You can do these kinds of experiments a thousand different ways and the answer almost always comes out the same way. This happens even when you give people a clear and immediate environmental explanation of the behavior they are being asked to evaluate: that the gym, in the first case, has few lights on; that the Contestant is being asked to answer the most impossibly biased and rigged set of questions. In the end, this doesn't make much difference. There is something in all of us that makes us instinctively want to explain the world around us in terms of people's essential attributes: he's a better basketball player, that person is smarter than I am.

We do this because, like vervets, we are a lot more attuned to personal cues than contextual cues. The FAE also makes the world a much simpler and more understandable place. In recent years, for example, there has been much interest in the idea that one of the most fundamental factors in explaining personality is birth order: older siblings are domineering and conservative, younger siblings more creative and rebellious. When psychologists actually try to verify this claim, however, their answers sound like the Hartshorne and May conclusions. We do reflect the influences of birth order but, as the psychologist Judith Harris points out in The Nurture Assumption, only around our families. When they are away from their families — in different contexts — older siblings are no more likely to be domineering and younger siblings no more likely to be rebellious than anyone else. The birth order myth is an example of the FAE in action. But you can see why we are so drawn to it. It is much easier to define people just in terms of their family personality. It's a kind of shorthand. If we constantly had to qualify every assessment of those around us, how would we make sense of the world? How much harder would it be to make the thousands of decisions we are required to make about whether we like someone or love someone or trust someone or want to give someone advice? The psychologist Walter Mischel argues that the human mind has a kind of "reducing valve" that "creates and maintains the perception of continuity even in the face of perpetual observed changes in actual behavior." He writes:

When we observe a woman who seems hostile and fiercely independent some of the time but passive, dependent and feminine on other occasions, our reducing valve usually makes us choose between the two syndromes. We decide that one pattern is in the service of the other, or that both are in the service of a third motive. She must be a really castrating lady with a facade of passivity — or perhaps she is a warm, passive-dependent woman with a surface defense of aggressiveness. But perhaps nature is bigger than our concepts and it is possible for the lady to be a hostile, fiercely independent, passive, dependent, feminine, aggressive, warm, castrating person all-in-one. Of course which of these she is at any particular moment would not be random or capricious — it would depend on who she is with, when, how, and much, much more. But each of these aspects of herself may be a quite genuine and real aspect of her total being.

Character, then, isn't what we think it is or, rather, what we want it to be. It isn't a stable, easily identifiable set of closely related traits, and it only seems that way because of a glitch in the way our brains are organized. Character is more like a bundle of habits and tendencies and interests, loosely bound together and dependent, at certain times, on circumstance and context. The reason that most of us seem to have a consistent character is that most of us are really good at controlling our environment. I have a lot of fun at dinner parties. As I result, I throw a lot of dinner parties and my friends see me there and think that I'm fun. But if I couldn't have lots of dinner parties, if my friends instead tended to see me in lots of different situations over which I had little or no control — like, say, faced with four hostile youths in a filthy, broken-down subway — they probably wouldn't think of me as fun anymore.

Some years ago two Princeton University psychologists, John Darley and Daniel Batson, decided to conduct a study inspired by the biblical story of the Good Samaritan. As you may recall, that story, from the New Testament Gospel of Luke, tells of a traveler who has been beaten and robbed and left for dead by the side of the road from Jerusalem to Jericho. Both a priest and a Levite — worthy, pious men — came upon the man but did not stop, "passing by on the other side." The only man to help was a Samaritan — the member of a despised minority — who "went up to him and bound up his wounds" and took him to an inn. Darley and Batson decided to replicate that study at the Princeton Theological Seminary. This was an experiment very much in the tradition of the FAE, and it is an important demonstration of how the Power of Context has implications for the way we think about social epidemics of all kinds, not just violent crime.

Darley and Batson met with a group of seminarians, individually, and asked each one to prepare a short, extemporaneous talk on a given biblical theme, then walk over to a nearby building to present it. Along the way to the presentation, each student ran into a man slumped in an alley, head down, eyes closed, coughing and groaning. The question was who would stop and help? Darley and Batson introduced three variables into the experiment, to make its results more meaningful. First, before the experiment even started, they gave the students a questionnaire about why they had chosen to study theology. Did they see religion as a means for personal and spiritual fulfillment? Or were they looking for a practical tool for finding meaning in everyday life? Then they varied the subject of the theme the students were asked to talk about. Some were asked to speak on the relevance of the professional clergy to the religious vocation. Others were given the parable of the Good Samaritan. Finally, the instructions given by the experimenters to each student varied as well. In some of the cases, as he sent the students on their way the experimenter would look at his watch and say, "Oh, you're late. They were expecting you a few minutes ago. We'd better get moving." In other cases, he would say, "It will be a few minutes before they're ready for you, but you might as well head over now."

If you ask people to predict which seminarians played the Good Samaritan (and subsequent studies have done just this) their answers are highly consistent. They almost all say that the students who entered the ministry to help people and those reminded of the importance of compassion by having just read the parable of the Good Samaritan will be the most likely to stop. Most of us, I think, would agree with those conclusions. In fact, neither of those factors made any difference. "It is hard to think of a context in which norms concerning helping those in distress are more salient than for a person thinking about the Good Samaritan, and yet it did not significantly increase helping behavior," Darley and Batson concluded. "Indeed, on several occasions, a seminary student going to give his talk on the parable of the Good Samaritan literally stepped over the victim as he hurried on his way." The only thing that really mattered was whether the student was in a rush. Of the group that was, 10 percent stopped to help. Of the group who knew they had a few minutes to spare, 63 percent stopped.

What this study is suggesting, in other words, is that the convictions of your heart and the actual contents of your thoughts are less important, in the end, in guiding your actions than the immediate context of your behavior. The words "Oh, you're late" had the effect of making someone who was ordinarily compassionate into someone who was indifferent to suffering — of turning someone, in that particular moment, into a different person. Epidemics are, at their root, about this very process of transformation. When we are trying to make an idea or attitude or product tip, we're trying to change our audience in some small yet critical respect: we're trying to infect them, sweep them up in our epidemic, convert them from hostility to acceptance. That can be done through the influence of special kinds of people, people of extraordinary personal connection. That's the Law of the Few. It can be done by changing the content of communication, by making a message so memorable that it sticks in someone's mind and compels them to action. That is the Stickiness Factor. I think that both of those laws make intuitive sense. But we need to remember that small changes in context can be just as important in tipping epidemics, even though that fact appears to violate some of our most deeply held assumptions about human nature.