Again, it’s not been shown to be a significant factor in the drop in crime in the 20th century. There’s more explanation needed than just lead.

https://www.sciencedirect.com/science/article/pii/S0166046222000667?via%3Dihub

The lead-crime hypothesis: A meta-analysis

Does lead pollution increase crime? We perform the first meta-analysis of the effect of lead on crime, pooling 542 estimates from 24 studies. The effect of lead is overstated in the literature due to publication bias. Our main estimates of the mean effect sizes are a partial correlation of 0.16, and an elasticity of 0.09. Our estimates suggest the abatement of lead pollution may be responsible for 7–28% of the fall in homicide in the US. Given the historically higher urban lead levels, reduced lead pollution accounted for 6–20% of the convergence in US urban and rural crime rates. Lead increases crime, but does not explain the majority of the fall in crime observed in some countries in the 20th century. Additional explanations are needed.

https://en.wikipedia.org/wiki/Lead–crime_hypothesis#%3A~%3Atext%3DA_2007_report_published_by%2C%25_decline_in_violent_crime".?wprov=sfla1

Systematic reviews / meta-analysis

The first meta-analysis of the lead-crime hypothesis was published in 2022. “The Lead-Crime Hypothesis: A Meta-Analysis”, authored by Anthony Higney, Nick Hanley, and Mirko Moro consolidates findings of 24 studies on the subject. It found that there is substantial evidence linking lead exposure to a heightened risk of criminal behavior, particularly violent crimes. This aligns with earlier research suggesting lead exposure may foster impulsive and aggressive tendencies, potential precursors to violent offenses. The study concluded that, while a correlation between declining lead pollution and declining criminality is supported by research, it is likely not a significant factor in reduced crime rates, and that the link is generally overstated in lead-crime literature.

The study’s implications point towards the potential benefits of reducing lead exposure to decrease crime rates. Such reductions could be achieved through initiatives like removing lead from products like gasoline and paint, water pipes and enhancing lead abatement measures in schools and residences.