Last year, lawmakers in New Jersey and Pennsylvania proposed legalizing recreational marijuana in their states. A debate ensued. Some argued that legalizing pot would make crime go up; others claimed that it would make crime go down. There is evidence to favor the optimists: a recent paper in the Journal of Economic Behavior & Organization reports that, after Washington State legalized recreational marijuana, in 2012, rapes there decreased by as much as thirty per cent, and thefts by about twenty per cent.

And yet there are plenty of pessimists about legalization, too; many of them work in law enforcement. Curious about their views, I reached out to more than seventy-five county sheriffs in California, Colorado, Maine, Massachusetts, Michigan, Nevada, Oregon, Vermont, and Washington—states where recreational marijuana is legal. (It’s also legal in Alaska.) Of the twenty-five sheriffs who got back to me, half said they hadn’t noticed a trend, and the rest were certain that legalizing marijuana had made crime go up. “We can just tell you from our experience that any time you’re around marijuana, or the marijuana industry, the likelihood that you’ll be the victim of some type of crime is higher,” Ray Kelly, a sheriff’s sergeant in Alameda County, California, which is home to Oakland, said. Paul Bennett, a captain in the Riverside County Sheriff’s Department, in California, told me, “I can certainly say that cops in the field, on the streets, and specifically narcotics officers, have experienced an increase in violent crime, all related to marijuana trafficking, sales, and cultivation, both legal and illegal.” I asked the sheriffs about the paper in the Journal of Economic Behavior & Organization. “Whoever gave you those statistics is so full of crap that they can’t even see how ludicrous these statements are—you can quote me on that,” Kendle Allen, the sheriff of Stevens County, Washington, said. Frank Rogers, the sheriff of Okanogan County, Washington, had a different hypothesis: “Maybe when they wrote it they were indulging in a little of the green stuff themselves.”

Whether smoking marijuana causes crime is an important question. It informs opinion on whether smoking marijuana should be a crime. According to the F.B.I., there were more than six hundred thousand arrests for marijuana possession in 2018—about six per cent of all arrests nationally. Even if an arrest doesn’t lead to imprisonment, it creates a criminal record, disrupts work and family life, and piles up legal fees and other costs; the enforcement of anti-marijuana laws is, moreover, overwhelmingly focussed on poor communities of color.

Given the high stakes of the question, it’s tempting to take sides: either legalizing pot leads to more crime or it doesn’t. And yet the truth may be unknowable. “We do not have a good mechanism in place for tracking why a person commits crime,” Timothy Tannenbaum, a sheriff’s lieutenant in Washington County, Oregon, told me. “I’m not sure most of the data you seek is available.” In an e-mail, the spokesman for Sheriff Joseph McDonald, of Plymouth, Massachusetts, cautioned that “it’s often hard to identify marijuana as either the cause or the deterrent for criminal conduct.” I brought all these responses to David Weisburd, a criminologist at George Mason University. “The sheriffs raise an important question,” Weisburd said. In his view, marijuana’s effects on crime are likely to remain hazy; in fact, the effect of pretty much anything on crime is rarely crystal clear.

Certainly, we know a few things about what causes and prevents crime. The “Handbook of Crime Correlates,” from 2009, a reference book compiled by three criminologists, lists more than a hundred demographic, economic, relational, institutional, cognitive, and biological risk factors; in aggregate, they suggest that young men in hard times find trouble. A 2015 report from the Brennan Center for Justice identifies a dozen plausible explanations for the major decline in crime that unfolded across America from 1990 to 2010—among them, more police officers, a decline in alcohol consumption, a stronger economy, and the adoption of CompStat, a statistics-based approach to managing police departments, pioneered by the N.Y.P.D. But each of these factors can explain only a few per cent of the broader change. After analyzing a hundred and sixty-nine criminology studies published from 1968 to 2005, Weisburd found that, on average, each study—despite combining many variables—could explain only a third of a given change in crime. A 2018 report in the Annual Review of Criminology concluded that the findings in one out of ten crime studies couldn’t be replicated, and that another fifteen per cent were only partially replicable.

“The world is complicated,” Weisburd said. Many people are sure that they know how to reduce crime. They urge the adoption or repeal of laws based on that conviction. But crime and crime statistics are more mysterious than they seem.

The first problem with understanding crime is that measuring it is harder than it sounds. The Department of Justice approaches the problem in two ways. The F.B.I.’s Uniform Crime Reporting Program, or U.C.R., solicits data from about twenty thousand law-enforcement agencies around the country. Simultaneously, the Bureau of Justice Statistics’ National Crime Victimization Survey, or N.C.V.S., interviews about a hundred and fifty thousand nationally representative citizens, asking them whether they have been victims of a crime.

Both datasets have problems. An obvious one is that there’s no consensus about what counts as criminal activity. In some jurisdictions, only offenses worthy of incarceration are considered crimes. In others, fined infractions also count. (Is speeding a crime? What about manspreading, for which one can be fined seventy-five dollars in Los Angeles?) Because the U.C.R. draws its data from investigators, and the N.C.V.S. relies on victims, they can present starkly different pictures of crime. According to the U.C.R., the incidence of rape nearly doubled from 1973 to 1990. The N.C.V.S., by contrast, shows that it declined by around forty per cent during the same period. Researchers at Vanderbilt University looked into the discrepancy; they found that the upward trend in the U.C.R. data correlated with upticks in the number of female police officers, and with the advent of rape crisis centers and reformed investigative styles. It could be, in short, that a modernized approach to the policing of rape drastically increased the frequency with which it was reported while reducing its incidence. But coherent stories like these only sometimes emerge from the conflicting data.

In 2016, a panel convened by the National Academies of Sciences, Engineering, and Medicine on modernizing the nation’s crime statistics concluded that we need to incorporate additional data that complements the U.C.R. and N.C.V.S.—data that is more fine-grained and carefully categorized. But the panel’s chair, Janet Lauritsen, a criminologist at the University of Missouri-St. Louis, acknowledged that collating and reporting detailed statistics require resources and training that aren’t widely available in police departments. In the meantime, insights derived from one dataset often prove incompatible with the other. In 2007, for example, a study conducted by Jessica Reyes, a professor at Amherst College, concluded that the removal of lead from gasoline in the late nineteen-seventies and early nineteen-eighties could explain fifty-six per cent of the reduction in violent crime between 1992 and 2002. (Reyes drew upon a large body of literature that theorized that lead, by damaging children’s brains, had made them more likely to become criminals.) That study used U.C.R. data. When Lauritsen and her colleagues re-ran the analysis with N.C.V.S. data, they found that leaded gasoline had approximately zero effect on violent-crime trends.