T HE TWO mass shootings within 24 hours of each other last weekend, one in El Paso, Texas, the other in Dayton, Ohio, were horrifying. Yet at the same time they were not surprising—at least in a purely statistical sense. So far this year America has averaged one shooting in which four or more people are killed or injured every single day. The death toll at the El Paso Walmart was 22. And that awful number made it only the fifth-deadliest shooting this decade. The ten people killed in Dayton put the murder spree there down at number 11 on the same list.

When police officers are trying to solve a murder they look at motive and opportunity. That framework is useful for thinking about mass murders, too. The shooter in Dayton left no explanation for his actions. His social-media accounts show he was a misogynist with an interest in leftish causes. The El Paso killer posted a manifesto filled with racist anxiety about the replacement of whites by Hispanics, as well as language that could have been drawn from a Trump rally (see article).

After the killings, people have blamed any number of causes—from mental illness and video games to the internet and the social alienation of young men. Yet cause and effect are hard to pin down, as shown by the row about Donald Trump’s culpability for what happened in El Paso. His role matters not just because, as president, he has a responsibility to unite the country, but also because America’s biggest mass shootings come in patterns. In the 1980s there was a wave of post-office shootings. Later, shootings at schools and universities became a way for a certain type of young man to achieve fame. More recently there has been an increase in acts of terrorism perpetrated by white men who believe they are locked in a struggle against non-whites and Jews. This thread connects the shooting at a Charleston church in 2015 to the one at a Pittsburgh synagogue last year and to the El Paso Walmart shooting.

That is where Mr Trump’s language comes in. His presidential campaign began with an impromptu speech in which he said Mexico was sending rapists across the border, and it continued in that vein. The White House has not changed him. At a rally in Florida in May, where he denounced migrants at the southern border, someone in the crowd shouted that the solution was to shoot them. “That’s only in the Panhandle you can get away with that kind of statement,” responded Mr Trump, to laughter and cheers. After the El Paso shootings, as after Charlottesville, the president, reading from a teleprompter, condemned white supremacists and bigots. Yet the next time he is in front of a big crowd he will be at it again.

If you accept that the words people say have some effect, then the words that a president says must matter more. There is no way to calculate the probability of such racially divisive language encouraging someone to act out violent racist fantasies, but it is not one and it is not zero. Run the experiment enough times with enough people and at some point it becomes lethal.

Yet it is also true that mass shootings were common before Mr Trump took office and will continue after he has gone. The El Paso shooter’s main fixation was immigration, but he also wrote in his manifesto about excessive corporate power and environmental damage. The Dayton shooter was not a Trump supporter at all. In such cases it is impossible to know whether the ideology makes the person violent, or whether the violent desires come first and the half-baked justification follows after.

If motive can be hard to attribute precisely, and policy correspondingly hard to design, the same is not true of opportunity. White nationalists can be found in many Western countries, as can politicians who exploit racial divisions. But in a society where someone with murderous intent can wield only a kitchen knife or a baseball bat, the harm he can do is limited. When such a person has access to a semi-automatic weapon, which can hold 100 rounds of ammunition and discharge them in under a minute, it is grievous—and hence, lamentably, more seductive.

The answer is obvious: restrict the ownership of certain types of guns, as New Zealand did after the shootings in Christchurch, and introduce proper background checks. Such measures will not prevent all gun deaths. The constitution will not be rewritten and too many weapons are in circulation. Yet given the number of fatalities, even a 5% reduction would save many innocent lives. Mass shootings in America have become like deforestation in Brazil or air pollution in China—a man-made environmental hazard that is hard to stop. Such hazards are not cleaned up overnight. That should not prevent people from making a start.■