Rick Nevin is an economist. He has no training in medicine or public health. He’s not a statistician either. What this claim needs is the equivalent of the Surgeon General’s Report on Smoking (and the earlier British Royal College of Physicians Report). Proving this claim requires looking at many different studies. If lead is as bad as claimed, it should have affected many things, not just the crime rate. I suspect that the effects of lead are as bad as claimed, which is why this claim needs to be publicized as widely as possible, and the best way to do that is to have a big official report on the subject pulling together everything known about what the effects of lead are:
What’s so implausible about it? I’m not saying that it’s settled, but we know there’s a correlation, and (unlike the other conspiracy theories you bring up) we actually have a mechanism of action (we know a lot about the effects lead can have upon the brain, and it includes behavioral and developmental problems).