Belgian soldiers guard a memorial site at the Place de la Bourse in Brussels on March 27. (Geert Vanden Wijngaert/AP)

After major terrorist attacks hit Brussels and Paris, Europe is still reeling. These atrocities prompted a heated dispute between two noted French scholars of political Islam, Gilles Kepel and Olivier Roy, over why a handful of European Muslims become terrorists.

Their disagreement isn’t just an intellectual squabble. It strikes to the heart of how the West understands violent Islamist extremism — and what the appropriate policy responses might be.

European terrorists are mostly homegrown

Most of the assailants in the recent Brussels and Paris attacks were raised in either France or Belgium. As were those who carried out the January 2015 assault on the Charlie Hebdo magazine in Paris. Why did these people turn against their own countrymen — and who or what is to blame?

Both Kepel and Roy are leading experts on political Islam whose works are widely read by policymakers. And they disagree vigorously about why French and Belgian citizens of Muslim extraction turn to terrorism.

Is Islam to blame?

In the wake of the Paris attacks last November, Roy wrote in Le Monde that we shouldn’t be talking about the “radicalization of Islam” but rather the “Islamization of radicalism.” Roy doesn’t see radicalization as the result of a country’s failure to integrate immigrant communities. Even more controversially, he downplays the role of religion.

Roy says that we should stop looking for religious or cultural explanations, given that only a tiny fraction of European Muslims have been drawn to such extremism. He also refutes the idea that racism or discrimination is what radicalized Muslims in European societies. If this were the case, he argues, why do a substantial number of white European converts join groups such as the Islamic State? If these converts were brought up outside the Muslim faith, how can we blame “Muslim culture” for their radicalization?

Instead, Roy argues that these terrorists are engaged in a generational revolt — much as when a handful of angry young Europeans in the 1970s turned to left-wing terrorist groups such as the Red Brigades and the Red Army Faction. According to Roy, today’s nihilists are instead turning to a warped version of Islam as the best way of rebelling against society. They simply use the promise of paradise to justify their actions after being manipulated by extremist organizations.

These ideas are anathema for many who research terrorism and the rise of the Islamic State. Still, Roy raises some salient points. The biographies of many of Europe’s homegrown terrorists suggest that few were particularly religious. They were more likely to be found in nightclubs than in mosques and often had a history as petty criminals. Roy agrees that religion has some relevance, but he argues that psychological factors may be more important in radicalization.

The implication? We might compare the Paris attackers who massacred concertgoers with assault rifles to teenagers who carried out mass shootings in U.S. schools.

Or should we specifically blame Salafism?

Kepel was finishing his latest book, “Terreur dans l’Hexagone” (“Terror in the Hexagon,” a reference to the hexagonal shape of France) when Paris was attacked in November. Rushed into print the following month, the book has since sold thousands of copies.

Kepel thinks that the social, economic and political marginalization of French-born Muslims helped create what he calls the “third generation of jihad” — those who emerged between 2005 and 2015. Their marginalization has pushed them toward extreme forms of Islam, including Salafism, a highly conservative version of Islam imported from the Middle East with the help of Saudi Arabian petrodollars. Kepel thinks that one cannot separate violent jihadism from the nonviolent forms of Salafism, and stresses the importance of such religious beliefs in creating the conditions for being drawn to terrorism.

These academic heavyweights are now trading blows

Early this year, Kepel openly mocked Roy’s “Islamization of radicalism” concept in TV and radio interviews. In a March article for the left-leaning newspaper Libération, he accused Roy of being “ignorant of social realities.” Roy responded a few weeks later in the pages of the bestselling magazine L’Obs, claiming that Kepel was waging a war of egos to secure research funding and comparing him to Eugène de Rastignac, an opportunistic money-grubber in Balzac novels.

A feature article in Libération allowed Roy and Kepel to defend their positions. Roy said that he does not ignore religion and that he agreed with Kepel about the extent of Salafism in France’s poor suburbs. However, he said that this cannot explain the radicalization of people, such as Salah Abdeslam, the key suspect whom France will put on trial after he is extradited from Belgium. Roy refuses the notion that Salafism helps incubate violent extremism — or that the perpetrators of the Brussels and Paris atrocities can even be classified as Salafis. Kepel, for his part, admitted that some similarities exist between different forms of extremism but reiterated his belief in the link between jihadi terrorism and Salafism. He also criticized radicalization as a concept for lumping together disparate groups and ideologies including leftists, neo-fascists and Islamic extremists.

Here’s why we should care

This debate is not simply academic. It has crucial implications for Western nations’ policy choices. Does the term “radicalization” tell us anything, or does it just confuse very different kinds of groups and actors? Should we seek to compare Islamist extremism to other forms of radical behavior?

It may well be, of course, that Kepel and Roy’s views are complementary. It is important that researchers work together across different disciplines to illuminate why people are drawn to such extreme behavior as terrorism.

When France prosecutes people like Abdesalam and Mohamed Abrini, both accused of involvement in last November’s terrorist attacks in Paris, we may glean some answers about why people raised in Europe become radicalized. But it would be still more important for policymakers to support research that might explain or even prevent such violent extremism. French Prime Minister Manuel Valls has expressed exasperation at those who “search for excuses and cultural or sociological explanations.” Policymakers may wish to accept that these are complex debates — and that we can only avert future attacks if we try to understand why some people are drawn to radical acts.

Timothy Peace is a lecturer in politics at the University of Stirling in the U.K. He is the author of “European Social Movements and Muslim Activism” and “Muslims and Political Participation in Britain.”