Hey John. I absolutely agree. However, we know pretty well that elevated blood lead levels in children tend to reduce IQ and increase behavioral issues. The lead-crime issue was first noticed about 10 years ago and there have been some studies that seem to support it:
https://jamanetwork.com/journals/jamapediatrics/fullarticle/190628
https://www.brookings.edu/blog/up-front/2017/06/01/new-evidence-that-lead-exposure-increases-crime/
https://www.motherjones.com/environment/2016/02/lead-exposure-gasoline-crime-increase-children-health/