When Andre Poulin, a Canadian who converted to Islam, first appeared on YouTube in Syria last year, he said his family didn’t understand why he had moved to the country to fight with other jihadists against President Bashar al-Assad’s forces. Allah, he said during his video message, had pulled him to Syria. Poulin was just one of several Westerners to join the Islamic State, which at the time was just a small group of extremists vying for power. Now the Islamic State (also called ISIS) has obtained millions of dollars in new weaponry and is gaining more followers like Poulin, pledging allegiance to a group so barbaric that even al-Qaida has denounced it.  

“We tend to think they are crazy,” John Horgan, a psychologist and professor at the University of Massachusetts Lowell’s Center for Terrorism and Security Studies, said. “Because of what terrorists do, we assume that can be explained via the pathology of those people, but trying to explain terrorism as mental illness is misleading.”

The recent beheadings of U.S. journalists James Foley and Steven Sotloff set off a wave of fury on social media and forced Western leaders to publicly address ISIS’ barbarity. At the same time, news broke that dozens of young men -- neighbors, sons, friends, from places like London and Minnesota -- had left their homes to join the group.

Horgan is one of the few psychologists in the U.S. who study the minds of terrorists. In the more than 20 years he has been researching the topic, he said he had never seen a message by a member of a terrorist organization as compelling as Poulin’s.

In the video message, which ISIS later used in a propaganda video, Poulin explained why he had joined the Sunni militant group. “Before I come here to Syria, I had money, I had a family, I had good friends. It wasn’t like I was some anarchist or somebody who just wants to destroy the world and kill everybody. I was a regular person,” Poulin, who later began calling himself Abu Muslim, said in the message. “We need the engineers, we need doctors, we need professionals. Every person can contribute something to the Islamic State.” 

“Very often we see radicals decide they want to become a terrorist turn away at the last minute, but [Poulin’s] message hit the nail on the head, which is to say there is a road for everyone. It makes radicalization and recruitment much easier,” Horgan said. “It is an equal opportunity organization. It has everything from the sadistic psychopath to the humanitarian to the idealistic driven.”

As far as foreign fighters are concerned, Horgan said, they are driven to join ISIS by the need to “belong to something special.”

“They want to find something meaningful for their life,” he said. “Some are thrill seeking, some are seeking redemption.”

For many, the only way to learn about ISIS is through the news, or through social media. It is not often we hear honest accounts of why people join terrorist organizations, Max Abrahms, an expert on terrorism from Northeastern University, said.

“If you ask terrorists why they joined an organization after they have been in it, they will pair it with the official line of the group,” Abrahms said. “But in reality they don’t join the group for that reason.” 

For the members of ISIS, joining the group means promoting the creation of an Islamic caliphate and ridding it of infidels. Last month Vice Media gained exclusive access to some ISIS fighters. In a documentary, Vice interviewed Iraqi and Syrian children who said they wanted to become part of ISIS so they could kill infidels.

No one knows exactly how many fighters ISIS has; estimates vary anywhere between 10,000 to 40,000. The majority of them come from Middle Eastern countries, particular Iraq and Syria, but about 2,300 are foreigners.

ISIS has been particularly successful in recruiting its members through social media. In that sense, Horgan said, there is a "truly global appeal of ISIS" that is new. "They have become so adept at social media that they are reaching out to disaffected individuals on a global scale," he said.

When al-Qaida first began to form under Osama Bin Laden, members of the organization were recruited from communities that already had a large presence in the organization. They were then taught and essentially radicalized in the infamous madrasas, partnered with a mentor, and eventually worked their way up in the ranks of the organization.

Today, terrorist organizations including ISIS rely heavily on Twitter and Facebook to reach out to potential recruits -- those who are friends or family with someone already affiliated with the organization. From most of the terrorism research available, Abrahms said, those who join terrorist groups like ISIS are the most "ignorant people with respect to religion and they are generally the newest members to the religion." 

"They would probably fail the most basic test on Islam," Abrahms said. 

There is still a lot we don't know about ISIS members, Horgan said. We only ever see the end products like the beheadings and the mass executions. "The commission of those kinds of acts takes a long process of radicalization and recruitment," he said. "We don’t get to see the gradual radicalization process or the interaction between a leader and a follower. It would work differently depending where the fighter is coming from and how it is getting to the region."

Over the past two weeks, the U.S. and its allies in Europe and the Middle East have been working to build an international coalition to fight ISIS. President Obama said at the NATO summit Thursday that one of the ways the coalition would stop the threat of the Sunni militant group was to cut off its recruitment from Western countries.

"We are struggling to control the stem that recruits to ISIS," Horgan said. "It is important to look at accounts of individuals that have become disallusioned as a result of joining ISIS. It is key to preventing the next generation from joining."