According to the dictionary, racism is antagonism directed against a person on the basis of their membership in a racial or ethnic group that is different from your own. As a neuroscientist, I view racism through a unique, but not necessarily clearer, lens.
Given the apparent universality of racism around the world, it stands to reason that racism must be an essential emergent property of brains that has survived throughout our evolution. Given that brains evolved to perform only two things—survive and procreate more brains—then racism must have been conserved as a behavior that supports one of these two goals. The most obvious choice is survival. How does being aggressive towards others that do not share your physical or genetic traits improve your survival?
The answer, from the standpoint of the brain’s functioning, depends upon knowing about the function of a small almond-shaped structure, the amygdala, and its companion, the hippocampus. They both live on the lateral sides of the brain, not far from each ear. The amygdala and hippocampus receive information from your eyes and ears (as well as many other inputs).
These two brain regions assess the information and answer a simple question: is this familiar? If what you are seeing or hearing is unfamiliar according to your hippocampus, the amygdala generates fear. For example, if you are in an unfamiliar location and you see people you do not know or look unfamiliar, then your brain may generate fear. If the information you are seeing or hearing is ambiguous or inappropriate, the amygdala may generate fear. If you notice that someone you do not know is staring at you, your brain may generate fear. Humans hate being stared at. Giving a speech in front of a crowd is horribly frightening to most people. If you were seated with friends in a familiar location and a small child kept staring at you from across the room, you may become fearful. We fear the unknown, the unfamiliar, or the unexpected. Because of this, we may fear people who do not share our appearance or genes or behaviors.
Almost without fail, and regardless of the nature of the information gathered by your vigilant brain, the amygdala usually comes to the same conclusion: be afraid. If everyone is considered dangerous until proven otherwise (such as they are known family or friends), you are much more likely to survive the experience and pass on your be-fearful-first genes to the next generation. Thus, humans tend to fear everything that is unfamiliar or not-like-me: we fear unfamiliar dogs, people who look, speak or dress differently, unfamiliar places, unfamiliar odors, things that go bump in the night, dark alleys, unknown people who follow us, etc. You get the idea. We all have witnessed the consequences of fear: we hide behind closed doors, we hide in protected or gated communities, we keep a loaded gun by every door and under the pillow, we hire bodyguards, or we install security systems or build walls to keep out the others who are not like us.
By now you have gotten the point that being frightened of everything all of the time was an effective way to maintain our species. In order to defend their territory or family, most animals become aggressive when they are frightened. Humans react with the same aggression because of the nature of how our brains evolved to increase our likelihood of survival and the procreation of our species. Humans tend to fear anyone who is different. We fear that they will reduce the likelihood of our survival, our way of life, the safety of our family or friends and we become aggressive in response to that fear. The be-fearful-first approach to survival has worked well for the past two hundred thousand years—but in modern times, it has contributed significantly to racism.
Wenk GL (2017) The Brain: What Everyone Needs to Know (Oxford University Press).