Psychologists went to war-torn Northern Iraq to find out why some fighters will sacrifice everything for their cause

Why are some people willing to risk their own lives – and even their children's lives – to fight an enemy? An extraordinary study involving interviews with frontline fighters against the Islamic State, as well as IS fighters, finds that three crucial factors are at play. The most important was the strength of commitment to a "sacred" or deeply-held value or idea – but not necessarily a religious one. The findings "may help to inform policy decisions for the common defense," wrote Ángel Gómez and his colleagues in their new paper in Nature Human Behaviour.

Traditionally, most analyses of why groups choose to fight have focused on relative material strength – those with greater firepower and manpower are more willing to go to battle. But the authors of the new research point out, ever since World War II, insurgent groups have prevailed with as little as a tenth of the military muscle of state forces. "One plausible reason resides in the motivations of the combatants," the researchers noted. "When group interests become sacred and non-negotiable, spiritual considerations trump material ones."

To examine this idea, the researchers, all associated with Artis International, a research institution established to explore politically motivated violence, based in Arizona, US, set up a series of studies. In 2015, members of the team travelled to northern Iraq, to interview combatants fighting against so-called Islamic State forces (IS; also known variously as ISIS, ISIL and Daesh), including fighters from the Kurdistan Worker's Party, and they also interviewed some captured IS fighters. They also ran another field study in February and March this year on the same frontline, interviewing Peshmurga (Kurdish Regional Government forces), Iraqi army Kurds and Arab Sunni militia.

Based on earlier research, the team identified a likely set of "sacred values" for these groups. These included "Kurdeity" (a cultural concept denoting a sense of Kurdish language, heritage and land) and, for the Sunni Arab fighters, "Arabness" (also a cultural concept). A value was deemed "sacred" if an individual indicated that no amount of material gain would persuade them to give it up. For other people, 'democracy' might represent a sacred value.

The researchers examined how willing the fighters would be to make costly sacrifices, by asking if they'd be willing to die, let their family suffer, kill civilians, undertake a suicide attack and/or torture women and children for that value.

Since previous studies of combat soldiers have stressed devotion to comrades over cause, the researchers also systematically explored the extent to which the fighters were willing to sacrifice members of a group that they belonged to – including their comrades and their family – to fight for their value.

In addition, the researchers ran 14 online studies with more than 6,600 non-combatants from various regions of Spain, a country that has experienced recent IS terror attacks, to explore whether civilians may have similar attitudes. "Strikingly, the frontline and online studies converge on key determinants of willingness to make costly sacrifices," the researchers noted.

Across groups, the three critical factors underlying people's willingness to make sacrifices to fight for a cause were:

• having a commitment to non-negotiable sacred values and to groups that individuals were 'wholly fused with'
• a readiness to forsake kin for those values
• a perception that the spiritual strength of the in-group versus the foe is more important than relative military might
   

More than half of the fighters, when asked what was most important to them, stated at least one sacred value, such as "Arabness", as being more important than their comrades or even their family.  This was generally not the case for the non-combatants from Spain (more than 77 per cent of whom said their family was more important to them than their most important "sacred value"). However, like the fighters, those minority of civilians who said a sacred value or values was more important to them than the groups they belonged to reported being more willing to make costly sacrifices.

For some anti-IS fighters, the sacrifices they made in prioritising their values over their family were horrific. "In some cases, ISIS has divulged children's public executions for parents who opposed the Caliphate and its leader," the researchers wrote. These choices were very difficult: "Indeed, frontline fighters would be highly emotional when discussing making such tragic choices." But they still made them.

Results from both the frontline and the online studies also found that when it came to a willingness to make sacrifices, participants were less interested in the military might of an opponent than in the concept of "spiritual formidability". Avowedly religious ISIS fighters and avowedly secular PKK fighters (the only force that held fast against the ISIS onslaught in summer 2014) alike firmly disregarded a consideration of their foe's manpower and firepower, relative to their own, if they believed they had the upper hand in spiritual terms. The Kurdish fighters, for example, stated that what was important was "ruhi bi ghiyrat" – "spirituality with bravery".

Some other studies – of opponents of the Gadaffi regime in Libya, for example, as well as of soldiers in the West – have concluded that a deep commitment to comrades is key to an individual's fighting spirit. This study finds that commitment to a value "can trump group loyalty in willingness to fight". As such, the researchers hope their findings will help policymakers wrangling with how best to deal with the threat posed by groups like IS. They add that an important subject for further study is why some groups are better able to inspire loyalty to an abstract cause than others.