The Goetz case has become a symbol of a particular, dark moment in New York City history, the moment when the city's crime problem reached epidemic proportions. During the 1980s, New York City averaged well over 2,000 murders and 600,000 serious felonies a year. Underground, on the subways, conditions could only be described as chaotic. Before Bernie Goetz boarded the number two train that day, he would have waited on a dimly lit platform, surrounded on all sides by dark, damp, graffiti-covered walls. Chances are his train was late, because in 1984 there was a fire somewhere on the New York system every day and a derailment every other week. Pictures of the crime scene, taken by police, show that the car Goetz sat in was filthy, its floor littered with trash and the walls and ceiling thick with graffiti, but that wasn't unusual because in 1984 every one of the 6,000 cars in the Transit Authority fleet, with the exception of the midtown shuttle, was covered with graffiti — top to bottom, inside and out. In the winter, the cars were cold because few were adequately heated. In the summer, the cars were stiflingly hot because none were air-conditioned. Today, the number two train accelerates to over 40 miles an hour as it rumbles toward the Chambers Street express stop. But its doubtful Goetz's train went that fast. In 1984, there were 500 "red tape" areas on the system — places where track damage had made it unsafe for trains to go more than 15 miles per hour. Fare-beating was so commonplace that it was costing the Transit Authority as much as $150 million in lost revenue annually. There were about 15,000 felonies on the system a year — a number that would hit 20,000 a year by the end of the decade — and harassment of riders by panhandlers and petty criminals was so pervasive that ridership of the trains had sunk to its lowest level in the history of the subway system. William Bratton, who was later to be a key figure in New York's successful fight against violent crime, writes in his autobiography of riding the New York subways in the 1980s after living in Boston for years, and being stunned at what he saw:

After waiting in a seemingly endless line to buy a token, I tried to put a coin into a turnstile and found it had been purposely jammed. Unable to pay the fare to get into the system, we had to enter through a slam gate being held open by a scruffy-looking character with his hand out; having disabled the turnstiles, he was now demanding that riders give him their tokens. Meanwhile, one of his cohorts had his mouth on the coin slots, sucking out the jammed coins and leaving his slobber. Most people were too intimidated to take these guys on: Here, take the damned token, what do I care? Other citizens were going over, under, around, or through the stiles for free. It was like going into the transit version of Dante's Inferno.

This was New York City in the 1980s, a city in the grip of one of the worst crime epidemics in its history. But then, suddenly and without warning, the epidemic tipped. From a high in 1990, the crime rate went into precipitous decline. Murders dropped by two-thirds. Felonies were cut in half. Other cities saw their crime drop in the same period. But in no place did the level of violence fall farther or faster. On the subways, by the end of the decade, there were 75 percent fewer felonies than there had been at the decade's start. In 1996, when Goetz went to trial a second time, as the defendant in a civil suit brought by Darrell Cabey, the case was all but ignored by the press, and Goetz himself seemed almost an anachronism. At a time when New York had become the safest big city in the country, it seemed hard to remember precisely what it was that Goetz had once symbolized. It was simply inconceivable that someone could pull a gun on someone else on the subway and be called a hero for it.

This idea of crime as an epidemic, it must be said, is a little strange. We talk about "epidemics of violence" or crime waves, but it's not clear that we really believe that crime follows the same rules of epidemics as, say, Hush Puppies did, or Paul Revere's ride. Those epidemics involved relatively straightforward and simple things — a product and a message. Crime, on the other hand, isn't a single discrete tiling, but a word used to describe an almost impossibly varied and complicated set of behaviors. Criminal acts have serious consequences. They require the criminal to do something that puts himself at great personal peril. To say someone is a criminal is to say that he or she is evil or violent or dangerous or dishonest or unstable or any combination of any of those things — none of which is a psychological state that would seem to be transmitted, casually, from one person to another. Criminals do not, in other words, sound like the kind of people who could be swept up by the infectious winds of an epidemic. Yet somehow, in New York City, this is exactly what occurred. In the years between the beginning and the middle of the 1990s, New York City did not get a population transplant. Nobody went out into the streets and successfully taught every would-be delinquent the distinction between right and wrong. There were just as many psychologically damaged people, criminally inclined people, living in the city at the peak of the crime wave as in the trough. But for some reason tens of thousands of those people suddenly stopped committing crimes. In 1984, an encounter between an angry subway rider and four young black youths led to bloodshed. Today, in New York's subways, that same encounter doesn't lead to violence anymore. How did that happen?

The answer lies in the third of the principles of epidemic transmission, the Power of Context. The Law of the Few looked at the kinds of people who are critical in spreading information. The chapter on Sesame Street and Blue's Clues looked at the question of Stickiness, suggesting that in order to be capable of sparking epidemics, ideas have to be memorable and move us to action. We've looked at the people who spread ideas, and we've looked at the characteristics of successful ideas. But the subject of this chapter — the Power of Context — is no less important than the first two. Epidemics are sensitive to the conditions and circumstances of the times and places in which they occur. In Baltimore, syphilis spreads far more in the summer than in the winter. Hush Puppies took off because they were being worn by kids in the cutting-edge precincts of the East Village — an environment that helped others to look at the shoes in a new light. It could even be argued that the success of Paul Revere's ride — in some way — owed itself to the fact that it was made at night. At night, people are home in bed, which makes them an awful lot easier to reach than if they are off on errands or working in the fields. And if someone wakes us up to tell us something, we automatically assume the news is going to be urgent. One can only imagine how "Paul Revere's afternoon ride" might have compared.

This much, I think, is relatively straightforward. But the lesson of the Power of Context is that we are more than just sensitive to changes in context. We're exquisitely sensitive to them. And the kinds of contextual changes that are capable of tipping an epidemic are very different than we might ordinarily suspect.

During the 1990s violent crime declined across the United States for a number of fairly straightforward reasons. The illegal trade in crack cocaine, which had spawned a great deal of violence among gangs and drug dealers, began to decline. The economy's dramatic recovery meant that many people who might have been lured into crime got legitimate jobs instead, and the general aging of the population meant that there were fewer people in the age range — males between eighteen and twenty-four — that are responsible for the majority of all violence. The question of why crime declined in New York City, however, is a little more complicated. In the period when the New York epidemic tipped down, the city's economy hadn't improved. It was still stagnant. In fact, the city's poorest neighborhoods had just been hit hard by the welfare cuts of the early 1990s. The waning of the crack cocaine epidemic in New York was clearly a factor, but then again, it had been in steady decline well before crime dipped. As for the aging of the population, because of heavy immigration to New York in the 1980s, the city was getting younger in the 1990s, not older. In any case, all of these trends are long-term changes that one would expect to have gradual effects. In New York the decline was anything but gradual. Something else clearly played a role in reversing New York's crime epidemic.