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When John F. Kennedy was 17, he was part of a prank club. At Connecticut’s elite Choate school in 1935, word spread that the group was planning to pile horse manure in the gymnasium. Before this “prank” could happen, the school’s headmaster confronted the troublesome boys. The scheme was the culmination of a list of offenses at the school, and young Kennedy was expelled.

Though the sentence was eventually reduced to probation, the headmaster suggested that Kennedy see a “gland specialist” to help him “overcome this strange childishness.” The doctor Kennedy ended up seeing was Prescott Lecky, a young, mutton-chopped psychologist. Lecky had made a name for himself at Columbia University as a skeptic of psychoanalytic theory, running up against Carl Jung and the Viennese establishment’s approach at the time. Instead of tracing Kennedy’s rebellious instincts to repressed motives or early-life stress, Lecky interrogated the boy’s sense of self.

Lecky paid particular attention to Kennedy’s talk of sibling rivalry. “My brother is the efficient one in the family, and I am the boy that doesn’t get things done,” Kennedy says in one of Lecky’s records. This constituted what Lecky considered a “self view”—a deeply held belief about oneself. He wrote that Kennedy had a reputation in the family for “sloppiness and inefficiency, and he feels entirely at home in the role. Any criticism he receives only serves to confirm the feeling that he has defined himself correctly.”

Kennedy’s case fit into a new idea Lecky was developing, called self-consistency theory. It posited that people are always striving to create a world in which their ideas of themselves make sense. We are motivated, sometimes above any sense of morality or personal gain, simply to hold our views of ourselves constant. This allows us to maintain a coherent sense of order, even if it means doing things the rest of the world would see as counterproductive.

The idea was never fully formed, and Lecky died at just 48, his work unpublished. But today, the basic concept is seeing a renewed interest from scholars who think Lecky was truly onto something. When the psychologist’s students compiled his writing posthumously, in 1945, the postwar world was grappling with how humans were capable of such catastrophic cruelty. Surely entire armies had not been motivated by their relationships with their mothers. The early science of the mind was beginning to delve into the timeless questions of philosophy and religion: Why do we do destructive things—to others, and to ourselves? Why do we so often act against our own interests? Why would a young boy risk his acceptance to Harvard to pile manure into a school gym?

These questions meant studying the roots of identity, and how a person could be at peace with being hateful and even dangerous. Now, decades later, an emerging explanation points to something more insidious than the possibility that someone simply identifies with a malicious group or blindly follows a toxic person. Instead, out of a basic need for consistency, we might take on other identities as our own.

“I have always been intrigued with the surprising things people will do in the service of preserving their identity,” says William Swann, a social- and personality-psychology professor at the University of Texas at Austin. He took up Lecky’s ideas and, in the 1990s, built them out into what he called self-verification theory. It asserts that we tend to prefer to be seen by others as we see ourselves, even in areas where we see ourselves negatively. As opposed to cognitive dissonance—the psychological unease that drives people to alter their interpretation of the world to create a sense of consistency—self-verification says that we try to bring reality into harmony with our long-standing beliefs about ourselves.

Swann’s theory offers an explanation for all sorts of seemingly counterproductive things that people do, from procrastination to poisoning relationships. Swann has noted that people with negative self-views tend to withdraw or flee from romantic partners who treat them too well. Some would call this “self-sabotage”—the basis for why some people ignore those who seem to genuinely appreciate them.

As Swann sees it, outwardly appearing self-injurious behaviors like these might actually be part of a fundamental “desire to be known and understood by others.” Self-views enable us to make predictions about our world and guide our behavior. They maintain a sense of continuity and order. Stable self-views also, ideally, help facilitate relationships and group dynamics. When people know their role in any particular dynamic, they predictably play the part, even when doing so is self-destructive.

Self-views seem to have their basis in how others treat us, and they solidify as we accept our position and behave to further warrant similar treatment. An overall sense of identity comes together like a patchwork quilt of group and self, defined by where we fit into the world. Each of us is someone’s child, someone’s neighbor; a member of some community or religious sect; we are the work we do, the dogs we have, the places we’ve lived, the bands we listen to and teams we cheer for and authors we keep on our shelves.

Sometimes we bond especially strongly with some of our associations, such as family, a military group, or a religion. We say we can’t imagine existing without something. Even in cases of extreme identification, however, people typically maintain a sense of their own identity. There is a distinct conceptual border between self and other. You are a part of the team, and you are you.

Occasionally, though, this border becomes permeable. Over the past decade, a new conceptualization has gained attention. It began with the seeds of an idea after the attacks on 9/11, Swann says, in that the terrorists’ actions seemed to him to be driven by unusually powerful group identities. A willingness to die—and to kill thousands of others in the process—goes beyond simple allegiance. He reasoned that these people had essentially taken on the group identity as their own.

Swann gradually developed the concept and deemed it “identity fusion.” Along with a collaborator named Angel Gómez, he defined it in 2009 as when someone’s “personal and social identities become functionally equivalent.” The border between self and other, as Swann sees it, “become[s] porous.” The phenomenon is sometimes described as a visceral feeling of oneness with a group or person, and sometimes as an expansion of the self.

“When people are fused, your personal identity is now subsumed under something larger,” says Jack Dovidio, a psychology professor at Yale. One way researchers test for fusion is to ask people to draw a circle that represents themselves, and a circle that represents another person (or group). Usually people draw overlapping circles, Dovidio explains. In fusion, people draw themselves entirely inside the other circle.

“This isn’t the normal way most people think about identity,” says Jonas Kunst, a psychology researcher at the University of Oslo. In disagreements over politics, for example, many people believe they can change someone else’s mind with a thoughtful-enough argument. Typically that’s the case; people are willing to challenge their group identities, if reluctantly. In fusion, though, a perceived challenge to the group’s ideology is a challenge to the self. Arguments about climate change, for example, might not actually be about climate change, and instead about people protecting their basic sense of order and consistency.

By a similar token, pundits often chalk “radical” behavior up to pathology, or simply to a vague “mental illness” or religious or political extremism. But fusion offers a framework that involves an ordered thought process. It is thought of as distinct from blind obedience (often assumed to be the case in cults and military violence), in which a person might follow orders and torture a prisoner, either unquestioningly or out of fear for personal safety. In fusion, people become “engaged followers.” These people will torture because they have adopted the value system that views the torture as justifiable. Engaged followers do so of their own volition, with enthusiasm.

Fusion is not a bunch of individuals contorting their way of thinking, but a bunch of individuals suspending their way of thinking. “It makes us more likely to do extreme things that aren’t consistent with our normal identity,” Kunst says. “It allows you to do things you couldn’t conceive of doing.”

Identity fusion might be a new name for a timeless phenomenon, but Swann and others find it helpful as part of an explanation for current social divides. Swann believes that the political landscape accounts for growing interest in the concept, and that better understanding how and why fusion happens could have serious global consequences. It could also just make it easier to understand other people, and to be aware of one’s own susceptibility.

This month, in the journal Nature: Human Behaviour, Kunst and Dovidio examined fusion specifically involving Donald Trump. In a series of seven studies using various surveys, including Swann and Gomez’s “identity fusion scale,” the Yale and Oslo team found that Americans who fused with Trump—as opposed to simply agreeing with or supporting him—were more willing to engage in various extreme behaviors, such as personally fighting to protect the U.S. border from an “immigrant caravan,” persecuting Muslims, or violently challenging election results.

The fusion might explain some apparent contradictions in ideology, Dovidio says. Even people who typically identify as advocates of small or no government might endorse acts of extreme authoritarianism if they have fused with Trump. In fusion, those inconsistencies simply don’t exist, according to Dovidio: Value systems are only contradictory if they’re both activated, and “once you step into the fusion mind-set, there is no contradiction.”

Fusion seems most likely to happen when there is a charismatic leader, particularly of an authoritarian bent. “Humans are social, and the individual person has a power over us that abstract thought doesn’t,” Dovidio says. “The leader is a concrete manifestation of ideas, but allegiance to individuals will trump allegiance to ideas.” In that sense, the idea of fusion might help some people explain how family members or colleagues whom they view as fundamentally good people might seem to suspend their typical sense of morality and do things like downplay Trump’s bragging about groping women; enriching himself at taxpayer expense; defending white supremacists in Charlottesville, Virginia; or failing to release his tax returns despite multiple promises to do so.

The idea of identity fusion is not, the researchers assure me, some effort to use science to overlook or excuse bigotry or racial hatred, which are distinct elements in the formation of identity. Though fusion tends to happen with authoritarian leaders, the fusion is not itself antisocial or bad. It can be seen in political movements of all sorts; Kunst cites followers of Mohandas Karamchand Gandhi. Fusion might have arisen as a psychological adaptation to facilitate cooperation among kin in the face of extreme adversity, explains Harvey Whitehouse, chair of social anthropology at Oxford. Even so, Whitehouse warns, “social institutions could hijack the fusion mechanism in novel ways.”

A sense of deprivation—real or perceived threats to socioeconomic status—also seems to leave people inclined to fuse. “When we primed people to think of relative deprivation, this increased their likelihood of fusion with the leader,” Kunst says, noting that economic recessions have often preceded authoritarian movements. The findings from Kunst and Dovidio’s study suggest that Trump’s continued emphasis on the relative deprivation of his base—and his promise of the power and resources presumably under his control as an apparently wealthy Manhattan real-estate developer and reality-TV star—probably helped his election by increasing his followers’ fusion with him.

Even if this personal enrichment didn’t come to fruition for his voters, the researchers found that fusion with Trump only increased after his election. The presidency itself made him more powerful, and hence a more attractive target to fuse with.

Fundamentally, fusion is an opportunity to realign the sense of self. It creates new systems by which people can value themselves. A life that consists of living up to negative ideas about yourself does not end well. Nor does a life marked by failing to live up to a positive self-vision. But adopting the values of someone who is doing well is an escape. If Donald Trump is doing well, you are doing well. Alleged collusion with a foreign power might be bad for democracy, but good for an individual leader, and therefore good for you. “Fusion satisfies a lot of need for people,” Dovidio says. “When you fuse with a powerful leader, you feel more in control. If that person is valued, you feel valued.”

The process of de-fusing, then, might involve offering alternative systems of creating consistency and order. If people who are inclined to fusion have the option to fuse with entities that do not wish to exploit them, and that are generally good or neutral for the world, they might be less likely to fuse with, say, a demagogue. “But, of course,” Dovidio says, “that’s hard.”

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