The answer is in the EO Wilson article you posted:

Posted on: November 23, 2018 at 15:09:32 CT

Today, the social world of each modern human is not a single tribe but rather a system of interlocking tribes, among which it is often difficult to find a single compass. People savor the company of like-minded friends, and they yearn to be in one of the best—a combat Marine regiment, perhaps, an elite college, the executive committee of a company, a religious sect, a fraternity, a garden club—any collectivity that can be compared favorably with other, competing groups of the same category.

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When in experiments black and white Americans were flashed pictures of the other race, their amygdalas, the brain’s center of fear and anger, were activated so quickly and subtly that the centers of the brain were unaware of the response. The subject, in effect, could not help himself. When, on the other hand, appropriate contexts were added—say, the approaching African-American was a doctor and the white his patient—two other sites of the brain integrated with the higher learning centers, the cingulate cortex and the dorsolateral preferential cortex, lit up, silencing input through the amygdala. Thus different parts of the brain have evolved by group selection to create groupishness, as well as to mediate this hardwired propensity.