The prevalence of handguns
The most frequently cited reason mass shootings — not necessarily in schools — are more frequent in some countries than in others is the prevalence of handguns. In his famous study, Public Mass Shooters and Firearms: A Cross-National Study of 171 Countries, University of Alabama criminology professor Adam Lankford found a link between the number of guns and mass shootings between 1966 and 2012 that killed four or more people.
The study indicated that fewer weapons would probably result in a decrease in shootings. That's exactly what happened in Australia after the country tightened gun legislation following a mass shooting in 1996. It would also explain why countries where gun ownership is rare, such as France or Britain, have largely been spared such catastrophic incidents.
Apart from arguing that Lankford's overall data set is misleading because it doesn't take into account politically motivated violence, critics also questioned whether the number of weapons is really the most significant factor. They point to one nation in particular: Switzerland.
Access as the key factor?
Switzerland has one of the world's highest ratios of firearms per person, with an estimated 45.7 guns per 100 residents, according to the Small Arms Survey. Only two countries have a higher ratio: Yemen, with 54.8 guns per 100 residents, and the United States on 88.8. Other studies have even indicated the share of households with weapons may almost be the same in Switzerland as it is in the United States. Those statistics have big margins of error, but they still point to a legitimate question: why has there never been a school shooting in Switzerland, despite the Swiss enthusiasm for weapons?
With about eight million citizens, Switzerland is of course much smaller than the United States. But it's still more populous than Finland, a European country that has fewer weapons but more school shootings.
Many of Switzerland's weapons are distributed to citizen soldiers, as they are known. Conscription is mandatory for Swiss males, and conscripts can keep their semiautomatic assault rifles at home even after returning to their nonmilitary careers. (They still have to report for a short annual training.) Meanwhile, those who wish to buy weapons themselves have to undergo a weeks-long background check.
Swiss authorities have a list of about 2000 individuals they suspect of being willing to commit shootings. All of them are frequently approached by authorities, along with psychologists, and are forced to hand over their weapons immediately or are barred from buying new ones.
Some sociologists say that Switzerland's military service comes close to an extended background check, too, and that the country's education system teaches children early on to search for compromises instead of risking open conflicts. Hence, although almost every home in Switzerland may have a weapon, access is still indirectly regulated and the use of weapons usually follows strict societal norms.
There's also another crucial difference with the United States: extensive, mandatory health insurance, which allows schools to have direct and immediate access to psychologists and intervention teams.
Growing awareness of the need for psychological support
Similar measures are still being implemented in Germany, the nation with the most school shootings in contemporary European history. After a string of attacks, the country tasked a number of academics and professionals to come up with guidelines how to spot potential attackers early on.
When US President Donald Trump took to Twitter on Thursday morning, he urged students and others to alert authorities to anyone whose behaviour strikes them as suspicious.
"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behaviour," Trump tweeted. "Neighbours and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"
But experience from abroad shows that awareness alone may not be sufficient.
Other countries, including Germany, have tried to set up government-led national networks dedicated to spot potential attackers and to stop them before they can carry out their plans.
In a first step, funding for in-school psychologists was increased exponentially. Teachers at every school are now being trained to act as "trusted personnel", as a first point of contact either for students who want to seek psychological support themselves or for others who want to raise alarm over the behaviour of an individual. Psychologists are then called in to examine each case further.
Psychological tests are also standard practice for Germans younger than 25 who want to buy firearms. Age restrictions were tightened and a national registry of weapons was created in 2013.
This hasn't stopped other attackers, such as a 2016 right-wing shooter who killed nine people in Munich's city centre, from obtaining weapons illegally online. Even the best prevention programmes, government experts agree, won't provide 100 per cent safety — but they can at least do something.
