tl;dr: "Taken together, these findings suggest that the effects of the War on Drugs are often relatively slight compared to other causes, and that they are certainly not as big as many often assert. In reality, a majority of prison growth has come from locking up violent offenders, and a large majority of those admitted to prison never serve time for a drug charge, at least not as their “primary” charge. These results pose a challenge to those who wish to aggressively scale back incarceration, since the current politics of reducing sanctions for drug offenders is less complicated than that for reducing punishment for violent or property offenders. Reforming drug statutes is easier, but doing so will likely not effect significant change in the overall incarceration rate." p.6