Many times in the history of the United States, rising prevalence of use of a drug has been responded to with increased arrests of people in possession of that drug. As Peter Reuter and Rob MacCoun pointed out some years ago, this makes the rise of marijuana possession arrests in the U.S. particularly puzzling: The rate of marijuana use was historically low and stable prior to the growth of arrests.

The trend began in the 1990s and has continued to the present day. It has been almost entirely an urban phenomenon, with the best example being New York City (For a wonderful series of charts on NYC’s arrest patterns, see this piece by Michael Keller).

No informed observer seems to believe that this rise in arrests resulted from heightened concern about marijuana use per se. Rather the explanations invoked range from the arrests being a side effect of stop-and-frisk policing, to them being a way for those police who are racially prejudiced to hassle people of colour, to them being a tactic to punish malefactors who have gotten away with something worse (e.g., someone who has repeatedly beaten a spouse who is too afraid to testify against the perpetrator). Some people are sure they know the explanation with absolute certitude, but if like me you admit the possibilities of your own ignorance and life’s complexity, please read on.

In studies of drug use, researchers employ the concept of “substitutes” for various drugs (i.e., a different drug with a similar effect). It works out more neatly in theory than in everyday life, but a well-founded aspect of the theory holds that removal of a drug from the market can result in its users seeking a substitute. For example, if a state tightens up regulations on doctor shopping, people addicted to opiate pain medication may start seeking a substitute, such as heroin. It would be useful to know whether this same phenomenon occurs not just for drugs, but for drug possession arrests.

The question of interest is whether legalisation or decriminalisation of marijuana (e.g., in a state or nation) would affect the total number of arrests for all crimes, or, whether any drop in marijuana possession arrests would simply be substituted for with other charges. That is, if you were a police officer bent on hassling young males of colour, would marijuana legalisation/decriminalisation affect you in any way, or could you just start charging the same people with disorderly conduct/vagrancy (followed by, if you were really in a bad mood, resisting arrest). Alternatively, if you were a policeman who took sympathy on a young male of colour caught with cocaine and marijuana by only charging on the latter, would you respond to marijuana legalisation by substituting the cocaine charge rather than letting the person go free?

The answers to these sorts of questions have huge implications for understanding the impact of marijuana legalisation/decriminalisation, both in terms of fiscal impact but also in terms of whether the population of users will have less, more or the same amount of contact with the criminal justice system. So, how would one get those answers?

My own approach would be to interview a random sample of 1000 police officers who had recently made a marijuana possession arrest. The arrest record would be examined by the officers and the interviewer (under strict confidentiality), with the interviewer asking “If you could not have arrested this individual for marijuana possession, what would you have done instead?”. Granted, some drug policy reform advocates do not care about data, but there are enough who do to make such a study of the substitutability of marijuana possession arrests of great value.

[Cross-posted at The Reality-Based Community]

Keith Humphreys

Keith Humphreys is a professor of psychiatry at Stanford University. He served as a senior policy advisor at the White House Office of National Drug Control Policy from 2009 to 2010.