A 13-year-old girl who attended the Ariana Grande concert looks at floral tributes and messages in Manchester, England, on May 24th, 2017. (Photo: Jeff J Mitchell/Getty Images)

As crowds filed out of the Ariana Grande concert at the Manchester Arena on Monday night, 22-year-old suicide bomber Salmon Abedi detonated a homemade device just inside the building, killing 22 and injuring dozens more.

Following an investigation that found Abedi was part of a wider terrorist network, and the Islamic State claiming responsibility for the Manchester attack, British Prime Minister Theresa May raised the nation's terror alert to its highest level.

Such tragic events inevitably make us question how terrorists can carry out these atrocities. A new study published today in Nature Human Behavior may offer some insights: Their moral reasoning is fundamentally different from the rest of ours.

The study looks at how terrorists' moral judgment differs not just from that of the rest of us, but from other violent criminals as well.

For insights into the mindset of terrorists, an international team of researchers from Argentina, Colombia, Chile, Australia, and the United States turned to Colombian prisons, where decades of violent conflict between right-wing paramilitary groups and left-wing insurgents have left the country with one of the highest terrorism rates in the world.

The researchers compared 66 members of a paramilitary terrorist group who had been convicted and incarcerated for murder (with a mean of 33 victims) with 66 non-criminal controls and 13 non-terrorist murderers on tests for IQ, aggression, emotion recognition, and moral judgment. Perhaps unsurprisingly, the terrorists were more aggressive than their non-criminal counterparts, and worse at detecting others' emotions. But the most significant distinction was in their moral judgments of four imaginary scenarios between two women named Grace and Alice.

In each case, Grace adds a white powder to Alice's coffee. Sometimes, that white powder is toxic and Alice dies, and sometimes it is sugar and Alice gets to enjoy her coffee. But Grace's intent also varied: Sometimes she meant to harm Alice, and other times she genuinely thought the toxic white substance was sugar.

"Terrorist moral code actually approves of any action insofar as it contributes to achieving a given aim."

Both non-criminals and non-terrorist criminals in the study followed this pattern, deeming accidental harm (cases where Grace didn't know the white powder was toxic) as more permissible than attempted harm (cases where Grace thought the white powder she gave Alice was toxic, but it was actually sugar).

This finding gels with past research that has found most people believe intent matters as much, if not more, than outcome when evaluating the morality of one's actions. Look no further than our legal system for a real world example: Attempted murderers are usually judged harshly, while those who cause accidental harm are often granted more leniency.

But the terrorists found accidental harm to be less permissible, and attempted harm—in cases where Alice still escaped unscathed—to be more permissible. Terrorists, in other words, cared more about outcomes.

"Our results are consistent with the view that the terrorist moral code actually approves of any action insofar as it contributes to achieving a given aim," Agustín Ibáñez and Adolfo García, both from Favaloro University in Argentina, write in an email exchange. However, they caution that this study does not show that identifying this unusual moral judgment in individuals might predict who will join terrorist organizations, or which terrorists might be more likely to relapse upon release from prison. For one thing, we don't know which comes first—atypical moral reasoning or participation in terrorism.

"Terrorism and radicalization are … molded by group dynamics, biological predispositions, cultural constraints, and socio-psychological factors," Ibáñez and García write. "It may even be the case that this abnormal form of moral cognition is the result of participating in terrorist practices."

Unfortunately, that abnormality may have reared its head on Monday.