On the one hand, you have a meta-analysis estimating 7–28% of the U.S. homicide decline, depending on the model, after correcting for publication bias. That’s 4–15 percentage points of a 54% decline.[1]

On the other hand, you have the Dunedin study, where lead exposure was uncorrelated with measured socioeconomic status, which found weak links with offending overall and no significant link to violent crime.[2]

In any case, people talk about this like lead was the dominant factor in the violent crime decline, when these results don’t establish that. Even the meta-analysis leaves most of the homicide decline unexplained by lead.

[1] https://eprints.gla.ac.uk/332830/1/332830.pdf

[2] https://jamanetwork.com/journals/jamapediatrics/fullarticle/...