Scientists debunk the enduring myth of the human "lizard brain"

For decades, popular psychology and business culture have embraced the notion that a primitive "lizard brain" lurks beneath our rational thoughts, driving impulsive behaviour. This idea traces back to 1959, when scientist Paul D. MacLean introduced his "triune brain" model.

The theory proposed that the brain consists of three layers stacked on top of each other: a deep, ancient "reptilian" core responsible for survival, foraging and reproduction; a surrounding "mammalian" layer containing the limbic system, which governs emotion and nurturing behaviour; and an outer "neomammalian" layer — the cortex — providing humans with reasoning and language. MacLean argued this final layer kept our primitive impulses in check.

The concept spread widely into self-help books, business training and pop psychology, built around the idea that people could use their "rational" brain to suppress unhelpful emotional reactions. However, contemporary neurobiologists now consider the model deeply misleading, since evolution doesn't produce such neatly separated mental layers. Research shows many reptiles actually possess limbic-like structures, while mammals display features once thought unique to "higher" species.

Guy Leschziner, a consultant neurologist in London, says the idea of distinct brain systems separately driving instinct, emotion and logic simply doesn't hold up against evolutionary or physiological evidence. Lisa Feldman Barrett, a psychology professor at Northeastern University, argues instead that the brain functions as one integrated system focused on predicting and managing the body's energy needs, rather than as a battleground between instinct and reason.

Despite the evidence against it, the triune brain model continues to appear in introductory psychology textbooks, which critics say fosters a flawed understanding of evolution and reinforces the misconception that only humans are capable of truly rational behaviour.