Saturday, June 8, 2019

Lead and Crime, Causality

I've been interested in the lead-crime hypothesis for a while, which basically says that taking lead out of gasoline (and paint) caused the decrease in crime that started in the 1990s. Mother Jones particularly has written about this extensively over the years.

Today this series of tweets caught my eye. It's a discussion of a recent research paper based on a randomized, controlled study of lead effects.

In this paper they explicitly measured the blood lead levels of children, and then took a group of those with the highest lead exposure and intervened to lower the blood lead level of some of those with the highest lead levels.

If lead in the blood stream causally causes crime then you would expect that crime rates should be correlated with measured lead except for the intervention group who should have lower crime than their high-lead comparators.

That is just to say, that if lead causes crime and you remove the lead, then crime rates in those individuals should be lower. And that is exactly what they find.

Here is the key chart:

(click to enlarge to readable size)

You can see both that measured lead levels (BLL) does show a smooth relationship with a variety of anti-social behaviours, and also that the intervention to lower lead levels massively lowers those behaviours relative to the control group.

This is the holy grail for causality. It conclusively proves that the lead crime effect is causal and not just a proxy for other factors. It also shows its large size. Interventions to lower lead halve the rates of anti social behaviour.
So that's something. The study is from 2018, however, so I guess it's not getting the attention it deserves because so many other things are burning all around us.

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