Wait, you are telling me that in 1960s we had 1-2% of the population in prison or a mental asylum? Why do no other countries have near that rate?

Mental problems do not need to be inherent from birth; whereas minority groups are highly correlated with crime and low income as a result of American class divisions, at a glance, I can see several ways to argue that minorities are at least more susceptible to mental illness. This isn't ethnic so much as it is environmental living conditions in poverty. For one, you have malnutrition, which is an entire precursor to mental illness in of itself.

Of course you can't reasonably argue that minorities have an inherent predisposition to mental illness (it'd be frankly racist, and without much evidence I believe), but in practice, they probably do have a predisposition to it over the course of their lifetimes from a statistical standpoint.

(Also worth noting: there's a strong correlation between cannabis (marijuana) use and schizophrenia. The drug warriors tend to get alarmed that this means cannabis use causes schizophrenia. But schizophrenics also tend to smoke tobacco, drink, and use other drugs. What if they're really self-medicating, and the cannabis correlation is there because smoking a joint makes the voices go away for a bit? I suspect the answer to this question is again down to cultural norms: privileged white folks determine the norms and set the laws, and this works against underprivileged/minority folks. Oh, and privileged white folks get to pay lawyers to either get them off drugs charges, or get them a lighter sentence. Hence their under-representation in prisons. Full disclosure: I am a privileged white person.)

Also, lead lowers IQ. Various estimates have been in the several point range, but at least one researcher has estimated an increase of 6 IQ points as a result of the lead ban: http://www.nytimes.com/2012/12/13/opinion/kristof-its-a-smar....

I imagine it's the former, but I'm wondering if you have any other IQ-related sources.

Everybody involved has been trying to figure out why crime rates dropped so hard when they were expected to grow. There's a ton of proposed reasons, and not much in the way of solid proof behind any of them. Lead in the water is one of them, also legal abortion, drug market changes, gun law changes, and many others. Maybe it was really all one of them, maybe all of them played a part, maybe just a few. We'll probably never know for sure - it's not like you can perform experiments to really isolate one of those variables properly.

Just because Kevin Drum likes it doesn't make it "widely accepted." You oversell his theory. It's proven about as well as abortion reducing crime, which has good evidence behind it but is hardly a slam-dunk case.

http://www.sciencedirect.com/science/article/pii/S0013935107...

Maybe it's a complete coincidence.

Or maybe (keeping in mind that somoene above suggested the correlation has been shown in different countries at different times) -- societies that work to reduce lead also tend to be societies that work on other social issues, and those other social issues reduce crime. So the lead is correlated to crime historically, even though neither causes the other directly.

Now, i think the study showing lead/crime correlation is _awfully_ interesting, and I don't dismiss it.

But "correlation does not prove causation" does not just mean "B could have caused A instead of A causing B." And this is important to understand in order to evaluate statistical findings properly. There are all sorts of things that can cause correlation rather than either A or B causing the other -- including things nobody's actually even thought of yet, but may still be what happened. And complete coincidence is always possible as well.

The word "correlated" means that it is NOT just a complete coincidence. One might not cause the other, they might be correlated because of a separate factor that interacts with both, but if they are "correlated" then complete coincidence can be ruled out.

> [...] So the lead is correlated to crime historically, even though neither causes the other directly.

Yes, this is what I mean by a third factor affecting both. But lead is a physical effect -- there are only certain sorts of things that can cause it. Things like eating lead, breathing lead, or having a diet that causes one to incorporate less lead into neurons. Thinking happy thoughts cannot change your lead levels (directly). So there are only certain categories of separate factors, C, that could be the cause of both A (lead) and B (crime). Where your name comes in the alphabet might conceivably cause crime, but it simply cannot cause lead. C, if it exists, must be a physical effect.

That's why I mentioned controlling for location where people live. That is a likely candidate for C: it is plausible to correlate with crime and it is possible to correlate with lead. If you control for it and still find an effect, then either A causes B, B causes A (not reasonable), or there is a different C'. Perhaps a genetic mutation that increases lead absorption and also alters brain chemistry to increase propensity to crime? That one is already a bit of a stretch.

I guess what I am saying is that it is true that correlation does not imply causation, but that there are reasonableness constraints on the kinds of interactions one can hypothesize, and after a certain point Occam's razor implores us to consider causation.

How do you figure? The word 'correlated' means that a relationship has been shown to be statistically _unlikely_ to have happened purely by chance. It can never be shown to be impossible however. And there can (and often are) subtle mistakes in the calculations that mean even the correlation isn't what one first thought.

And obviously statistics alone can never prove (or rule out)_ a causation, I think we agree there.

I haven't examined the research under discussion in detail enough to know if they controlled for all the things you suggest would be good to control for, etc., I have no idea.

But yeah, in the end we use our judgement as to what statistical correlations actually mean (such as a causation, and what sort through what mechanism). We can also do experiments or analysis to try and rule out (or confirm) other plausible alternate explanations. That's science, yep.

I think we mostly agree. I still object to what I think was an overly facile implication on your part that a few minutes of thinking of alternate explanations and deciding they are implausible -- on your own without seeing what other people's responses to the study in the literature have been, without an in depth analysis and consideration of the original research -- is sufficient to determine that there's "quite strong evidence" for correlation. That's not science.