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 upside-down U pattern... The two curves looked eerily identical, but were offset by about 20 years.
They claim a strong methodology: they studied during the phaseout of leaded fuel (which was not implemented uniformly across places, crucially) and subsequent violence levels. The same trend was proved across the world. Lead shrinks the part of the brain responsible for empathy, hence where lead poisoning was more common, people are more predisposed to violence. Big cities thus become more violent.
Do widely-spread brain toxins explain more violence? Are many violent criminals people who got their prefrontal cortexes shrunk by toxins? Do big cities make people aggressive and violent due to increased concentration of toxins?