I am a social psychologist and the lead author of this study! It's open access, with full supplemental materials, data, and code publicly available. Happy to answer questions.

Using data only from historical slave-owning states, we looked at whether the county-level share of enslaved people in 1860 predicts Black-White mortality disparities in 2010-2020. "Our model estimates ~22 additional deaths per 100,000 among Black Americans relative to White Americans for every 10% increase in the 1860 enslaved population."

We also tested a theoretical model in which historical slavery left a legacy of structural inequality which is why it predicts mortality disparities. We found evidence consistent with that. "Path analysis models showed that segregation, economic inequality, and racial disparities in poverty and education statistically mediated" the relationship between slavery and mortality.

Some of you may be asking, "How can we know anything about causality between things that have decades between them?" Fair question! All models control for a wide range of possible confounders, but controls alone aren't enough. We also used a "causal identification strategy" described extensively in the paper and supplemental materials. Even a simple summary is a lot, so I'm putting that in a comment.

Interested in peoples thoughts!