“The ‘lead-crime hypothesis’ posits a direct correlation between crime and lead, or as you put it, ‘More lead, more murder.’ What’s the connection? During the post-war period, an enormous amount of lead was in the air from mainly two sources: Leaded gas, which everybody used for decades, and heavy industry like smelting… Many studies connect lead exposure to a particular kind of frontal cortex damage that leads to heightened aggression… The higher the lead exposure, the greater the brain volume mass, and reduced brain volume has been linked to higher levels of psychopathy.”