I don’t know anything about the effect of lead on human cognition and behavior, I’m asking you to tell me about it

If that’s even what their comment was about

How does that effect work?

your assumption wasn’t a “lead free” response, am I doing it right?

Lead exposure gives you brain damage: https://en.wikipedia.org/wiki/Lead_poisoning. If you damage your frontal lobes, you generally become more impulsive and less measured in your response to things. Ergo, chronic lead poisoning causes populations to become more aggressive and more likely to engage in crime.