zimmerman is accused by his detractors of having acted upon a stereotype. He saw a young black man allegedly pursuing an erratic course in a gated community and he concluded that he was up to no good, that quite possibly or even probably he was a burglar on the prowl. If only he had kept another stereotype in his mind, things might not have turned out so disastrously: It was raining that evening and burglars do not like the rain. In fact, the principal cause of certain kinds of crime is clement weather, because the statistical association between such weather and those types of crime is the strongest known to me, stronger even than those between smoking and criminality (more than 90 percent of prisoners, at least in Britain, smoke), and between tattooing and criminality (an even higher percentage of white criminals are tattooed, except for those charged with fraud, embezzlement, etc.).

I first learned of the meteorological causes of crime on the walk that I took most afternoons for 15 years, between the hospital where I worked in the morning to the prison where I worked in the afternoon. It was about 600 yards, and on fine summer days up to six or seven cars parked on the way would have been broken into, the little shards of shattered glass sparkling, almost with the color of peridots, on the curbside. In winter, or in the rain, not a single car was ever broken into, and I was surprised that the police had not issued a warning to car owners to park their cars only in bad weather. Criminals may be tough, but they are not hardy.

Now, if George Zimmerman had realized this, it would have neutralized his alleged stereotype and the whole tragedy would not have occurred. He didn’t realize that it was unlikely (though not absolutely impossible) that the young man was on a criminal enterprise because he wasn’t hurrying to get out of the rain, as most true criminals would have done if they had been caught in it.

There is no doubt that most people’s view of the case accords with the stereotypes they carry around with them in their minds almost as intimately as their cells carry their DNA. Polls showed that the disagreement between America’s whites and blacks on this cannot be a purely intellectual one, for both had access to the same information, the same records, and so forth. A survey conducted by Pew found that 86 percent of blacks and 30 percent of whites were dissatisfied with the verdict; and an ABC News/Washington Post poll found that 87 percent of blacks and 33 percent of whites thought the shooting was unjustified.

There were no facts in the case so unambiguous that they compelled any rational person to come to one and only one conclusion about George Zimmerman’s guilt or innocence of the charge, such that we could condemn anyone who didn’t come to that conclusion as mad, malign, or mentally deficient. But I should be inclined to add that no rational person could say that George Zimmerman’s guilt had been proved beyond a reasonable doubt, and therefore that his acquittal was both right and inevitable, were it not that by doing so I should be condemning 86 percent of blacks to the realms of irrationality.

For most people, the willing suspension of disbelief is difficult and requires effort not when, for example, they see a film or read a novel, but when they hear or read about events in the real world. And the belief that they cannot, will not, or desperately do not want to suspend is their belief in the picture of the world that they have formed for themselves. For them, the world is an assembly of stereotypes the abandonment of any one of which threatens the whole worldview that is precious to them and causes them discomfitingly to doubt their ability to understand the world.

An Internet commenter on an article that tried merely to give the factual account of the trial of George Zimmerman wrote: “This trial is a joke. You don’t need a trial to establish his [Zimmerman’s] guilt.” For this reader, Zimmerman was guilty ex officio, just as for others Trayvon Martin was ex officio nothing but a young thug who got what he deserved. This reader is the kind of person, of whom there are many, who cannot distinguish between O. J. Simpson’s guilt and the prosecution’s failure to prove it.

In a bad-tempered exchange that followed a Web article on the case, a reader whose cybername is “Yowolowo” wrote: “Zimmerman didn’t ‘introduce himself.’ If he had, probably none of this would have happened. He didn’t follow Neighborhood Watch protocol, which is to be the eyes and ears, not the enforcer. He brought a loaded gun and got out of his car and approached a young man who had no idea what his intent was. He acted foolishly, resulting in the death of an innocent person.” A man who calls himself “White Hunter” replied: “Good thing he had the gun. Otherwise an innocent person would have died.” Yowolowo countered: “An innocent person did die, the one who got stalked by a wannabe-cop with a loaded gun. There is nothing courageous or righteous about what Zimmerman did. And your comment is hateful and despicable.”

#page#One would have been surprised, I think, if Yowolowo had written what White Hunter wrote, and vice versa. Both Yowolowo and White Hunter chose their respective names to convey something important about themselves, and the fact that we would be surprised if Yowolowo thought Zimmerman was fully justified in shooting Martin, or if White Hunter thought that Zimmerman murdered the totally innocent Martin, suggests that they both succeeded in doing so. In other words, they were appealing to stereotypes, which brings us to an important but often overlooked point about stereotyping: that many, perhaps even most, people want to be stereotyped. Indeed, they do their best to ensure that they are, for, unlike the detractors of stereotyping, they appreciate something deeply embedded in human nature — namely, that man is a creature for whom symbolism is as inescapable as breathing.

It may be, of course, that we usually don our clothes as second nature, without thinking. But that does not mean that they symbolize nothing. For me it is as natural to put on a tweed jacket as for a youth of a certain kind to don a hoodie or a baseball cap sideways; neither of us gives much thought to the matter on any individual occasion, though both of us convey a message thereby.

But often we do give thought to such matters. The fashion among young males for low-slung trousers, for example, originated as a symbolic identification with prisoners, who have their belts removed from them on arrival in prison for fear that they will hang themselves with them or perhaps use them as a weapon. The results are obvious; and those who see, or rather intuit, in this fashion an insolent defiance, a deliberate rejection of what would once have been called respectability, are surely right to do so even if they do not know the origin of the fashion. The same is true, incidentally, of those who obey the fashion; they may not know its origin, but they are fully aware of the effect it is likely to have on those whom they wish to offend. Such, indeed, is its whole purpose: The fashion is a symbol of an attempted creation of a mirror-image moral universe, in which what is held to be good by one part of society (that to which we, dear reader, belong) is held in contempt by the other, and vice versa.

Now it is obviously true that not all young men who dress in, say, hoodies are thugs; but if you were walking down an inadequately lit alleyway and a young man in a hoodie came toward you, it is likely that you would experience a greater frisson of fear than if he were dressed in a tweed jacket. And that may be precisely what he wants, even if he has no intention of attacking anyone. He wants you to stereotype him.

I learned this lesson from my patients. In England, young people who have committed so many crimes that they are sent to reformatories often tattoo an Indian-ink blue spot on their cheek, in the same way that Old Etonians wear ties. This marks them out after their release as people not to be trifled with. But among my patients were a number of young men who had never been in trouble with the law who tattooed themselves with the blue spot. They did this for one of two reasons, or for both: Breaking the law sufficiently frequently to have been sent to reformatory carried a kudos in the area in which they lived, evil in those areas having become the new good; or they wished to appear more aggressive and violent than they were, as certain butterflies mimic poisonous varieties so that birds will not eat them. If you want to be left alone, look dangerous.

Attempting to look tougher than you are, or more aggressive than you are inclined to be, has its dangers. I had a patient who had the words “NO FEAR” tattooed prominently on the side of his neck, supposedly to convey the message that he was, despite his less-than-great size or muscular development, of ferret-like ferocity, and likely to attack at any moment for no reason. Unfortunately his supposed fearlessness was taken as a challenge by those who like to think of themselves as bad bastards, and he was four times landed in hospital by unprovoked or prophylactic attacks on him when he entered pubs. Removal of the offending tattoo was beyond his means.

The symbolic importance of apparel, and the simultaneous desire to be stereotyped for wearing it while avoiding the negative consequences of that stereotyping (that is to say, having your cake and eating it), was vividly illustrated recently in an incident in France, where it is now illegal to wear any form of dress that conceals the face. A young woman in Trappes, a town near Versailles, who was an Antillean convert to Islam, was walking in the town in a niqab when the police stopped her. What happened next is a matter of dispute (as it usually is in such incidents, with everyone believing what he wants to believe); but before long there were riots.

Commenting in the left-wing national newspaper Libération, a Muslim activist, Mohammed Mechemache, whose organization, ACLefeu (L’Association collectif liberté, egalité, fraternité ensemble unis, which is a homonym for Assez le feu, “enough of the fire”) was founded after the riots all over France in 2005, said that people were being excluded from society in France because of a bit of cloth, the niqab: to which one might reply, If it is only a bit of cloth why insist upon wearing it?

#page#To read the website of ACLefeu is to hear the American arguments over the meaning of the Trayvon Martin killing in a French accent. Discussing the case of Zyed Benna and Bouna Traoré, aged 17 and 15 respectively, who were killed during a police car chase, the site says: “On Tuesday, 4 June 2013, the prosecutor asked for the case against the two policemen to be dismissed. . . . This decision, unfortunately one of many, undermines the honesty and the guarantee of impartiality of justice in our country, and once more this ‘verdict’ leaves a bitter taste of the reality of an unequal justice. The request for a dismissal strengthens the feeling expressed in working-class areas that the justice they confront is unequal.”

The question boils down to this: Are stereotypes generalizations drawn from experience of actual behavior, or is that behavior a response to the stereotypes? In an unguarded moment, Jesse Jackson famously (or notoriously) revealed which side he came down on, what he really believed, when, rhetoric aside, he said: “There is nothing more painful to me at this stage in my life than to walk down the street and hear footsteps and start thinking about robbery. Then look around and see somebody white and feel relieved.” In other words, Jackson shared the stereotype that George Zimmerman is now so severely criticized for allegedly having had. (Actually, Jackson’s account of his own experience is inaccurate. What he was relieved at not seeing behind him was not a black person in general, but a young black man; a black woman or a middle-aged black man wouldn’t have worried him in the least.)

Stereotyping is inevitable in complex societies in which not everything can be known about every individual. It is a kind of mental shorthand, and like shorthand can be either accurate or inaccurate. The stereotype can be wrong in general or wrong in a particular case. Moreover, most people are apt to forget that from the mere fact that most a’s are b’s, it does not follow that most b’s are a’s — that is to say, from the fact that most people who get lung cancer smoke it does not follow that most people who smoke get lung cancer, any more than it follows from the fact that most white criminals are tattooed that all white tattooed people are criminals.

But stereotypes are useful as rough guides. It so happens that on the day before I wrote this, I appeared as an expert witness at a murder trial. There was a security check at the entrance to the courthouse but the security men gave me only the most cursory of inspections, on the grounds that 60-year-old men in business suits carrying bags full of documents are very unlikely to be bent on causing (physical) mayhem. But they searched very thoroughly the man behind me, more than 30 years my junior, tattooed, with a chunky gold chain around his neck and several rings that might have doubled as knuckledusters, a shaved head, scarred face and scalp, unnecessary gold dentistry, and eyes that sparkled with malignity. Lombroso would have had a field day with him.

In short, they profiled him, without necessarily knowing that that was what they were doing; and since violence and intimidation in public areas of the courthouse are far from unknown, it is difficult to see the different way in which we were treated as completely unreasonable. If you don’t want to be taken for a thug, why go to such efforts to look like one? It is very unlikely that the man did not know that he looked like a thug.

Once inside the courthouse, it was not very difficult to distinguish, physically, between those on the wrong side of the law and their legal advisers. There were a lot of people present (all white, incidentally) to whom, on the basis of stereotyping, you would have given a wide berth on a dark night. Indeed, you would have been a fool not to. It was, however, far more difficult to distinguish, physically, the families of the perpetrators from the families of the victims. They looked, on the whole, very much the same types. And this was a timely reminder that the main victims of crimes are those who are close, geographically and socially, to the criminals who commit them.

In other words, the main victims of crimes committed by blacks in the United States are blacks, not whites. And the statistical chance of a young black man’s being killed by a white assailant is very slender (now that the dreadful days of lynching are over) by comparison with his chance of being killed by another young black man. Jesse Jackson’s stereotyped thinking when he heard the footsteps behind him acknowledged this truth.

It is not wrong to stereotype; it is wrong — foolish and sometimes wicked — to allow stereotypes inflexibly to trump evidence. There is no reason to think that George Zimmerman did this. He must have known that not all black people in the gated community should be suspect, because he knew that 20 percent of the residents were black. Moreover, a few years earlier he had protested against the maltreatment of a black man by the police. His suspicion of Trayvon Martin was therefore specific to Martin, even if he thought that young blacks were more likely to be criminal than young whites. (Would he have reacted in the same way had Trayvon Martin been white? I cannot prove it, of course, but I think he would have.) If Zimmerman’s conduct is to be reprehended, it is for foolishness or worse, not for acting unjustifiably on a stereotype.

President Obama’s inelegant and imprecise impromptu remarks illustrate, however, the dangers of inflexible thinking in stereotypes (though also, possibly, the political usefulness to demagogues of such thinking). He said: “I think it is important to recognize that the African-American community is looking at this issue through a set of experiences and a history that doesn’t go away.” Of course it doesn’t go away if you believe, a priori, that everything that happens is a reenactment of it.

– Mr. Dalrymple is a contributing editor of City Journal and the author, most recently, of Farewell Fear.