The mass causality terrorist attacks in Paris and now in Brussels underscore an unsettling truth: Jihadists pose a greater threat to France and Belgium than to the rest of Europe. The body counts are larger and the disrupted plots are more numerous. The trend might be explained by the nature of the Islamic State (ISIS) networks in Europe or as failures of policing in France and Belgium. Both explanations have merit. However, our research reveals that another factor may be at play: French political culture.

Last fall, we began a project to test empirically the many proposed explanations for Sunni militancy around the globe. The goal was to take common measures of the violence—namely, the number of Sunni foreign fighters from any given country as well as the number of Sunni terror attacks carried out within it—and then crunch the numbers to see which explanations best predicted a country’s rate of Sunni radicalization and violence. (The raw foreign fighter data came from The International Centre for the Study of Radicalisation and Political Violence; the original attack data came from the University of Maryland’s START project.)

What we found surprised us, particularly when it came to foreign fighter radicalization. It turns out that the best predictor of foreign fighter radicalization was not a country’s wealth. Nor was it how well-educated its citizens were, how healthy they were, or even how much Internet access they enjoyed. Instead, the top predictor was whether a country was Francophone; that is, whether it currently lists (or previously listed) French as a national language. As strange as it may seem, four of the five countries with the highest rates of radicalization in the world are Francophone, including the top two in Europe (France and Belgium).

An Islamic State fighter gestures from a vehicle in the countryside of the Syrian Kurdish town of Kobani, after the Islamic State fighters took control of the area, October 7, 2014. Reuters Knowledgeable readers will immediately object that the raw numbers tell a different story. The English-speaking United Kingdom, for example, has produced far more foreign fighters than French-speaking Belgium. And fighters from Saudi Arabia number in the several thousands. But the raw numbers are misleading. If you view the foreign fighters as a percentage of the overall Muslim population, you see a different picture. Per Muslim resident, Belgium produces far more foreign fighters than either the United Kingdom or Saudi Arabia.

So what could the language of love possibly have to do with Islamist violence? We suspect that it is really a proxy for something else: French political culture. The French approach to secularism is more aggressive than, say, the British approach. France and Belgium, for example, are the only two countries in Europe to ban the full veil in their public schools. They’re also the only two countries in Western Europe not to gain the highest rating for democracy in the well-known Polity score data, which does not include explanations for the markdowns.

Adding support to this story are the top interactions we found between different variables. When you look at which combination of variables is most predictive, it turns out that the “Francophone effect” is actually strongest in the countries that are most developed: French-speaking countries with the highest literacy, best infrastructure, and best health system. This is not a story about French colonial plunder. If anything, it’s a story about what happens when French economic and political development has most deeply taken root.

An important subplot within this story concerns the distribution of wealth. In particular, the rate of youth unemployment and urbanization appear to matter a great deal too. Globally, we found that when between 10 and 30 percent of a country’s youth are unemployed, there is a strong relationship between a rise in youth unemployment and a rise in Sunni militancy. Rates outside that range don’t have an effect. Likewise, when urbanization is between 60 and 80 percent, there is a strong relationship.

These findings seem to matter most in Francophone countries. Among the over 1,000 interactions our model looked at, those between Francophone and youth unemployment and Francophone and urbanization both ranked among the 15 most predictive. There’s broad anecdotal support for this idea: consider the rampant radicalization in Molenbeek, in the Paris banlieus, in Ben Gardane. Each of these contexts have produced a massively disproportionate share of foreign fighters, and each are also urban pockets with high youth unemployment.

Broken windows of the terminal at Brussels airport are seen during a ceremony following bomb attacks in Brussels in Zaventem, Belgium, March 23, 2016. Yorick Jansens / Reuters As with the Francophone finding overall, we’re left with guesswork as to why exactly the relationships between French politics, urbanization, youth unemployment, and Sunni militancy exist. We suspect that when there are large numbers of unemployed youth, some of them are bound to get up to mischief. When they live in large cities, they have more opportunities to connect with people espousing radical causes. And when those cities are in Francophone countries that adopt the strident French approach to secularism, Sunni radicalism is more appealing.

For now, the relationship needs to be studied and tested by comparing several cases in countries and between countries. We also found other interesting relationships—such as between Sunni violence and prior civil conflict—but they are neither as strong nor as compelling.

Regardless, the latest attacks in Belgium are reason enough to share the initial findings. They may be way off, but at least they are based on the best available data. If the data is wrong or our interpretations skewed, we hope the effort will lead to more rigorous explanations of what is driving jihadist terrorism in Europe. Our initial findings should in no way imply that Francophone countries are responsible for the recent horrible attacks—no country deserves to have its civilians killed, regardless of the perpetrator’s motives. But the magnitude of the violence and the fear it engenders demand that we investigate those motives beyond just the standard boilerplate explanations.