Other than their alleged interest or participation in the Islamic State group, psychologists say perhaps not much.
"There is never, ever one single factor that explains why people go," said John Horgan, a professor at the University of Massachusetts in Lowell who studies the psychology behind terror movements and the people who join them.
"It's religion, it's politics ... it's the push factors and the pull factors, the thrill of adventure, the excitement and the comfort you get from camaraderie," he explained. "Those are very, very powerful motivating factors,"
Horgan said "there's no doubt in my mind that if you look at the individual who becomes involved with ISIS [Islamic State] or wants to become involved, whether it's three years ago or three months ago or three hours ago, it can be very difficult to disentangle why they want to become involved."
A friend of “Jihadi John,” the man identified this week in a Washington Post article as Mohammed Emwazi, told the newspaper the 26-year-old was upset by what he perceived as personal slights — including being blocked by counterterrorism officials in London from traveling to Kuwait.
“He at some stage reached the point where he was really just trying to find another way to get out,” the unnamed friend told The Washington Post.
Although the U.S. and British governments have not confirmed Emwazi’s identity as the man featured in several highly publicized Islamic State videos of brutal beheadings, the U.K.-raised computer science graduate is believed to have ultimately traveled to Syria to join the militant group.
Other would-be IS members have been intercepted before reaching the self-proclaimed caliphate based in Syria and Iraq. Three men were arrested in the United States this week in connection with an alleged plot to join the group — including one as he was boarding a flight to Turkey from New York.
According to court documents, one of the men, Abdulrasul Hasanovich Juraboev, expressed concerns about his family's lack of religiosity, saying he wanted to "become a martyr under the Islamic Caliphate against the polytheists and infidels."
Why they join
There are an estimated 20,000 foreigners who have joined the Islamic State, with several thousand coming from Western countries. The thread that unites them is they all find a reason — personal, or much broader — to pledge allegiance to the Islamic State.
“You've got real-deal fanatics who really do believe this is end of days, apocalyptic, millenarian stuff," said Horgan. "For those guys, this is fantasy come to life. This is the kind of stuff that jihadis in the 1970s and 1980s could only have dreamt of."
Horgan said he has no doubt that there are "hardcore believers that are there and will be the first to go and the last to come back, if ever. But then, you've got the dregs. You've got the people who do fall under the spell of recruiters and who do believe they can achieve something out there that they can never achieve at home.”
In short: the Islamic State — which redefines what “right” and “wrong” are within its borders and re-brands “success” — can make its members feel like they are accomplishing something.
James Piazza, a political science professor at Pennsylvania State University who focuses on terrorism and political violence, said the attention that Islamic State garners through bloody, graphic killings is a key recruitment tool.
“They're trying to show a wider audience of people that are disaffected with the status quo that they're willing to break norms and show how dedicated they are,” Piazza told VOA.
Arie Kruglanski, a professor at University of Maryland, began researching the psychology of terror groups after the September 11 attacks. He said would-be IS members may have micro or macro grievances and be distanced or spurned from the society they are living in — but to different degrees and fueled by varying motivations.
"There are two types of disenfranchisement," explained Kruglanski. "The first is ... feeling personal humiliation and failure; the other is the disenfranchisement of a social group. Al-Qaida and propaganda have been harping on that successfully — 'you may have a successful life, but your brothers are being massacred; your social identity is being humiliated.'"
"There’s no profile though," he added. He said "personality is important to some extent, but argument, group pressure — [they] can affect everybody."
That group-thinking generally starts well before Islamic State supporters buy a plane ticket.
"When people radicalize, it's very often a group process,” said Sam Mullins, counterterrorism professor at the George C. Marshall European Center for Security Studies. "The number of people who entirely self-radicalize with no social interaction — even online — is very, very rare.”
Why they stay
The psychology behind what causes men and women to join groups like Islamic State differs from the group mindset that keeps them inside, and in some cases, prompts them to carry out vicious, publicized attacks like those that made ‘Jihadi John’ the masked face of IS brutality.
What Kruglanski calls the “great cleverness” of the Islamic State group is that it “removes the veneer of civilization by providing an ideological warrant for criminality.”
“It turns sinners into saints, allowing that under the guise of religious ideology," he added.
In other words, it is the inversion of the civilization that the men and women of the Islamic State grew up with before they joined. Violence, crime and justice no longer mean the same thing.
Once they are inside the Islamic State's territory — in parts of northern and eastern Syria, and northern Iraq — getting out is also, by anecdotal accounts, extremely difficult.
"From what we can see right now, the Islamic State is terrified about people leaving its ranks," said Horgan, who is researching how former terror group members reintegrate into society.
He said "a lot of disillusioned jihadis basically say, 'well, you know, we came out here two or three years ago, and it wasn't quite what we expected.'"
In the years to come, Horgan believes more stories of the disgruntled, disillusioned IS supporters will emerge. And they will be a source of invaluable information on how the Islamic State group functions, and what can be done to counter its psychological tactics.
How to redirect them
For now, Hogran says paying closer attention to who recruiters select to travel, join, carry out beheadings, and play other roles in the self-declared caliphate may be more important than unraveling why foreign fighters decide to join.
Earlier this week, France announced it blocked six citizens from leaving the country for Syria, and intended to confiscate the passports of dozens more in an effort to curb foreign fighters exodus.
But experts warn that travel bans aren't a cure-all.
"There's obviously no sort of foolproof solution," said Mullins, who studies the flow of foreign fighters from Europe, North America and Australia to militant groups like the Islamic State.
"The best approach ideally speaking — is to be as flexible as possible — and use different tools for different people, depending on the level of risk they present," he said.
He was quick to point to the flaw in that logic, though, saying "there's basically no such thing as a reliable, standardized risk assessment tool for people like this."
Kruglanski suggests countries find a way to channel the energy of would-be Islamic State supporters — including the ones caught acting on travel plans. It involves reframing what the “fight” is, just as the Islamic State has reframed what is right and wrong.
“Mobilize the motivation that young people have — that quest for significance, that aggression,” he said. “It would mean mobilizing for a cause — but the cause, rather than being the cause of jihadism, would be the cause of fighting jihadism.”