PSA: Simpson's paradox by ethnicity approximately never explains health outcomes differences across states.
Here's an annoying pattern of internet arguments I've come across at least half a dozen times.
Alice: Southern states have worse health outcomes X than other states. Therefore they should...
Bob: Woah, woah, woah. Let me stop you right there! Did you know that Southern states have more black people, and black people have worse health outcomes than white people? So we can have a situation where the health outcomes are better for every ethnicity across the board in State A while its overall results are worse than State B. What you think of as a treatment effect is actually entirely a selection effect. I am a very sophisticated statistical reasoner.
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This argument is on the face of it possible, even plausible. But every single time I looked into it (life expectancy, obesity, maternal mortality, etc), the rank order among states is essentially preserved if you only look at data for white Americans. So it simply cannot be the case that the effect is entirely selection rather than treatment.
AFAICT, the case for the Simpson's Paradox explanation being correct is even weaker for cross-national differences, at least among Western countries. If the UK had the same demographics as the US, not only would the UK still have longer life expectancies than the US, but the difference will be even larger (since black Brits have a longer life expectancy than white Brits).
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Note: I'm restricting this PSA to health outcomes. Education and crime rates might be different. I'm less sure.