This prediction is based on decades of research that my colleagues and I have conducted at the University of Oxford to determine what makes people willing to fight and die for their groups. We used a variety of methods, including interviews, surveys, and psychological experiments, to collect data from a wide range of groups including tribal warriors, armed insurgents, terrorists, regular soldiers, religious fundamentalists, and fans of violence.

We find that life-changing and group-defining experiences cause our personal and collective identities to blend together. We call this “identity fusion.” Integrated individuals will do whatever it takes to advance the interests of their group, and this applies not only to actions we laud as heroic—such as rescuing children from a burning building or taking a bullet for a comrade—but also to acts of suicide terrorism.

Integration is usually measured by showing people a small circle (representing you) and a large circle (representing your team) and placing pairs of such circles in order so that they have varying degrees of overlap: no overlap at all, Then just a little bit, then a little more, and so on, until the smaller circle is completely enclosed within the larger circle. People were then asked which pair of circles best represented their relationship to the group. The person who chooses the small circle within the larger circle is said to be "fusion". These people love their group so much that they will do anything to protect it.

This is not unique to humans. Some birds will pretend to have broken wings to attract predators away from their chicks. One such species - Australia's beautiful fairy wrens - lure predators away from their young by imitating the behavior of cute mice and making squeaking noises. Humans also generally go to great lengths to protect their genetic relatives, especially if their children (except identical twins) share more genes than other family members. But, unusually in the animal kingdom, humans often go further and put themselves at risk to protect groups of genetically unrelated members of their tribe. In ancient prehistory, such tribes were small and everyone knew everyone else. These local groups were united by shared hardships, such as painful initiations, hunting dangerous animals together, and heroic battles on the battlefield.

Today, however, integration has expanded to much larger groups, thanks to the ability of the world's media, including social media, to fill our minds with images of horrific suffering in conflicts in far-flung regions.

When I met a former leader of the terrorist group Jemaah Islamiyah in Indonesia, he told me that he first became radicalized in the 1980s after reading newspaper reports about the treatment of fellow Muslims by Russian soldiers in Afghanistan. Yet two decades later, nearly one-third of U.S. extremists were radicalized through social media, and by 2016 that proportion had risen to about three-quarters. Smartphones and immersive reporting have shrunk the world to such an extent that shared suffering in face-to-face groups can now be recreated to a large extent and spread thousands of miles with the click of a button millions of people outside.

Integration based on shared suffering may be powerful, but it is not by itself sufficient to inspire violent extremism. Our research shows that three other ingredients are needed to produce this deadly cocktail: threat from outside groups, demonization of enemies, and a perceived lack of peaceful alternatives. In areas like Gaza, where the suffering of civilians is often captured on video and shared around the world, it's only natural that the rate of integration among those watching in fear increases. If people believe that a peaceful solution is impossible, violent extremism will spiral.