Since the election of Donald Trump, President Barack Obama has shifted into a prophylactic stance. He is warning that the world is complex, not a simple collection of binaries where things are either completely fantastic or the absolute worst.

Obama sees, rather, an ecosystem, a global community where borders are increasingly illusory, where prosperity for one economy means prosperity for others. Still this is a difficult concept to impart even to competitive students or coworkers, much less a population of seven billion. Recounting how he explained the election outcome to his daughters, Obama said in The New Yorker this week, “This is not mathematics; this is biology and chemistry. These are living organisms, and it’s messy.”

If biology is messy, neurobiology is a dumpster filled with smaller dumpsters, all ablaze. We are competitive by nature, as a matter of survival, and this tendency is easily goaded into hate.

This goading isn't always so overt as Trump's sweeping claims about immigrants, Muslims, and “the media.” The us-versus-them construct is a tool he leverages in less obvious ways, as when he said this weekend, “I watched parts of Saturday Night Live last night. It is a totally one-sided, biased show - nothing funny at all. Equal time for us?” Who is us?

This is a stoking of tribalism, a play on our biology and chemistry. Historical odds favor Trump’s success. Dividing is easy. But these tactics can be understood and anticipated. A population determined to stay united can be immunized.

For most of our evolutionary history, tribal humans acted on cues from appearances, drawing information about who to trust based on how people looked. Similarity was a cue to kinship, and kinship a cue to safety. We came to make these judgments very quickly, according to Alexander Todorov, a psychology professor who runs Princeton University’s Social Perception Lab. His work has shown that we make up our minds about others after seeing their faces for a fraction of a second. These snap judgments have been shown to predict economic, legal, and other decisions even today.

As tribes came together into societies, new systems of self and other came to be. Institutionalized religion arose as societies grew centralized—from tribes with particular supernatural beliefs into states with dogma that helped maintain order. As Jared Diamond wrote in Guns, Germs, and Steel, “Religion helps solve the problem of how unrelated individuals are to live together without killing each other.”

Snap judgments based on appearances may once have been beneficial and evolutionarily adaptive from a health perspective—such as aversions to faces that are unfamiliar to us because they may indicate illness. A genetic disorder, for instance, might come with facial disfigurement. There was once some merit to that concept, but in today’s global community, those snap judgments are at best rarely accurate. Leslie Zebrowitz and Gillian Rhodes have called this the “anomalous face overgeneralization effect.”

Societies have globalized, but instincts to demarcate self and other—and to rush to judgments accordingly—persist. As Obama told The New Yorker, “Your job as a citizen and as a decent human being is to constantly affirm and lift up and fight for treating people with kindness and respect and understanding. You should anticipate that at any given moment there’s going to be flare-ups of bigotry that you may have to confront, or may be inside you and you have to vanquish.”

There is, now, a flare-up. But the problem can be approached empirically. This month in the journal Nature Human Behavior, Todorov’s group at Princeton has backtracked on some of its prior work about what makes people judge certain faces certain ways, saying that tendencies can quickly be reprogrammed by new exposures.

I’ve covered Todorov’s work on facial bias before (“The Introverted Face”), in which he and fellow psychologists Christopher Olivola and Friederike Funk  found that people tend to project characteristics like introversion and extroversion onto people based on features like this:

(Trends in Cognitive Sciences, Olivola et al. / The Atlantic)

Todorov first became interested in how these stereotypes came about after studying the judgments people made about elected officials. When people were relatively uninformed, the appearance of a candidate mattered a lot. The image at the top of this piece is a rendering of the faces that he found to be least and most attractive on average—and that attractiveness went along with being deemed trustworthy and competent.

Of course, Todorov’s years of research showed that inferences people made based on appearances alone—about who was trustworthy, who was competent, et cetera—were grossly inaccurate. Still when we “go with our gut,” these sorts of judgments are often what that comes down to. And in their new study, Todorov and colleagues found that across cultures, these biases are inconsistent. The research team showed people hundreds of faces and asked them to judge people (in terms of trustworthiness, attractiveness, competence, et cetera). The scientists found that the true underlying factor was our own experience of normalcy. We see faces more negatively the more they deviate from a learned idea of a “typical face.”

“Although there is no ‘average’ human face, you like faces that are closer to your own definition of a typical face,” Todorov told me. “It’s like a tribal bias. I like people that look like the people around me.

That might not be especially revelatory, but this is: The idea of a “typical face” is malleable, and rapidly so. When his experimental subjects stared at faces for just a half hour, their sense of normalcy was partially reprogrammed.

“The idea is it’s not just about who looks like you, or your lifelong learning about— say, who looks masculine or feminine, and what that means,” Todorov said, “but it’s a lot about who you’ve been around in the very recent past. The positive finding that we can overcome biases by exposure more easily than once thought.”

Exposure to different faces not only shifts what faces people perceive as typical, but also what faces they evaluate more positively. The more segregated we become, the more we’re likely to narrow our understanding of normal appearances––to appraise The Others more negatively.

The maps of where people voted for Clinton and Trump are clear. In urban areas where people live in close proximity to great diversity, the vote was overwhelmingly Democratic. Trump’s support came from monolithic non-urban areas.

If you have a diverse friend/colleague group in a multicultural city, I asked Todorov, do you have a broader set for judging normalcy?

“I would think so,” he said. “Of course, in real life, it’s much more complicated. You can live in a diverse city where there are strong divisions between groups, and that would affect how you evaluate people. But assuming that you have kind of egalitarian relationships and a big social network that's diverse, and you live in a place like New York City, yes, your notion for typicality would be very different than if you grew up and spent most of your time in a rural area that has primarily one particular race or ethnicity.”

The traditional “contact hypothesis” in sociology is that if you put people together from different social and economic backgrounds, they get to know each other, and they rely on each other, and unity follows. Classically that came with qualifying conditions: You can put people together, but they have to be equal status. And they have to be dependent on each other, as in the army. When one group has more than the other, stereotypes are more likely to be confirmed than disconfirmed. (This has recently been disputed, with some experts arguing that contact alone is sufficient.)

In this way, the safeguard of the American idea may be contact. We may be wired to be competitive and tribal, but we are also wired to depend on one another in communities. We are wired to be reliant on others, to feel pain when we see people in pain, to cry when others cry, to perceive something as funny when we hear a laugh track. The limit of our biology is where we draw the lines around what we consider our community. And this, it seems, comes down to exposure.

President Obama has assured Americans that Donald Trump’s success is our success. On this history and biology are less reassuring. Trump proposes to build a tremendous wall; his chief strategist, Steven Bannon, proposes to pit the United States against other nations. In 50 years, the Senate Judiciary Committee has rejected only two federal judges. One, in 1986, was denied  by a Republican-dominated Senate on grounds of racism. That man, Jeff Sessions, is now Trump’s nominee for attorney general of the United States.

The success of people who oversimplify and divide does not mean success for a population. Against the instincts of these men’s neurochemistry and limited experiences—or manipulation of ours—the biological truth is that there is only one population.