On November 18, 2016, ten days after Donald Trump won the Presidential election, graffiti appeared on a Brooklyn Heights playground named after Adam Yauch, a founding member of the Beastie Boys. Yauch, who died in 2012, was Jewish; a vandal had spray-painted two swastikas on the equipment and, beneath them, had written, “Go Trump.” The incident received national attention not just for its hateful nature but because it happened in a liberal enclave. To me, though, one of the most disturbing aspects wasn’t the swastikas themselves but the fact that they had been drawn incorrectly—one was backward and the other was misshapen. Apparently, the person engaging in hate speech didn’t know what a Nazi swastika looked like. This was someone trying on the role of anti-Semite for size—someone who hadn’t been a rabid neo-Nazi his whole life but who felt emboldened by the election of the new President. It was a new behavior prompted by a new event. Were such incidents, then, the new “normal”? And, if this shift of norms could happen with such speed, in such an improbable location, then how quickly and how much might the norms of our whole society change?

Understanding the psychology of changing norms starts from a simple insight: although we may wish to be perfectly rational and impartial, bias is an inescapable part of what it means to be human. At three months old, we already prefer the faces of people who share our skin color over the faces of those who don’t. By five years old, we’re aware of our group’s status and have imbibed certain community ideas about how various groups are perceived and treated. As we grow older, these ideas are constantly reinforced by popular culture, our social environments, and even our language and symbolism. The question, therefore, isn’t “Do biases exist?” but, rather, “How much do we let them affect our behavior?” In 1990, Susan Fiske and Steven Neuberg, then psychologists at the University of Massachusetts, Amherst, and Arizona State University, respectively, described the process by which bias sways behavior using what they called the “continuum model of impression formation.” According to their model, our reliance on stereotypes in decision-making exists on a continuum and shifts by degrees, rather than operating in absolutes. No one is ever bias-free, but some people let their biases influence their actions more than others. “You can’t help it if you live in a certain culture,” Fiske, who is now at Princeton University, explained to me. “But are you motivated to go beyond stereotype?”

To a large extent, our motivation to overcome our biases depends on implicit social norms, which we assimilate from a variety of sources. Sometimes we find them in the environment; people are more likely to litter in a dirty place than in a clean one, for instance. We also find them in the behavior of people we respect, or who occupy positions we respect. If someone in a powerful position acts in a certain way or expresses a certain view, we implicitly assume that those actions and views are associated with power, and that emulating them may be to our advantage. As a result, while our biases may be slow to change—they’re based on long-standing stereotypes, and we have been learning them since birth—our norms can shift at the speed of social life. We might think of anti-Semitism as stemming from deeply rooted beliefs, and, in some sense, that’s true, but the expression of anti-Semitism depends on highly changeable facts about our social environment.

Betsy Levy Paluck, a psychologist at Princeton University who just received a MacArthur “genius” grant, has spent her career studying how shifting social norms affect behavior. In the early aughts, she studied the profound shift in relations between the Hutus and the Tutsis at the time of the Rwandan genocide. Prior to the carnage, Paluck told me, “Hutus reported such good relations with their Tutsi neighbors”; then, in an instant, one group massacred the other. What Paluck observed during her time in Rwanda wasn’t the power of age-old hatreds—Hutus and Tutsis had always had stereotyped ideas about one another—but of quickly shifting social norms. To a great extent, the norms in Rwanda shifted so rapidly because they did so from the top: influential radio stations broadcast a powerful, persuasive, and constantly repeating message urging listeners to join killing squads and organize roadblocks. “That was the voice of authority,” Paluck explained. Suddenly, people saw violence as something that wasn’t just possible but normal.

The voice of authority speaks not for the one but for the many; authority figures have a strong and rapid effect on social norms in part because they change our assumptions about what other people think. In the United States, one way to study that effect is to examine the decisions of the Supreme Court, a universally acknowledged source of authority. In a study in the September, 2017, issue of Psychological Science, Paluck and Margaret Tankard, of the RAND Corporation, look at the change in American attitudes toward same-sex marriage before and after the Supreme Court decision that established it as a constitutional right, in June, 2015. In the months before the decision, Paluck and Tankard surveyed people in cities all over the country; they then repeated the survey after the decision was announced. They found that, while personal opinions on same-sex marriage hadn’t shifted in the wake of the ruling, people’s perception of others’ opinions had changed almost immediately. Americans, whether liberal or conservative, thought that their fellow-citizens now supported same-sex marriage more than before, even though, in reality, the only thing that had changed was the ruling of a public institution. The impression created by the ruling was that “more Americans currently support same-sex marriage, and that even more will support it in the future,” Paluck said.

Our tendency to extrapolate the opinions of others from the opinions of authority figures helps explain phenomena like the incorrectly drawn swastikas on the playground in Brooklyn. The psychology of norms suggests that you don’t “need a nation of raging anti-Semites to license the use of anti-Semitism as a social weapon,” Paluck said. Instead, an authority figure could make the expression of anti-Semitism—an old bias that had previously been subtle, implicit, and almost imperceptible—suddenly appear to be one of the broadly “acceptable” ways of showing pent-up anger. “A leader could whip up everyone’s frustration and channel it to these scapegoats and make it normative to use this language,” Paluck said, “encouraging people to say, ‘Ah, this is how to express my frustration, to lash out against liberalism and so-called élites.’ ” Such an authority figure can create the impression of a social consensus where none exists.

What can we do to counteract the rise of violent and hateful new norms? The social psychologist Bibb Latané argues that norms are more readily transmitted when the person modelling them has a high degree of personal influence and is physically close by the person absorbing them; a student, for example, is more likely to be affected by her professor than by a fellow-student or a professor at another school. One possibility, therefore, is to call upon influential people in small communities to fight the perceived consensus created by larger authority figures. If the President suggests that some neo-Nazis are “very fine people,” but those in positions of power closer to you—such as a pastor, principal, or governor—speak out against him, you’ll be more likely to call into question the new normal that the President has modelled. The new behavior will look more like an outlier than like a norm.

Last year, Kevin Munger, a Ph.D. candidate at New York University, found a novel way to test this hypothesis. He created Twitter bots that would speak out against racist harassment by automatically tweeting at users who had previously tweeted anti-black slurs. All the bots were made to appear male, but they varied along other dimensions: they were either white or black, and they had either few followers (that is, not much of a perceived influence) or many. Munger found that one particular group was able to shift behavior: white men who appeared to be influential. After receiving just one admonishment from such a user (for example, “Hey man, just remember that there are real people who are hurt when you harass them with that kind of language”), people significantly reduced their use of slurs over a period of two months.