Researchers have offered several potential explanations for historically higher rates of violent crime in big cities: from the idea that there's more valuable stuff to steal in big cities to the rise of the crack epidemic. Even if other potential reasons strike you, you probably haven't considered a new persuasive hypothesis: exposure to gasoline lead. In an article for Mother Jones and in a follow-up blog piece, Kevin Drum examines the connection between lead exposure and crime.
"In a nutshell, the lead-crime hypothesis is simple: exposure to gasoline lead in small children produces heightened aggressive tendencies. When an entire generation of children was exposed to lead in the 40s, 50s, and 60s, thanks to the boom in auto sales after World War II, it led to a huge rise in violent crime when the children grew up in the 60s, 70s, and 80s. The more lead they were exposed to, the more crime you got."
"So where did we see the most exposure to gasoline lead? Answer: in places with the densest concentration of automobiles. And that's in the inner core of big cities. In the early 60s, big cities had double the ambient air lead levels of mid-size cities, which in turn had air lead levels 40 percent higher than small cities. (Nevin, p. 316.) So if lead exposure produces a rise in crime, you'd expect to see a bigger rise in big cities than in small ones. Over time, big cities would become increasingly more dangerous than small ones."
"Likewise, when lead was removed from gasoline, and children started to grow up normally, you'd expect to see a bigger crime decrease in big cities. Over time, crime rates would start to converge."
"And that's exactly what we see in the data."
"Only gasoline lead," concludes Drum in his article, "with its dramatic rise and fall following World War II, can explain the equally dramatic rise and fall in violent crime."