A woman at a meeting of the Union of Islamic Organisations of France, northeast of Paris, in May. PHOTOGRAPH BY MARTIN BUREAU / AFP / GETTY

Late last fall, just a few weeks before the coördinated attacks in Paris, a Brookings Institution researcher named Chris Meserole assembled all the data he could find about which countries ISIS fighters came from, and began to run programs looking for correlations. Much of the scholarship in the evolving field of terror analysis emphasized jihadists’ networks and their psychological profiles, but Meserole and his collaborator, Will McCants, were interested in a separate line of questions. What was the social position of Sunni Muslims in each country that sent jihadis to Syria, and did any aspects of that position seem to correspond with the number they sent? Meserole thought that some new analytics techniques could help cut through the data, and once he applied them he found several correlations. Two were not especially strong or surprising: countries where Sunni Muslims were densely concentrated in cities, and where they had especially high rates of youth unemployment, tended to produce more ISIS fighters. But the third was striking. The most powerful variable by far in predicting how many jihadis a country would produce was whether the people in that country spoke French.

The Francophone connection was not, in itself, a satisfying conclusion. Surely the language must be standing in for something else. Eventually, Meserole and McCants thought they saw a more meaningful explanation for France, Belgium, and Tunisia’s high rates of ISIS fighters: the campaigns against the veil across the Francophone world in 2010 and 2011. In April, 2010, after a high-profile debate, France’s government began enforcing a national law that effectively prohibited Muslim women from wearing the niqab or burqa, which cover the face, in public. In July, 2011, Belgium passed a similar law. (Tunisia, which long had a ban on the veil on the books, had begun enforcing it in 2006.)

Meserole met with a terrorism researcher named Amarnath Amarasingam, a fellow at Dalhousie University, in Nova Scotia, who had been studying the sixteen people who left Quebec to join the fight in Syria. Amarasingam believed that a high-profile, failed campaign to ban the niqab at citizenship ceremonies in Quebec may have played an important role. Fourteen of the sixteen jihadis had left the province after the bill was debated. In interviewing the fighters’ family members, Amarasingham found that the bill had been a “big catalyst” in their radicalization, he told Meserole. It was at that point, Meserole told me, that “I thought we might actually be on to something.” Spokesmen for ISIS have sometimes said that the group’s goal in recruitment is to eliminate the “gray area” in which people feel both Muslim and part of the West. Maybe the debates over the veil, Meserole and McCants came to believe, had helped do that for them. (Quebec has not had the same strictly secular tradition, which to Meserole made this an especially interesting case of what happened when political ideas were imported directly from France.)

In March, Meserole and McCants published a brief account of their findings in Foreign Affairs. (Meserole gave a more detailed description of their research on his blog last week.) Then, as ideas tend to in Washington, it began to amble slowly downhill, referred to in op-eds and at conferences, acquiring supporters and dissenters, without really being proven or disproven. The study exhibited “a surprisingly shallow understanding of the French-speaking world,” Olivier Decottignies, a career French diplomat now in residence at the Washington Institute, insisted. (If Washington looks at Paris and sees lax security services, Paris looks at Washington and sees naïfs with spreadsheets.) Others pointed out that many of the Belgian terrorists were from predominantly Dutch-speaking regions, and so might have been more insulated from the debates over the veil.

The criticism that Meserole took most seriously was that their methodology meant that they emphasized causes that could be identified at the national level, when the real action was taking place within jihadis’ social networks. When most of the terrorists were of Moroccan descent and many were from a few neighborhoods, why would Meserole and McCants ask how France was different from England, or Belgium from Denmark?

Meserole had compared the number of Moroccan immigrants in each European country to the number of the country's jihadi recruits, and he found that it did not bear out excluding the Francophone connection. There were approximately one and a half million Moroccans in France, and that nation had sent an estimated twelve hundred jihadis to the Middle East. But there were eight hundred thousand Moroccans in Spain, which had sent only seventy-five. Belgium and Italy each had about half a million Moroccan immigrants, but Belgium had supplied more than five times as many jihadis. “You can say it has something to do with Moroccan networks, but I think you have to say that it has something to do with the Moroccan experience in France and Belgium particularly,” Meserole told me.

Meserole’s spreadsheet had included a variable for forced secularization, but it lumped together the Francophone countries and repressive nations like China and Turkmenistan, and it hadn’t been especially predictive. Meserole told me that his running theory was that radicalization took advantage of two central French political ideas: “laïcité et liberté.” France and Belgium forced secularization on Muslims, but also gave them the freedom to organize against it.

You could draw a lot of lines out of this, into a lot of places. Writing about the study in Haaretz last week, Gershom Gorenberg wondered if Israel’s secular founders and their descendants had pushed the ultra-Orthodox into the settlements and toward extremism. He urged Israelis “to learn from the voters of London," who just elected a Muslim mayor, Sadiq Khan, “not the fashion police of Paris.”

Americans, by contrast, may find a kind of national relief in McCants and Meserole’s hypothesis. Compared to “the fashion police of Paris,” we are comfortable with religious diversity and religious expression, and perhaps this is a source of insulation against ISIS recruitment here. But the real pressure is not on the strength of religious freedom but on the possibility of hyphenated identity—on whether a person feels able to be both Muslim and French, or both Muslim and American. Formal enforcement of secular culture may be the force that works most directly against dual identity, but lower-amplitude advertisements of comfort and hostility, the daily interactions that a person has, matter, too. One common theme in the radicalization stories that foreign fighters told, Meserole said, was feeling ostracized at school for being Muslim.

For many years, one basic imbalance in the terrorist threat to Americans was that it emanated from so far on the periphery of daily American life. That threat was understood to depend on the relative brutality of dictators supported by the West, on the influence of more or less anti-Western clerics working in a different language in another part of the world, on the opportunities open to the children of the Middle Eastern middle class at a particular moment in time. To study accounts of the 9/11 hijackers as they gathered in Hamburg is to realize just how faint a conception they had of American society, and how abstracted their objections to it were.

The terrorists we worry about are now often Westerners themselves, and so their grievances with Western society are often more precise, and hit closer to home. If Meserole and McCants are right, radicalization has moved out of the lagoons of counterterrorism and foreign policy and into one of the main currents of Western experience: the fraught, essential project of pluralism, of people with many identities living as one.

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