To better understand the roots of racial division in America, think about this:

The human brain seems to be wired so that it categorizes people by race in the first one-fifth of a second after seeing a face. Brain scans show that even when people are told to sort people by gender, the brain still groups people by race.

Racial bias also begins astonishingly early: Even infants often show a preference for their own racial group. In one study, 3-month-old white infants were shown photos of faces of white adults and black adults; they preferred the faces of whites. For 3-month-old black infants living in Africa, it was the reverse.

This preference reflected what the child was accustomed to. Black infants living in overwhelmingly white Israel didn’t show a strong preference one way or the other, according to the study, published in Psychological Science.

Where does this ingrained propensity to racial bias come from?

Scholars suggest that in evolutionary times we became hard-wired to make instantaneous judgments about whether someone is in our “in group” or not --- because that could be lifesaving. A child who didn’t prefer his or her own group might have been at risk of being clubbed to death.

“It’s a feature of evolution,” says Mahzarin Banaji, a Harvard psychology professor who co-developed tests of unconscious biases. These suggest that people turn out to have subterranean racial and gender biases that they are unaware of and even disapprove of.

I’ve written about unconscious bias before, and I encourage you to test yourself at implicit.harvard.edu. It’s sobering to discover that whatever you believe intellectually, you’re biased about race, gender, age or disability.

What’s particularly dispiriting is that this unconscious bias among whites toward blacks seems just as great among preschoolers as among senior citizens.

Banaji’s research projects show that unconscious racial bias turns up in children as soon as they have the verbal skills to be tested for it, at about age 4. The degree of unconscious bias then seems pretty constant: In tests, this unconscious bias turns out to be roughly the same for a 4- or 6-year-old as for a senior citizen who grew up in more racially oppressive times.

In one set of experiments, children as young as about 4 were shown ambiguous photos of people who could be white or Asian. In some, the people in the photos were smiling; in others, they were frowning.

White American kids disproportionately judged that the people who were smiling were white and that those who were frowning were Asian. When the experiment was conducted in Taiwan with exactly the same photos, Taiwanese children thought that the faces when smiling were Asian, when frowning were white.

The American children were also shown faces that were ambiguous as to whether the person was white or black. In those cases, white kids disproportionately thought that the smiling people were white and the frowning ones were black.

Many of these experiments on in-group bias have been conducted around the world, and almost every ethnic group shows a bias favoring its own. One exception: African-Americans.

Researchers find that in contrast to other groups, African-Americans do not have an unconscious bias toward their own. From young children to adults, they are essentially neutral and favor neither whites nor blacks.

Banaji and other scholars suggest that this is because even young African-American children somehow absorb the social construct that white skin is prestigious and that black skin isn’t. In one respect, that is unspeakably sad; in another, it’s a model of unconscious race neutrality. Yet even if we humans have evolved to have a penchant for racial preferences from a very young age, this is not destiny. We can resist the legacy that evolution has bequeathed us.

“We wouldn’t have survived if our ancestors hadn’t developed bodies that store sugar and fat,” Banaji says. “What made them survive is what kills us.” Yet we fight the battle of the bulge and sometimes win --- and, likewise, we can resist a predisposition for bias against other groups.

One strategy that works is seeing images of heroic African-Americans; afterward, whites and Asians show less bias, a study found. Likewise, hearing a story in which a black person rescues someone from a white assailant reduces anti-black bias in subsequent testing. It’s not clear how long this effect lasts.

Deep friendships, especially romantic relationships with someone of another race, also seem to mute bias --- and that, too, has implications for bringing young people together to forge powerful friendships.

“If you actually have friendships across race lines, you probably have fewer biases,” Banaji says. “These are learned, so they can be unlearned.”