Can anyone explain to me why the group that had high exposure but an intervention that reduced their exposure to about 5 on the graph, has much lower violent crime levels than the group that had an exposure of around 5 and no intervention?
"Two primary channels emerge through which intervention affects antisocial behavior and cognitive outcomes. First, intervention may dramatically reduce the amount of continued childhood exposure to the dangerous neurotoxin by directly reducing exposure risks within the home environment. Second, long-term benefits may occur through improvements in early-life health unrelated to any changes in lead exposure. [11] ...
[11] The elevated BLL intervention package includes treatments previously demonstrated to impact later-life outcomes such as: visits from health workers; increased medical care; nutritional assessments and dietary modifications; and referral to the Special Supplemental, Nutrition Program for Women, Infants, and Children (WIC). Prior research documents long-term benefits from programs similar to each of these elements ..."
Why didn't they control for the other intervention treatments? One where they reduce BLL + other treatments, and one where they didn't reduce BLL but still gave the other treatments?
The data is from a program that aimed to mitigate problems from high lead. The program's primary aim wasn't to allow papers to be published about their research goal many years later. You make the best use that you can of the data that you've got. And don't insist on the perfect data that you don't have.
But even if this had been designed as a pure research program, the design that you suggest would never be approved on ethical grounds. We accumulated enough data many years ago on the negative impact of lead that we cannot in good conscience withhold treatment for the sake of collecting better data. Review the history of the Tuskegee Study for an example of why such ethical guidelines were put in place.
This is a good question, but keep in mind it's not a randomized control trial, they are using some clever techniques to look backwards on data from an existing program.
They could expand the study and compare to other children who had health workers in the home for unrelated reasons. But those unrelated reasons could themselves affect 'antisocial behavior and cognitive outcomes'
Plus, obvious results don't make the news https://www.xkcd.com/882/