Re: lead, they do say they ran the test in other countries. They're also not claiming that lead is the sole cause of criminality, but one explaining factors among many. As for the growth, apparently leaded gasoline used grew constantly until it was banned then fell down, so it explain the raise in criminality as well.
That's still doesn't fit the datas about other countries. Also some countries with a current higher rate of lead still have a lower criminality. Just look a China, which has a desastrous politics in terms of lead, and still one of the lower criminality of the world.
The studies are ok, but the articles out of this are completely bs. With the same kind of reasoning, you could say : "Violence lead to higher rate of lead in blood" (I'm talking about the reasoning here, before anyone else says "But the datas ...").
I would note that lead is a toxin that manifests several symptoms that can lead to an increased propensity for crime. Some of the the side effects of lead poisoning include an increase in aggression and impaired judgement.
From a statistics study ?
Also, "increased propensity for crime" isn't scientific. You want to say "increased aggressiveness" (and it's what they say). But there still is no proof. It's an hypothesis, with a correlation. If you want a proper reasoning, get some groups of white mouse (or others, rats may be good), and test their aggressiveness with differents exposure to lead.
The average blood lead level in rich countries is higher than the WHO limits. I guess we're all contamined with aggressiveness then.