"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 lead being used in leaded gasoline (to prevent engine knocking) was exposing millions of children to high levels of lead, which is linked to increased levels of violent behavior. The fact that environmental lead levels were high and crime was high does not necessarily mean that the lead caused the crime - but the authors of the article present some really good evidence of causality. Another example of how environmental pollution can have major effects on human wellness.
