reptilian brains

Reptilian brain refers to the oldest, most primitive part of the human brain—the brainstem and cerebellum—which controls basic survival functions like breathing, heartbeat, and fight-or-flight responses.

The term comes from the idea that this brain structure resembles that of reptiles.

In popular usage, acting from one's reptilian brain means responding with raw instinct rather than reason: Fear triggers the reptilian brain.

The plural reptilian brains typically refers either to multiple people's primitive neural structures or, in conspiracy theories, to alleged shape-shifting aliens disguised as powerful humans.