Came across a recent meta-study purporting that effect of lead on crime is negligibly small, effectively debunking the lead-crime hypothesis. From the abstract:
Does lead pollution increase crime? We perform the first meta-analysis of the effect of lead on crime by pooling 529 estimates from 24 studies. We find evidence of publication bias across a range of tests. This publication bias means that the effect of lead is overstated in the literature. We perform over 1 million meta-regression specifications, controlling for this bias, and conditioning on observable between-study heterogeneity. When we restrict our analysis to only high-quality studies that address endogeneity the estimated mean effect size is close to zero.When we use the full sample,the mean effect size is a partial correlation coefficient of 0.11, over ten times larger than the high-quality sample.
Overall, the results suggest that declines in lead pollution are not the cause of the majority of the fall in crime observed in many western countries. Our results however leave open the possibility that it may have a socially significant effect. The upper end of our range of elasticities would imply the lower lead pollution today saves around 6,000 lives a year in the US. The lower end, however, would mean lead has no effect and we must look to other causes entirely.
If I had to put money on it, I would wager that the lead-crime hypothesis is wrong or vastly exaggerated. But doesn’t even a small effect prove the hypothesis is correct? That would be goalpost moving, by creating an arbitrarily low predictive threshold for success. The lead-crime hypothesis  posits that the reduction of crime in the 90s is due to the reduction of lead, but if the effect is really tiny, which the above paper suggests, then it likely means that other factors, such as increased incarceration and or accessibility to abortion, may also be to blame for falling crime, or something else entirely.
It’s understandable why the lead-crime hypothesis was/is so popular, because it’s an alternative to the hypothesis that increased incarceration and abortion reduced crime, which, politically, are less popular. It’s more politically correct to blame lead poisoning than insufficient policing.
Lead has become a convenient explanation for all social problems and disaprties:
Crime? Blame lead
Low IQs, racial achievement gap? Blame lead
Academic underachievement? Blame lead
Lead has assumed a sort of magical property in which every societal ill or disparity, particularly for non-whites, can be attributed to it.
Part of my skepticism of the lead-crime hypothesis is I think pundits and academics greatly overestimate the deleterious effects of lead exposure. Lead has assumed properties like kryptonite, in which merely touching a lead pencil can doom a child to poverty, prison, and an early death. I think people erroneously think childhood lead exposure is like fetal alcohol syndrome, but the evidence suggests the effect of lead exposure in terms of reduction of IQ and other problems, to be much more mild. Whereas fetal alcohol syndrome can manifest in an IQ a full standard deviation below average, it takes considerable lead exposure to lower IQ even by just 5-8 points.
Studies show some effect, but it is small:
Every 5 microgram/dl increase in blood lead levels early in life was associated with a 1.61-point lower IQ by the time these children reached age 38, as well as reductions in perceptual reasoning and working memory, researchers report in JAMA.
However, such studies may also be beset by methodological errors that make it hard to establish causality between childhood lead exposure and low IQ:
A more correct statement is that the significant increase in prediction is due not only to lead level, but also to all other variables, known or unknown, not controlled in the study. As the history of lead–IQ research has revealed, the more that pertinent confounding and contaminating variables are identified and controlled, the smaller the relationship between lead level and IQ loss; it is a common finding for significant relationships between BLL and IQ to become much smaller (e.g., Baghurst et al., 1992), or to disappear altogether (e.g., Ernhart et al., 1989), when controlling for a diversity of relevant variables. Yet, so many variables that are usually uncontrolled in the lead research bear obvious potential relationship to the ingestion of lead, such as parental supervision of infants and toddlers. Furthermore, so many environmental variables associated with IQ are unknown — witness the increase in IQ across generations. It is possible that some or all of the IQ loss due to BLL is due to these other uncontrolled variables.
 Proponents of the lead–crime hypothesis argue that the removal of lead additives from motor fuel, and the consequent decline in children’s lead exposure, explains the fall in crime rates in the United States beginning in the 1990s. This hypothesis also offers an explanation of the earlier rise in crime in the preceding decades as the result of increased lead exposure throughout the mid-20th century.