Definition
Essentialism is the cognitive habit of treating a category — especially a social category — as if it picks out a natural kind whose members share a deep, fixed, inborn essence. Under essentialist thinking, a category is not a loose label but a claim about an unchanging inner nature that explains how its members look, act, and think.
Sapolsky describes essentialism as a default mode of human cognition that turns the malleable, statistical reality of human groups into something rigid and seemingly inevitable.
Why it matters
How it works
Essentialist cognition likely evolved as a useful shortcut for biological kinds, where category membership really does predict many hidden properties. The mind then over-applies the shortcut to social categories, projecting the same hidden-essence logic onto groups that are actually defined by culture, history, and statistics.
The result is a tendency to see a person primarily as an instance of a type, to expect uniformity within the type, and to treat differences between types as natural and unbridgeable. Recognizing essentialism as a cognitive bias is what allows it to be questioned rather than mistaken for an observation.