Does exposure to lead cause crime?

While driving to change the oil in my car yesterday, I listened to Philip and Alice Shabecoff discuss their book Poisoned Profits: The Toxic Assault on Our Children on the Diane Rehm show.

I haven't read it (don't plan to), but if the title and the interview are any indication, the book is polemic. The Shabecoffs have concluded, based on some vague anecdotal and correlational research, that a whole bunch of chemicals are toxic and should be banned.

Don't get me wrong -- I'm not a big fan of dumping/consuming random chemicals. But I'm also wary of beating up corporations just because they're easy political targets. And the evidence here is ... shaky. The problem is a statistical classic: correlation does not imply causation.

Case in point: a recent study on the effect of childhood lead exposure on adult criminal activity. The researchers targeted pregnant mothers in high-lead areas, took detailed readings of lead levels for the mothers and (later) their children, and then looked at the criminal histories of those children once they reached adulthood. They concluded that a little lead as a child makes a person much more likely to be arrested later in life.

The problem: which children are most likely to grow up in the homes with the worst lead problems? Those with the most impoverished, disadvantaged (and least health-conscious) parents. So when the researchers look at the correlation between lead exposure and arrests, it's impossible to tell whether lead or poverty is the real cause of crime.

So the study, like so many others is inconclusive. What do you think the standard of evidence for concluding that a chemical is harmful should be?