I didn't bother reading Drum's article the first time I saw it circulating. I've known about Nevin's papers for a couple of years, already decided they were interesting but weak evidence, and Drum didn't seem to be bringing much new to the table. But I've now looked at his article, and it does reference stuff I wasn't aware of, like the city-level correlations between vehicular lead emissions and assaults.

We now have studies based on MRI scans, ecological correlations at multiple levels of aggregation, and longitudinal studies of individuals. But they're still all observational, and could still be affected by individual-level confounding factors. Qualitatively the causal mechanism is obviously real,* but it's only backed up by actual experiments in vivo for blood levels of 10-50 μg/dL (as far as I know). Drum's aiming an order of magnitude lower, where I'd still expect some effect, but I don't trust observational studies or simple extrapolation to estimate it precisely.

The very very obvious thing to do now is run a nice, big RCT. There are RCTs for lesser interventions (e.g.) but I see none for window replacement or soil cleanup.

So: you select 100 city neighbourhoods. Send an army of researchers to each of them to bang on every door and recruit as many families with infants & toddlers as they can. Send a second wave of researchers to measure the soil in the recruited kids' houses, take blood samples, administer the relevant IQ/development tests (test the parents too if you're feeling hardcore), and write everyone's demographic details (sex, age, race, etc.) down on their clipboards. Now you can choose 50 neighbourhoods at random as the treatment group; don't forget to check it's statistically comparable to the control group!

Having done that, get another load of people to bang on every door in the 50 neighbourhoods in the treatment arm, and offer them money to replace their windows and their soil. (I don't know whether you can get away with not replacing windows & soil for the residents who don't have kids in the study. If you could skip them you'd save a lot of money.) Then do the cleanup jobs.

That's not even the hard part. You now have to keep track of all the families you've recruited for the next 3 decades. A lot of them will move elsewhere. (Boosting the value of their houses by eliminating the lead might even encourage them to sell up.) That's fine — you just pay some academics to keep track of them. A year down the line, visit all of the families and run the tests again to measure the short-term changes. Some families will refuse the follow-up visit. That's OK too — you just keep their details on file, 'cause another 5 years later, when the kids are in school, you're gonna do a third wave of testing. Then a few more years later, when the kids are hitting puberty, you're gonna do a fourth wave of testing, and you're gonna get their criminal records from the cops. And again, a decade later, once they're adults, you do the testing and criminal record check again. And you might do another one down the line just to check there aren't any late-breaking effects to bite you. At each step, you compare the control & treatment subjects and publish the results.

How much might this all cost? Say there are 100 families' houses to clean in each of the 50 treatment neighbourhoods. That's 5000 houses. Drum says that Nevin says that replacing 16 million houses' windows would cost about $200 billion in total, or $12500 per house. Soil cleanup costs about as much again, making the cleanup total $25000 per house, or $125 million for the 5000 treatment houses. Add the cost of the blood testing, paying a few academics to run the study for decades, and other odds & sods, and the total cost might be something like $150 million. Oh yeah, and your study still wouldn't give you an unbiased estimate of the benefits of lead abatement; it'd underestimate them because of the time the children spent being exposed to lead in their houses before you showed up to run the study. But it'd give you a robust answer to a $400 billion question, and you could get part of that answer by the end of the decade.

* Blood lead poisoning has been observed since antiquity; sufficiently high exposure causes death within weeks or months; lead-treated lab animals show behavioural deficits; non-fatal but high blood lead levels cause obvious symptoms in children that can be partially reversed with chelation.

"90 percent of the variation" is misleading when comparing the levels of one time-series against another. It's very easy to find two time-series that regress almost perfectly on one another because both steadily increase. Looking at first-differences is more informative about possible causal relations. The image Cyan posted is effectively two data points from a first difference perspective: both went up and then both went down.

Another graph from Nevin's website is slightly more persuasive:

There you can see 4-6 corresponding changes in trend. Still not that impressive, but maybe enough to start looking more closely.

The lead/crime theory seems to have entered blogospheric conversation-space thanks to an article by Mother Jones writer/blogger Kevin Drum. Here's a link to most of what he's written on the subject in the past month. Here's an excerpt from the short version/intro to the magazine article:

The chart [above] illustrates the basic data that inspired the lead hypothesis: it shows lead emissions starting in 1935 overlaid with the violent crime rate 23 years later. The two curves match almost perfectly.

Now, I know my readers, and first thing a lot of you are going to do is yell at me: "Correlation is not causation!" And that's true. If this curve were the only bit of evidence we had, the connection between lead and violent crime would be pretty thin. But it's not. You should read the story to understand just how many different studies confirm this relationship...

We now have a huge amount of evidence linking lead to violent crime. We have evidence not just at the national level, but also at the state level, the city level, and the international level. We have longitudinal studies that track children from birth to adulthood to find out if higher blood lead levels lead to more arrests for violent crimes. And perhaps most important, this is a theory that just makes sense. Everything we now know about the effects of lead on the brain tells us that even moderately high levels of lead exposure are associated with aggressivity, impulsivity, ADHD, and lower IQ. And right there, you've practically defined the profile of a violent young offender.

Does having a good grip on the causal mechanism help increase our confidence in the result?

It's pretty clear that lead causes cognitive damage. Cognitive damage in children (especially high functioning with emotion control issues) seems like a plausible cause of crime when the children grow up.

That doesn't tell us the magnitude of the change, but does tell us what direction to expect the effect to be.

Picking the time lag to maximise the best fit between the two data sets is the kind of thing that they teach you not to do in machine learning classes; it leads to overfitting.