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Scott Atran and his research colleagues were sitting face-to-face with captured ISIS fighters in Kirkuk, a city in northern Iraq, near the front lines of battle.
The researchers handed the fighters a set of cards, engraved with study questions to answer.
"They literally threw the cards down and said they refused to respond," said Atran, a cognitive anthropologist who is director of research at Artis International, a multidisciplinary research institution, and who holds teaching positions at the University of Michigan and the University of Oxford.
To Atran's surprise, the fighters were still willing to answer research questions; it was just that those particular questions were incomprehensible to some of them because the questions focused on the wrong motivations, he said.
Atran and his colleagues spent the past couple of years interviewing captured ISIS fighters, as well as Kurdistan Workers Party (PKK) members and other front-line fighters battling against ISIS, to better understand what drives anyone to willingly attack and die for a cause.
The researchers discovered that three crucial factors motivate both ISIS fighters and those fighting them: a deep commitment to sacred values, the readiness to forsake family for those values, and the perceived spiritual strength of the group or community that the fighter represents.
The findings, which were published in the journal Nature Human Behaviour on Monday, also help explain why the ISIS fighters whom the researchers interviewed were irritated with the initial line of questioning, Atran said.
That first round of research questions was not related to sacred values. Yet when Atran and his colleagues developed new questions and measurements based around spirituality and values, the ISIS fighters started opening up, he said.
The study could "help to inform policy decisions for the common defense" and fighting terrorism, the researchers write.