Here is a report from Scott Firestone. He does admit that much of the evidence carries some weight, but he is less persuaded when it comes to cohort studies:
It turns out there was in fact a prospective study done—but its implications for Drum’s argument are mixed. The study was a cohort study done by researchers at the University of Cincinnati. Between 1979 and 1984, 376 infants were recruited. Their parents consented to have lead levels in their blood tested over time; this was matched with records over subsequent decades of the individuals’ arrest records, and specifically arrest for violent crime. Ultimately, some of these individuals were dropped from the study; by the end, 250 were selected for the results.
The researchers found that for each increase of 5 micrograms of lead per deciliter of blood, there was a higher risk for being arrested for a violent crime, but a further look at the numbers shows a more mixed picture than they let on. In prenatal blood lead, this effect was not significant. If these infants were to have no additional risk over the median exposure level among all prenatal infants, the ratio would be 1.0. They found that for their cohort, the risk ratio was 1.34. However, the sample size was small enough that the confidence interval dipped as low as 0.88 (paradoxically indicating that additional 5 µg/dl during this period of development would actually be protective), and rose as high as 2.03. This is not very convincing data for the hypothesis.
For early childhood exposure, the risk is 1.30, but the sample size was higher, leading to a tighter confidence interval of 1.03-1.64. This range indicates it’s possible that the effect is as little as a 3% increase in violent crime arrests, but this is still statistically significant.
I don’t have any particular view on this matter, but if you wish can you read Drum’s response here.
Addendum: Andrew Gelman comments.