Family and friends describe them not as radicals, but as well-behaved and diligent students at a London private high school. So it came as a shock when the three British girls slipped their passports into handbags, casually walked out of their homes, and boarded a flight to Istanbul to join the Islamic State, or ISIS, in Syria last month.

British authorities believe that the teenagers, who disappeared last month, were likely aided by Aqsa Mahmood, a young woman originally from Scotland who helps recruit for the extremist group.

The trio’s highly publicized defection to Syria, as well as the apprehension of three young British males in Istanbul this week as they headed to join ISIS, are just the latest among a growing number of teenagers and young adults from middle-class, educated, often suburban backgrounds in Britain, the United States, Canada, and various European nations who have been enticed to abandon their comfortable lives and join the Islamic State since last summer. In late February, the Washington Post identified “Jihadi John,” the masked man seen in several ISIS videos beheading hostages, as a college-educated computer programmer from a well-off family in West London. Although a precise figure isn’t known, Lt. Gen. James Clapper, director of U.S. national intelligence, told Congress last month that an estimated 3,400 citizens from Western countries have traveled to Iraq and Syria, presumably to join ISIS.

Jessica E. Stern, Ph.D. ’92, is a fellow at the FXB Center for Health and Human Rights at the Harvard T.H. Chan School of Public Health (HSPH) and a lecturer in the Government Department at Harvard’s Faculty of Arts and Sciences (FAS). She serves on the Hoover Institution Task Force on National Security and Law and was a member of the National Security Council staff during the Clinton administration. Stern has written extensively about terrorism and violent extremists. Her latest book, “ISIS: The State of Terror,” co-authored with J.M. Berger, will be released March 24. Stern spoke with the Gazette about how and why ISIS has been increasingly effective in luring young Westerners to its side.

GAZETTE: We know that the so-called Islamic State is extraordinarily media-savvy. What social-media platforms have been most effective in reaching Western recruits?

STERN: There’s been a lot of activity on Twitter. Aqsa Mahmood is a good example. She’s been accused of enticing the three young women from London who apparently left their homes to join the Islamic State. She’s also known as Umm Layth, which means “mother of the lion.” She spoke to them on Twitter, and then they ended up moving to an encrypted platform to continue their discussion, which is a common recruitment tactic. [Mahmood] also answers questions on Somehow her postings are attracting young women, some of them very high-achieving, to leave home to join the jihad.

There’s a big debate about what should be taken off Twitter and whether Twitter is inadvertently facilitating terrorist recruitment. Twitter’s automated list of “who to follow” makes it easy for a person interested in ISIS to rapidly find additional ISIS supporters. Sometimes, ISIS accounts are suspended, but often, shortly afterward, a new account with a new name appears, which serious followers can find.

There’s a debate among those who think we should allow those accounts to remain active, and those who think that Twitter should be suspending terrorist accounts. Those who say that the accounts should be left alone argue that they’re a good way to gather intelligence, and that removing them would only result in recruiters moving to a less-transparent platform. Those who want the accounts shut down say that private companies should not allow ISIS and other groups to use social media to recruit followers, and that terrorists’ use of social media to promote violence does not constitute protected speech. Twitter recently suspended over 2,000 ISIS-related accounts. ISIS has now declared war against Twitter, threatening the lives of its staff.

GAZETTE: What is the pitch to male and female potential recruits?

STERN: For the men, it’s, “Come and fight if you can fight; if you can’t fight we also need doctors, we need social-media experts, engineers … We’re running a state, and so if you feel you can’t handle fighting, we can still use you.” The women are often recruited to marry jihadists: “You can participate in the jihad by marrying. You can be the mother of the next generation.” It is a fairly traditional female role.

There are tremendous social benefits for recruits: You’re making a world a better place, or so the group claims, which provides a kind of spiritual reward. There’s financial reward for the fighters. ISIS actually pays the fighters, gives them free housing, offers to provide them wives. Hence, the need to recruit young women. There’s also the tremendous lure of extreme fundamentalism. I think we can all understand the appeal: Wouldn’t it be nice to have easy answers to every morally complex question? Inside a group like ISIS, life becomes morally simple. The rules are clear. Good and evil come out in stark relief.

GAZETTE: What’s the psychological profile of those people most susceptible to their message?

STERN: We don’t have a profile of the Westerners joining ISIS yet because there haven’t been large studies. But I can tell you that [British intelligence agency] MI5 did a study of Westerners who were involved in or closely associated with extremist activity, prior to ISIS’s recent recruitment drive. They found that a surprisingly high number of them were converts to Islam. Many in the MI5 study were relatively ignorant of Islam, even if they were Muslim. Umm Layth is a good example. She grew up in a secular Muslim family and went from relative ignorance about Islam to recruiting for ISIS.

An important factor seems to be the desire to forge a new identity, an identity with dignity. I interviewed terrorists for many years and I can tell you that identity is often absolutely key. We also know that there is a higher rate of mental illness among so-called lone wolves, people who are inspired (often online) to commit terrorist actions without physically joining an extremist group. Studies of Westerners joining jihadi organizations, prior to ISIS’s recruitment drive, have shown that foreign fighters tend to be alienated or marginalized within their own societies; they may have had a bad encounter with police or distrust local authorities. They tend to disapprove of their nation’s foreign policies. If they’re living in an ethnic enclave, they’re likely to be alienated from people living alongside them, as well as the country as a whole, whether it’s the United States, or the U.K. or elsewhere in the West. For those who join ISIS, I think that there’s got to be an element of thrill-seeking as well, perhaps even an attraction to violence. It’s hard for me to imagine that anybody who gets recruited today doesn’t know about ISIS’s extreme brutality.

GAZETTE: Is the impulse to join the Islamic State very different from, say, the idealistic impulse of young people to join the Peace Corps or a nongovernmental organization, or any global organization they believe is doing important and uplifting work?

STERN: Many of the people who join terrorist organizations believe they are making the world a better place. They see pictures of [Syrian leader Bashar] Assad’s brutality against his own people, and they feel the desire to help. That sense of righteousness is a very appealing aspect of joining a terrorist group, for some. But I would say in some ways it’s more like joining the Weather Underground than the Peace Corps. At this point, it’s hard to imagine anyone joining without knowing that they’re going to be involved in real atrocities.

GAZETTE: But in their minds, those actions are righteous.

STERN: Absolutely.

GAZETTE: How effective is Mahmood as a recruiter, and what makes these Western recruiters so successful? Do they tend to be true believers or mere cynical mercenaries?

STERN: She is very effective. My guess is that it’s partly because she knows how to relate to young women like herself. She knows their lives. ISIS is using Westerners to run the social media campaign to recruit Westerners.

GAZETTE: The State Department has recently announced that it has stepped up its counter-messaging efforts. What are they doing, and is that likely to be sufficient, given the sophisticated and prolific nature of the Islamic State?

STERN: They have a program called “Think Again Turn Away,” and if you look at what they’ve been doing and compare it with what ISIS has been doing, it’s so boring. ISIS has professional cameramen. …

GAZETTE: The ISIS production values are quite high. It’s not like the old al-Qaida training videos we used to see.

STERN: No, it’s not. If you look at what the State Department puts out, sadly, you can tell that they didn’t have a lot of money. But the guy who ran that program told me, “Look, I know we can’t compete with the video imagery showing, ‘Here’s your chance to create this very pure state, and you’re going to get to kill infidels and Shiites.’”

GAZETTE: They can’t compete on the messaging or on the production values?

STERN: Both. ISIS has made an enemy of the entire world, other than those who join it. I hope that we’re going to get much more serious — we outside the government — to find ways to respond. There is a program that I’d like to bring to Harvard. I’ve been advocating for years to have young people design counter-messaging programs, rather than State Department employees or Madison Avenue. There is an organization, EdVenture Partners, that created a curriculum for students around the world to compete to create the most effective counter-messaging. The students will create digital platforms to amplify the messages of clerics who can argue against ISIS’ interpretation of Islam, or of former members of ISIS who turned against the organization. Those are just two examples; there are all kinds of things that can be done. The initiative is called “P2P: Challenging Extremism.” I would love to get students from across the University, students in engineering, students in political science, students who speak languages, or who are very good at communications … ideally we want a completely interdisciplinary group. I’m just so excited about this.

GAZETTE: Besides better coordinating the State Department’s fragmented messaging efforts, I wonder if that’s ever going to be sufficient compared to the prolific nature of ISIS. I understand they’re sending out as many as 200,000 social media messages per day.

STERN: No. It’s never going to be enough. I think the private sector has to get involved. I’m hoping Harvard alumni will be inspired to get involved.

GAZETTE: What is the Islamic State’s end game? Is it to provoke global Armageddon, or does it want to control the world and have everyone live under its terms?

STERN: They want to establish a worldwide caliphate. The dream is to take over the world. They are also obsessed with the Apocalypse. Although ISIS claims to justify its actions by referring to religious texts, ordinary Muslims have no idea what ISIS is talking about. The Quran is not an apocalyptic book, so ISIS has to borrow from different apocalyptic narratives. Their online English language magazine, it’s called Dabiq, which is the name of the town where ISIS believes the final battle of the Apocalypse will take place.

They believe that sexually enslaving women who are from religious minorities is a good thing; it’s a sign that the End Times are coming. They also justify sexual slavery as a way of avoiding the sin of adultery or premarital sex, because if you have sex with a slave, it’s not really sex, or so they claim. They can be pedophiles.

GAZETTE: Why is religion such a useful framework or pretext for terrorism, subjugation, and genocide?

STERN: ISIS is a millenarian movement. They want to create a new human being the same way the Soviets wanted to create a new human being. They want to recreate humanity, and they want to create a purified world. It’s a cosmic battle to them. It’s not totally different from communism or other ideologies, but God is a pretty compelling citation.

GAZETTE: Does religion give it a patina of righteousness or defuse any accusations that this is a mere power grab?

STERN: I think religion is often a patina or marketing strategy for terrorists to accomplish more worldly goals. In the case of ISIS, many of the leaders are former Baathists, the secular political party that ruled Iraq prior to the 2003 invasion. [Abu Bakr al-] Baghdadi, the “caliph” of the Islamic State, recruited former military and intelligence personnel from Saddam Hussein’s Iraq. They have important, useful skills. ISIS’s religious agenda is clearly intermingled with its more secular goals. ISIS is capitalizing on the feeling among Sunni Muslims that they are under threat in the new Iraq, and that ISIS is the only protection they have from the Iraqi leadership’s anti-Sunni, sectarian policies.

GAZETTE: In human history, where does ISIS rank in terms of what they’ve been able to accomplish — their lethality and their organizational strength — in such a brief amount of time?

STERN: Compared with modern terrorist organizations that we know, they rank very high. However, compared with the Khmer Rouge, the Nazis, the communists, they rank pretty low both in terms of their accomplishments and even in terms of their brutality. We’ve seen much worse. ISIS is not just a terrorist group; it is also an insurgent army. While it’s shocking to see how much territory ISIS acquired so quickly, we’re comparing it with terrorist groups that weren’t necessarily trying to acquire large amounts of territory. The ideology, the brutality of this group — I have to think they’re going to self-destruct before they manage to spread as far as, say, the communists or the Nazis. The Nazis weren’t advertising their atrocities; ISIS is publicizing its atrocities, flaunting its brutality. It’s part of the End Times narrative that ISIS hopes to spin.

This article has been lightly edited for length and clarity.