Julian Baggini: There is no one either good or bad, but circumstances make them so

As soon as the identity of the Cumbria killer became known, people immediately started to ask what kind of man Derrick Bird was. And there were always going to be only two possible answers. Almost every perpetrator of an atrocity is assigned the role of either a monster who was bound to do great evil eventually, or an ordinary person who inexplicably flipped.

Bird appeared to fall into the latter camp. Michelle Haigh, landlady of his local, the Hound Inn, spoke for many when she said he was a "normal bloke" and that his actions were "not in character". "We're talking about a different person from the Derrick we know." But there is something deeply unsatisfying, indeed disturbing, about this. Don't we know that bad people are fundamentally different? Surely it can't be true that tonight, you could be sharing a drink in the pub with any number of potential Derrick Birds?

And so the hunt is on for the signs that can reassure us that Bird was always a bit dodgy, and that despite all evidence to the contrary, people knew there was something not quite right about him. The Sun is leading this search, revealing that he stole from his employer 20 years ago and was a frequent sex tourist to Thailand. As evidence of his rage, the paper even claims that he would frequently complain about how hard it was to earn a decent living driving taxis. But if these are the hallmarks of a spree killer, they're everywhere. Indeed, his Thai jaunts were with fellow cabbies, some of whom are among the innocent, decent victims mourned by The Sun.

This conviction that there must have been tell-tale clues can lead people close to killers like Bird to feel tremendous guilt for not spotting them. As consultant forensic clinical psychologist Gerard Bailes told Radio 4's The World Tonight, "Of course after the event they can look back with a different light, but the reality is that you can't spot these things."

Newspaper profiles tend to list giveaway characteristics of spree killers. Typically the murderer is withdrawn, male, quite isolated, perhaps with an interest in guns. But as Bailes said, these can apply to a whole range of people, whereas spree killings are extremely rare. Character profiles are terrible predictors of such one-offs.

The desire to find the fatal flaw is based on a widespread but false assumption that character is the only way to unlock the secrets of a killer. Either the person is depraved, which explains everything, or the person acted completely "out of character" and it's all just a mystery, a case of someone going mad. There is no halfway house. We seem to have a deep faith that not only do people have a character which determines how they would act in different situations, but that we know what those characters are like. Several relatives of men on United Airlines Flight 93 for instance, which crashed in Pennsylvania before reaching its assumed target in Washington, attested that they just knew their husbands and sons would have done something to tackle the hijackers. On the other hand, friends and relatives of people accused, and even found guilty, of terrible crimes often insist they just know they could never have done such a thing.

In fact, the dominant school of thought in psychology and philosophy now is situationism, which claims that the best predictor of how people behave is the circumstance they find themselves in, not their predispositions. As the leading psychologist Philip Zimbardo told me, "We have underestimated the power of social situations because we overestimate the power of individual dispositions." Experiment after experiment seems to support the situationist case. For instance, one of the best predictors of whether people will help someone in distress or not is how many other people are also on the scene. What social psychologists call the "bystander effect" means that people are much more likely to help if only they can help, and more likely to do nothing if others are present.

One famous experiment also showed that simply being returned an unexpected coin from a pay phone made recipients much more likely to help an experimenter's stooge pick up dropped papers outside the booth. In another, trainee priests were much less likely to act as good Samaritans if they were in a bit of a hurry, even though their appointment was not crucial.

Situationist conclusions are not only compelling in the lab, but in horrific, historical events. Does anyone seriously believe, for instance, that the average German in 1939 had a worse moral character than the average Frenchman? We should have learned by now that concentration camp guards were just ordinary people who had the worst brought out of them by circumstances. The same people, in other situations, could have led blameless lives as pillars of the community.

One reason we resist this is that we worry that to explain all is to forgive all. As soon as we start arguing that situation is a critical factor in how we behave, individual responsibility goes down the drain. But this doesn't follow. For instance, when Philip Zimbardo was called as an expert witness in the trial of Sergeant Chip Frederick, one of the American soldiers charged with abusing prisoners at Abu Ghraib, he did not try to get him off. One of this first comments was that Frederick was guilty as charged. But Zimbardo told me, "He never would have done those things, based on everything I know about him, except that he was in the situation that corrupted him. In fact, every single prison guard on the night shift did similar things, nine out of nine. So you can't say this is a bad apple."

We need to understand that situation, rather than character, usually matters more in how people behave, not so we can just let everyone off, but so that we can better understand why people do wrong and do more to prevent it. Most people are neither bad apples nor good eggs, but soft fruit that can easily turn from ripe to rotten.

Left and right have traditionally made opposite mistakes in this regard. Conservatives have placed far too little stress on the role of social circumstances in criminality, imagining that poverty and social exclusion are mere excuses for criminality, not causal factors in it. The left, on the other hand, has tended to overplay the social, imagining that people have no control over how they respond to circumstances, that all criminality is the inevitable result of iron laws of economic determinism.

What we are now understanding is that it is not either/or, but that there is perhaps more to be said for the traditional left-wing view than the majority believes. Politics is abut creating the conditions in which the chances of people staying in a morally good condition is higher. Personal moral responsibility is about avoiding situations that bring out the worst in us and developing the self-awareness to resist the temptation to do wrong when such situations are unavoidable.

Mass killers always live on in the public imagination as symbols, never for the people they really were. Derrick Bird should serve as a reminder that moral character is fragile much more often than it is robust. The evidence suggests that most people blow with the ethical wind, but it does not show that it is not possible to hold fast, no matter what the moral climate. Most people are not of good or bad character, because strictly speaking, they have no robust character at all. The best response to that is to try to fortify our own, and not give in to the pessimism that says "there but for the grace of God go I", nor the naive optimism that only other people, never ourselves, are capable of doing terribly wrong things.

Julian Baggini is editor of 'The Philosophers' Magazine'; www.philosophersnet.com