There's been a general principle guiding drug policy for years: drug laws are enforced in tandem with the drug's use. But recent enforcement of marijuana laws has not followed that principle. The federal government’s National Survey on Drug Use and Health reports that in the aggregate Americans used marijuana on over 3.5 billion days in 2013, a  rise of 57 percent since 2007. But over that same period, the FBI Uniform Crime Report recorded that marijuana possession arrests fell 21 percent to 609,000.

The net effect of these two trends is a 50 percent drop in marijuana enforcement intensity, from roughly one arrest for every 2,900 days of use in 2007 to about one arrest for every 5,800 days of use in 2013. The average twice a week marijuana user could thus today reasonably expect to be arrested less than once per half-century.

Recreational marijuana wasn’t legalized anywhere in the U.S. until 2012, so why did marijuana enforcement begin waning well before that? Internationally respected marijuana policy expert Dr. Beau Kilmer of the RAND Corporation attributes it to multiple factors, including “the increasing legal protections offered by medical marijuana laws and the decriminalization of possession in California."

Kilmer also points out that the soaring number of days Americans use marijuana is only partially accounted for by more people using the drug: “The number of people who self-reported using marijuana for the first time increased from about 2 million in 2007 to 2.6 million in 2011 and then fell back slightly to 2.4 million in 2013. What's really driving the change in use days is the large increase in the number of daily/near-daily marijuana users.”

The marijuana using population is thus evolving to include a higher proportion of people who use the drug on a regular basis. In the coming years, this group will face a higher risk of negative health consequences as a result of their heavy, regular drug use, but will face a lower risk of legal consequences as the risk of arrest continues to fall.

Keith Humphreys is a Professor of Psychiatry and Mental Health Policy Director at Stanford University. Follow him on Twitter @KeithNHumphreys