Factors That Don’t Correlate

If mental health made the difference, then data would show that Americans have more mental health problems than do people in other countries with fewer mass shootings. But the mental health care spending rate in the United States, the number of mental health professionals per capita and the rate of severe mental disorders are all in line with those of other wealthy countries.

A 2015 study estimated that only 4 percent of American gun deaths could be attributed to mental health issues. And Mr. Lankford, in an email, said countries with high suicide rates tended to have low rates of mass shootings — the opposite of what you would expect if mental health problems correlated with mass shootings.

Whether a population plays more or fewer video games also appears to have no impact. Americans are no more likely to play video games than people in any other developed country.

Racial diversity or other factors associated with social cohesion also show little correlation with gun deaths. Among European countries, there is little association between immigration or other diversity metrics and the rates of gun murders or mass shootings.

A Violent Country

America’s gun homicide rate was 33 per million people in 2009, far exceeding the average among developed countries. In Canada and Britain, it was 5 per million and 0.7 per million, respectively, which also corresponds with differences in gun ownership.

Americans sometimes see this as an expression of deeper problems with crime, a notion ingrained, in part, by a series of films portraying urban gang violence in the early 1990s. But the United States is not actually more prone to crime than other developed countries, according to a landmark 1999 study by Franklin E. Zimring and Gordon Hawkins of the University of California, Berkeley.

Rather, they found, in data that has since been repeatedly confirmed, that American crime is simply more lethal. A New Yorker is just as likely to be robbed as a Londoner, for instance, but the New Yorker is 54 times more likely to be killed in the process.