Increased blood lead level in children has been correlated with decreases in intelligence, nonverbal reasoning, short-term memory, attention, reading and arithmetic ability, fine motor skills, emotional regulation, and social engagement. … The effect of lead on children’s cognitive abilities takes place at very low levels.

High blood lead levels in adults are also associated with decreases in cognitive performance and with psychiatric symptoms such as depression and anxiety.

https://en.wikipedia.org/wiki/Lead_poisoning#By_organ_system

https://en.wikipedia.org/wiki/Lead–crime_hypothesis