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Freakonomics? Or lead? Something for you to think about (03 Apr 18)

Two fascinating theories about cause and effect that relate to your work

If you stopped at any airport bookstand a few years ago you would have found it hard to miss the best-seller Freakonomics.

Among other pretty startling (and very convincing) theories, it suggests that the startling drop in U. S. crime rates has far less to do with better policing than the rise in abortion rates and a glut in crack cocaine.

To understand why, read the book, which is great fun, but the argument is almost all statistically based. Another explanation for crime rates, also deriving from statistics appeared recently in the Washington, DC magazine Washingtonian – and it talked about Great Britain.

A researcher claims the level of violent crimes and murders in the US is almost entirely (88%, actually) predicted by one factor -- exposure to lead in childhood. Furthermore, the researcher backs up his argument with data from Great Britain. This is relevant because we were slower then the U. S. to address this problem.

But can this be true? The case seems compelling. The researcher plotted many years’ worth of data on violent crimes and murders, comparing those crime rates to the lead exposure of children who were growing up 22 years earlier. He did this as the children exposed to lead had to grow up before they were old enough to commit violent crimes.

The patterns of the two trends were almost identical. For many years, children in the US were exposed to more and more lead, and the violent crime rate 22 years later went up almost exactly the same amount. And when the US finally wised up and started to reduce lead exposure, the rate went down 22 years later, too. So if we allow a “delay” of 22 years for the children to grow up, their childhood exposure to lead trends almost perfectly with the violent crimes they commit as young adults.

(This is exactly the same principle used in Freakonomics, incidentally, to prove a different thesis).

The data from Great Britain come in because the researcher measured lead exposure in two ways – in lead-based paint and in lead-based gasoline. In the US, both types of lead increased for a while, and then started to decrease. But we banned lead-based gasoline only in 2000, when its violent-crime rate was twice that of the US. The researcher predicts violent crimes here will start to decline as a new generation of lead-free babies grows up.

But is any of this true? Did the researcher show that exposure to lead “causes” violent crimes? Were other factors maybe involved, too? Was it anything to with drugs or abortion, for that matter?

All of us like to know why things happen. And especially whether what we do makes a difference. What does it take to prove that one thing “causes” another? More fundamentally, should we aim to prove “cause” in our work? What about showing a “plausible association” instead? If so, how could we make that case?

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