In late 2013, a 19-year-old girl named Aqsa Mahmood said goodbye to her parents in Glasgow and slipped away from home. The next time they heard from her, she was crossing the Syrian border to join the Islamic State. When her father begged her to come back, she said, "I will see you on the day of judgment." Three months later, she married an IS fighter.

Aqsa (who now calls herself Umm Layth) has become one of the Islamic State's chief recruiters. Her targets are ardent girls from across the Western world who dream of marrying an IS fighter. She tells them that the hardest part is leaving home – but that Allah requires it.

Three such girls, ages 15 and 16, left Britain last week. At least one is thought to have been in touch with Aqsa. They were last spotted in Istanbul, waiting for a bus to Syria. Several Canadian girls have disappeared too, including two from Quebec. One of Aqsa's besties is a 20-year-old Canadian who has adopted the name Umm Haritha. Aqsa has posted pictures of the two of them online, in identical head-to-toe black.

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The IS groupies weren't radicalized in the mosque but in their bedrooms, via Facebook, Twitter, Tumblr and other social media. Private messaging makes it easy for them to make personal contact with women like Aqsa. Everything they need to know is on the Internet – how to talk your way through the border, what life will be like in your new home (plenty of housework), even what to pack (boots and a solar adapter for your Android when the electricity fails).

Islamic State fangirls now number in the thousands, although just a few dozen have made it to Syria so far. What strikes you about these girls is how normal they were. They came from moderate religious backgrounds and did extremely well in school. They wanted to be humanitarians and make the world a better place. They were the apples of their devastated parents' eyes. They seemed to be role models for successful integration.

What could tempt a smart young woman to join a band of murderous fanatics who brutally oppress women, crucify their enemies and use mass rape as a weapon of war?

The answer is a mix of passionate idealism, combined with the absolutist world view of a convert and the desire to belong to something greater than themselves. Plus hormones. There is a transgressive thrill to the idea of marriage to a violent warrior. Some IS heartthrobs have acquired devoted bands of female followers who want nothing more than to submit to them and have their babies. "Messianic fervour, millenarianism and magnetism can whip up female hormones alarmingly," wrote Yasmin Alibhai Brown in The Independent.

Besides, the romance of jihad seems a lot more exciting than studying for your accounting degree. "There's a lot of that kind of mentality," Melanie Smith, who is in touch with hundreds of IS groupies, told the Daily Mail. "It's laziness, really."

It's tempting to see these young women as innocent, naive dupes. But that would be a mistake. The Islamic State's extreme self-publicized violence cannot possibly escape their notice. And the sisterhood in Syria give the brutality their wholehearted support. "OMG … Gut-wrenchingly awesome," tweeted one female recruit after she saw a video showing the beheading of 18 Christians. "More beheadings, please," tweeted another.

The flow of female recruits is picking up, and it's hard to see how to turn it off. You can't turn off social media, and you can't police a girl 24 hours a day. You'd hope that once they discover the harsh reality of life as jihadi brides, they would regret their choices and long to come back. But Ms. Smith says she has yet to be in touch with anyone who has regrets. "They see it as emigrating to a better life," she told the Guardian. "They say they feel free."