It wasn’t crack cocaine, poverty, or the breakdown of the nuclear family that caused the spike in violent criminality. Most likely, it was automobile fumes. Mother Jones has a bombshell story:
In city after city, violent crime peaked in the early ’90s and then began a steady and spectacular decline. By 2010, violent crime rates in New York City had plunged 75 percent from their peak in the early ’90s. Washington, DC['s] violent crime rate has dropped 58 percent since its peak. Dallas’ has fallen 70 percent. Newark: 74 percent. Los Angeles: 78 percent.
Lead emissions from tailpipes rose steadily from the early ’40s through the early ’70s, nearly quadrupling over that period. Then, as unleaded gasoline began to replace leaded gasoline, emissions plummeted. Intriguingly, violent crime rates followed the same pattern. The only thing different was the time period: the two curves looked eerily identical, but were offset by about 20 years.