Or maybe there might be an other confounder than skin color ? Maybe SES status of drug addict influences the way they finance their habit You might want to compare the impact of crack in black neighborhoods on violent crime and the impact of opioids in white neighborhoods on violent crime. Assuming the justice/police system is actually concerned about violent crimes, Occam's razor shaves off the racist assumptions pretty nicely. Also cf. Locking up our own https://www.nytimes.com/2017/04/11/books/review-locking-up-o...
+1 for Locking up Our Own - really interesting study of how the crack war led to massive incarceration of black youth, using DC as the primary case study
