Kevin Drum argues in Mother Jones that lead was responsible for the explosion of crime in the 1970s and 80s.
Experts often suggest that crime resembles an epidemic. But what kind? Karl Smith, a professor of public economics and government at the University of North Carolina-Chapel Hill, has a good rule of thumb for categorizing epidemics: If it spreads along lines of communication, he says, the cause is information. Think Bieber Fever. If it travels along major transportation routes, the cause is microbial. Think influenza. If it spreads out like a fan, the cause is an insect. Think malaria. But if it’s everywhere, all at once—as both the rise of crime in the ’60s and ’70s and the fall of crime in the ’90s seemed to be—the cause is a molecule.
A molecule? That sounds crazy. What molecule could be responsible for a steep and sudden decline in violent crime?
Well, here’s one possibility: Pb(CH2CH3)4. [http://www.motherjones.com/environment/2013/01/lead-crime-link-gasoline source]
If lead really was the cause of the wave, broken windows Broken Windows Theory Broken.
For a taste of the time, see Bernhard Goetz Incident