  Racial disparities in US drug arrest rates have been observed for decades,
but their causes and policy implications are still contested. Some have argued
that the disparities largely reflect differences in drug use between racial
groups, while others have hypothesized that discriminatory enforcement policies
and police practices play a significant role. In this work, we analyze racial
disparities in the enforcement of marijuana violations in the US. Using data
from the National Incident-Based Reporting System (NIBRS) and the National
Survey on Drug Use and Health (NSDUH) programs, we investigate whether
marijuana usage and purchasing behaviors can explain the racial composition of
offenders in police records. We examine potential driving mechanisms behind
these disparities and the extent to which county-level socioeconomic factors
are associated with corresponding disparities. Our results indicate that the
significant racial disparities in reported incidents and arrests cannot be
explained by differences in marijuana days-of-use alone. Variations in the
location where marijuana is purchased and in the frequency of these purchases
partially explain the observed disparities. We observe an increase in racial
disparities across most counties over the last decade, with the greatest
increases in states that legalized the use of marijuana within this timeframe.
Income, high school graduation rate, and rate of employment positively
correlate with larger racial disparities, while the rate of incarceration is
negatively correlated. We conclude with a discussion of the implications of the
observed racial disparities in the context of algorithmic fairness.
