Types of epidemics

A very interesting, and counter-intuitive, article on the cause of the crime epidemic of the 70s:

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.

Turns out the culprit is lead in gasoline. As we have switched to unleaded gasoline, the crime rate has subsided. I was skeptical, but the article is quite convincing.