One of the hallmarks of a good theory is that it answers questions you didn’t even know you had, and it turns out that the answer to this mystery might lie in the association between gasoline lead and violent crime. I mentioned this briefly in “Criminal Element,” my magazine piece about the lead-crime connection, but it deserves a little more explanation. So here it is.

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 midsize 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.

More: Are Big Cities More Dangerous Than Small Ones?