Why, according to Gladwell, did violent crime decline in the United States during the 1990s? Why does he see New York City’s situation as “more complicated”?

According to Gladwell, the decline of the illegal drug trade caused the decline in violent crime.

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 is responsible for the majority of all violence.

New York City was more complicated because there were multiple factors involved. First, the crime rate had already begun to decrease before the 1990s. Second, the decline in drug trade also affected violent crime statistics. Third, immigration shifted the age of the population.... the average citizen was younger.... not older. Lastly, Gladwell goes into detail about the Broken Windows theory, and the subsequent clean up that Mayor Guiliani put into place in the 1990s.

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.