After the ISIS attacks in Brussels and Paris, radicalization — the process by which people decide to become terrorists — has become perhaps one of the most discussed topics in the study of terrorism. How do people become terrorists, many are asking, and what can we do to stop it?

A new study, by the Brookings Institution's Will McCants and Chris Meserole, tried to examine this question by looking at the available data on people from around the world who have traveled to fight in Syria and Iraq.

They found something surprising: The countries most likely to produce people who leave to fight in Syria or Iraq tended to be French-speaking, or heavily influenced by French language and culture.

"As strange as it may seem," McCants and Meserole write in Foreign Affairs, "four of the five countries with the highest rates of radicalization in the world are Francophone, including the top two in Europe."

This is a preliminary finding — it hasn't yet "proven" anything. But if it ends up being right, it could potentially change how we understand radicalization and its causes.

What they found

McCants and Meserole began the study out of frustration: Despite the obvious importance of research about radicalization, studies on the topic hadn't found anything really firm.

"We felt the social science on Sunni radicalism was basically at a loss at this point," Meserole explained in an email to me. "Fifteen years after the Pentagon and WTC attacks, we still didn't really have a good handle on what was driving it."

So they decided to try something new. Using data on foreign fighters from the International Center for the Study of Radicalization, they figured out which countries were sending the most foreign fighters to Syria and Iraq relative to the overall Sunni Muslim population (not total population) in those countries.

"Our team had a whole lunch where we tried to make sense of what was going on"

The idea was that while not all people who radicalize go to fight in the Middle East, and not all people who go to fight in the Middle East are radicalized, the two are sufficiently related that they can be used as metrics for one another.

They then put together a bunch of variables, such as each country's youth unemployment or literacy rate, and used a complex computer algorithm to randomly test which variables best predicted the foreign fighter rate in each country.

They were shocked when, after running about 1,000 different tests, the computer ended up concluding that the Francophone variable — whether a country listed French as a national language — was by far the best predictor of its foreign fighter rates.

It wasn't even close: Francophone was well over twice as predictive, according to the algorithm, as any other variable.

"Neither of us went in to this project thinking Francophone was all that mattered," Meserole explains. "After we ran the model the first time, actually, Will and I and our team had a whole lunch where we tried to make sense of what was going on."

Why Francophone countries?

McCants and Meserole believe the most likely explanation is a unique Francophone political culture.

During the French Revolution, France developed a much more aggressive approach to church/state separation than other European countries. Instead of merely keeping religion out of state affairs, French secularism condemned religion's influence on political culture in general as pernicious. Formal separation of church and state isn't enough; you need to exclude religion from national identity and political conversation entirely.

This approach, called laïcité, really came into force in France's Third Republic, in the 1870s. Since then, it's been a major feature of French political culture — one that's bled over, to a lesser extent, to heavily French-speaking Belgium.

Both France and Belgium have also experienced large-scale Muslim immigration. Their political and social treatment of those Muslim communities, the theory goes, are affected by their preexisting culture of laïcité. You do see this play out in, for example, France's unusually strict laws restricting the practice of veiling. And this could extend in some way to former French colonies, such as Algeria and Tunisia.

This would perhaps be one way in which Francophone political culture would explain the apparent correlation between Francophone countries and radicalization.

McCants and Meserole hypothesize that this culture of laïcité has alienated French and Belgian Muslims from national culture, making them more vulnerable to radicalization. They argue that French influence in former colonies such as Tunisia and Lebanon, which are Francophone, could lead similar dynamics to play out there, exacerbating religious-secular divides and thus strengthening extremist narratives.

"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," they write. "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."

Interestingly, they also found a strong interaction between "the Francophone effect" and youth unemployment, and with urbanization: Francophone countries with higher youth unemployment rates and/or more urban populations tend to have higher rates of foreign fighters. From this, they told a pretty compelling story about what tends to attract young men to Islamist terrorist groups:

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.

What does this mean?

In our email conversation, Meserole pointed out that these findings are extremely preliminary. "Other than the potential connection with French political culture, especially its emphasis on laïcité, we're still not exactly sure what the finding means," Meserole told me.

These findings are also controversial. Perhaps the most vocal critic, somewhat amusingly, is Gérard Araud, the French ambassador to the United States.

"This text doesn't make any methodological sense. An insult to intelligence," Araud tweeted after reading the Foreign Affairs write-up.

His criticisms, which are worth hearing, include:

90 percent of Belgian foreign fighters come from Flanders, the country's northern, Dutch-speaking region, or from the capital Brussels, which is officially bilingual. Only 10 percent come from Wallonia, the French-speaking southern half.

More broadly, French and Belgian political cultures are too different for the comparison to be meaningful.

France's Muslim immigrants are more heavily Arab than those in other European countries, and thus should be more likely to go to Iraq and Syria.

These aren't necessarily fatal objections. As Meserole notes in a response, their study actually looked at an "Arabophone" variable, and found that heavily Arab countries weren't especially more likely (on a per capita basis) to send foreign fighters to Iraq and Syria.

The points about France and Belgium are stronger, and speak mostly to the fact that this study is in its infancy: Its authors are still working out what the data means, and haven't done the kind of granular research necessary to make their conclusions more ironclad.

However, if McCants and Meserole do turn out to be right, the findings would end up being quite significant. They suggest that political, social, and/or cultural practices associated with laïcité — say, for example, emphasizing secularism — could have something to do with radicalization. This would mean interrogating laïcité itself, which, after all, was developed at a time when France looked very different, and examining whether it might need to be adapted to the ways in which Francophone countries and the world have since changed.