The US incarcerates too many people. But comparisons with Europe are flawed.

One of the common arguments against mass incarceration is that the US incarcerates people at four to 12 times the rate of European nations, so it clearly has some room to bring down its jail and prison population. But while criminal justice experts widely agree that the US incarceration rate is too high, not everyone agrees comparing America and Europe is a good idea.

The contention is that while the US has more people in prison and jail than all other nations, including developed countries, it also has significantly more homicides, so higher levels of incarceration are justified. As data from the Organisation for Economic Co-operation and Development shows, the US homicide rate throughout the 2000s was more than three times the rate of Canada, four times that of the UK, and more than 10 times that of Germany.

As Keith Humphreys, a drug policy expert at Stanford University, suggested, this shows that the US will likely always need to keep a higher incarceration rate than other nations, due to the number of people who commit serious criminal offenses.

Marc Mauer, executive director of the Sentencing Project, acknowledged in an email that this explains some of the difference between US and European incarceration rates, but not all. Previous studies have found, for example, that the US generally punishes drug and property crimes, like theft, with harsher sentences than other nations. “Even taking into account higher rates of violent crime, it’s still clear that the US is far more punitive than other nations,” Mauer wrote.

John Roman, senior fellow at the Urban Institute’s Justice Policy Center, said it’s still fair to compare US and European incarceration rates. He argued that the US’s higher homicide rate is driven in part by laws and cultural views that make guns more widely available in the nation, referencing studies that show places with more guns tend to have more homicides. “That’s a choice we’ve made,” he said. “Making that comparison [with Europe] highlights the choice we’ve made, and whether that’s something we really want.”

So how should policymakers and the public evaluate incarceration levels to avoid potentially skewed comparisons between the US and Europe?

I asked Humphreys if looking at historical rates of incarceration in the US worked. He said no, because the historical rate is taken largely from a period before the equal rights movement for women in the 1970s. “That low rate was in part a function of not taking rape, spousal abuse, and other male violence against women seriously — and we should not seek to go back there,” Humphreys explained in an email.

Humphreys’s favored approach to measuring comparable incarceration rates, he said, is a bit abstract: the system should look at what makes the public safer, rehabilitates prisoners, and punishes people appropriately.

“I don’t know exactly what the right number is,” Humphreys acknowledged, “but what I would look for as indicators that we are there is when prisons are not so overcrowded that they can engage in rehabilitation, when incarceration going up makes us safer (versus putting less dangerous people in with very dangerous people and thus making them more dangerous), and when people are getting punished appropriately.”