The findings underline that the group’s appeal remains global, with foreign fighters from at least 86 countries represented in Syria and Iraq, according to official government estimates and other credible reports. In part, this influx is due to the infamous social media outreach and propaganda of ISIL, which downplays any territorial losses, and continues to offer an open-arm welcome to all Sunni Muslims — or willing converts — who wish to declare their allegiance to ISIL leader Abu Bakr al-Baghdadi.

But Soufan also found that deeper involvement in ISIL — namely, the decision to take up arms abroad — tends to come through interpersonal networks. Hence, the alarmingly high concentration of recruits from certain “hotbed” countries, cities and towns.

The highest per capita rate of fighters and greatest overall number continue to come from tiny Tunisia — 6,000 individuals, as of last count, or more than from the entirety of Western Europe — a high percentage of whom are natives of just two cities: Bizerte and Ben Gardane. As Soufan pointed out, these Tunisian towns exhibit a dangerous combination of poverty, high rates of criminality and other anti-government activity, plus easy access to cross-border travel. But they also show that the strongest draw to “jihad” in Syria and Iraq is a friend or family member who has personally made the trip.

The stream of fighters from Western Europe, which has doubled since June 2014 to approximately 5,000, is also highly concentrated in a few pockets. Three-quarters of Western European fighters come from just four countries: France, the United Kingdom, Germany and Belgium, each of which contains a handful of isolated hotbed communities that have contributed disproportionately high rates of fighters. The Molenbeek neighborhood of Brussels, from which several of the Paris attackers came, is perhaps the best-known.

One of the lessons Soufan gleaned from this data is the “personal nature of recruitment” to groups like ISIL. Though social media remains critical in spreading the group’s message, in most cases, “it appears more often to prepare the ground for persuasion, rather than to force the decision.” As hotbeds develop, “recruitment through social media becomes less important than via direct human contact, as clusters of friends and neighbors persuade each other to travel separately or together to join the Islamic State.”

Besides Tunisia, other countries in the top five include Saudi Arabia, Turkey and Jordan, each of which were accused of turning a blind eye in the early days of Syria’s uprising as their nationals streamed into Syria to fight their regional rival, President Bashar al-Assad. To Soufan, this concentration underlines that ISIL, at its core, remains “essentially a local and regional phenomenon.”

But the number of fighters from far away continues to climb. Contributing the third-highest number of fighters to Syria and Iraq, Russia appears to have an escalating problem on its hands. Since June 2014, the number of its citizens fighting in Syria and Iraq has more than tripled, to 4,700. Most of these people come from the North Caucasus regions of Chechnya and Dagestan — traditionally restive regions that have hosted a low-burning Islamic insurgency for decades.

Globally, as many as 30 percent of these fighters return home, though what danger they pose remains an open question. Hardliners argue they are dangerous and should all be locked up, while advocates of leniency say returned fighters that are truly disillusioned with life in the “caliphate” are a government's best tool against radicalization.

The new findings come as U.S. politicians wrangle over the domestic threat ISIL may pose in the wake of a mass shooting in San Bernardino, California, an event that may have been inspired by the group. In apparent response, GOP frontrunner Donald Trump on Monday called for a ban on all Muslim travelers entering the United States until “our country’s representatives can figure out what is going on.”

And yet, Soufan found that the U.S. is one country that has not seen an increase in the number of its citizens flocking to Syria and Iraq to fight. In fact, while 250 Americans have tried to travel to fight with radical groups in Syria and Iraq, and 120 have succeeded, U.S. officials say the rate of Americans trying to do so has ground to a halt in recent months — down to roughly two per month since July.

Some analysts have warned that this could be a sign ISIL intends to shift its focus towards encouraging “lone wolf” or even Al-Qaeda-style strikes at home — à la Paris — as its territory shrinks in Syria and Iraq. Still, a growing body of evidence suggests that ISIL is much less likely to gain traction in a country like the U.S., where the Muslim community tends to be highly diverse, and more educated and affluent than the Muslims in the banlieues of Paris — and therefore more inclined towards integration.