Attorney General Jeff Sessions’s latest explanation for the rise in violent crime is, well, exactly what you would expect from the head of President Donald Trump’s “tough on crime” agenda.

In comments at a conference for the anti-drug program DARE, Sessions referenced a memo by the Obama administration that told federal prosecutors to avoid charges for low-level drug offenders that could trigger lengthy mandatory minimums. He argued that this memo caused violent crime to spike for the first time in decades — and suggested his decision to revoke the memo will, in turn, cause violent crime to fall:

Under the previous administration, the Department of Justice told federal prosecutors not to include in charging documents the full amount of drugs being dealt when the actual amount would trigger a mandatory minimum sentence. Prosecutors were required to leave out true facts in order to achieve sentences lighter than required by law. This was billed as an effort to curb “mass incarceration” of “low-level offenders,” but in reality it covered offenders apprehended with large quantities of dangerous drugs. What was the result? It was exactly what you would think: sentences went down and crime went up. Sentences for federal drug crimes dropped by 18 percent from 2009 to 2016. Violent crime — which had been decreasing for two decades — suddenly went up again. Two years after this policy change, the United States suffered the largest single-year increase in the overall violent crime rate since 1991.

These claims ignore years and years of empirical research on this exact question — including some research done by US Department of Justice, which Sessions now heads.

In short, studies have found time and time again that harsher punishments — which mandatory minimums force on judges by requiring that they sentence offenders to a minimum amount of time in prison — and the higher incarceration rates they lead to don’t have a big impact on crime.

A 2015 review of the research by the Brennan Center for Justice estimated that more incarceration explained 0 to 7 percent of the crime drop since the 1990s, while other researchers estimate it drove 10 to 25 percent of the crime drop since the ’90s — not a big impact either way. A 2014 analysis by the Pew Charitable Trusts also found that states that reduced their imprisonment rates also saw some of the biggest drops in crime, suggesting that there isn’t a hard link between incarceration and crime.

In fact, longer stints in prison can actually lead to more crime. As the National Institute of Justice — an agency within the Justice Department — concluded in 2016, “Research shows clearly that the chance of being caught is a vastly more effective deterrent than even draconian punishment. … Research has found evidence that prison can exacerbate, not reduce, recidivism. Prisons themselves may be schools for learning to commit crimes.”

Harsh mandatory minimums for drug offenses don’t even seem to have an impact on the flow of drugs. A 2014 study from Peter Reuter at the University of Maryland and Harold Pollack at the University of Chicago found there’s no good evidence that tougher punishments or harsher supply-elimination efforts do a better job of driving down access to drugs and substance abuse than lighter penalties. So increasing the severity of the punishment doesn’t do much, if anything, to slow the flow of drugs.

More broadly, there’s a fundamental problem with the argument Sessions is making: The federal government just doesn’t have that much impact on crime. Most criminal justice policy is done at the state and local levels, where roughly 87 percent of prison inmates reside. As John Roman, a criminal justice expert at the University of Chicago, told me, “Federal drug sentencing affects the custody of only a tiny proportion of US offenders, so inferring a large, national increase in violence to any change in federal sentencing policy focuses on a butterfly effect rather than a direct cause.”

This isn’t the first time Sessions has made a comment like this. Previously, he said that New York City is “soft on crime” and as a result “continues to see gang murder after gang murder,” because it has adopted various criminal justice reforms. In reality, New York has seen its crime rate fall in the past few years — and its homicide rate is now below the national average.

So far, the Justice Department has not provided evidence to support Sessions's claims.

JUST IN—Asked for evidence to support Sessions's claim DARE worked & sentencing reform caused violence, spokesperson says: "Decline comment" pic.twitter.com/X7sH7wk9vt — Dominic Holden (@dominicholden) July 11, 2017

So what did cause the rises in violent crime over the past two years? The short answer is criminologists don’t really know yet. The most prominent, but unproven, explanation so far is that rising distrust in police, as well as police pulling back from proactive work in fear of mounting criticisms, led to more crime.

But it’s also possible, criminologists say, that the violent crime rise could be temporary statistical fluctuations — as occurred in 2005 and 2006 — or caused by something we just don’t know about yet. For more on that, read my full explainer.