Changes in childhood lead exposure are responsible for a 56% drop in violent crime in the 1990s, via effects on impulsivity, aggression, and IQ.

Evidence

NBER Working Paper Series (Reyes, lead exposure and crime) (NBER)

corroborated: 2 independent passages support this claim
XREF: connects to: Iodizing salt in 1924 closed a 15-point IQ gap cheaply; Lead poisoning costs the US billions of dollars and millions of IQ points yearly — Harriet A. Washington, A Terrible Thing to Waste, 37; Lead was a widely distributed developmental neurotoxicant with unknown but likely substantial consequences for population-level IQ. — Half of US population exposed to adverse lead levels in early childhood (+2 more)

Lead was a widely distributed developmental neurotoxicant with unknown but likely substantial consequences for population-level IQ. (similarity 0.89)
Childhood lead exposure impairs cognition and raises aggression and dropout (similarity 0.85)
Previous studies reported economic losses of $165-319 billion in lost wages from lead exposure (similarity 0.84)
Children absorb lead more readily and face higher exposure risk than adults (similarity 0.79)
Leaded gasoline exposure cost Americans born before 1996 an estimated 824 million IQ points, averaging 2.6 points per person, much smaller than iodization’s claimed 15-point gap-closing effect. (similarity 0.78)