Remarkably, even though participants knew they had been randomly assigned, their brains showed the same patterns of ingroup allegiance and outgroup disregard. “Us” versus “them” does not need to be predicated on deep meaning: even arbitrary labels rapidly create bias.
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My sister learned she was a carrier for a recessive disease, Bloom syndrome, late in one of her pregnancies. I remember the panicked call and the weeks of worry as she and her husband awaited his test results; if he was also a carrier, this meant their daughter had a one in four chance of being born with the disorder.