Hannah Critchlow writes:

In 1959 Paul D MacLean set out a new way of thinking about the evolution and function of the brain, which he christened the “triune” – three-in-one – model. It quickly became one of best known and most enduring concepts in human biology. His idea was that a trio of distinct brains are layered inside our skulls, one on top of the other, all fighting to generate our feelings and thoughts.

Deep inside, at the centre, lurks the primitive lizard brain, retained from an ancestor we share with reptiles. This contains ancient regions responsible for survival, helping us to hunt and forage, and driving our sexual impulses so that we reproduce. A second layer sits wrapped around the first, the so-called mammalian brain. It contains the limbic system, deemed to be the seat of our emotions. It enabled mammals to play, care for and nurse their young and communicate in some crude way.

The third and final layer, sheathing all of this and making humans in particular a cut above, is the “neomammalian” brain: the eminently rational cortex. This, it was proposed, provides us with our superior intellect, capacity for language and ability to reason. According to MacLean, it helped to keep in check our primitive, lizard-like impulses and rudimentary emotional reactions.

Human behaviour was thought to be a result of interactions between all three layers of the triune brain. Frayed tempers, impulsive acts and short termism were signs of the lizard brain winning out. The notion of reptilian impulses “breaking through” proved so popular it found its way into the worlds of pop psychology, business and self-help, and has proved remarkably tenacious. A mini-industry of books, talks and training grew up around the idea that you could draw on the power of the neomammalian brain to keep basic, unhelpful or destructive emotions in check.

The underpinning analogy – of an onion-like brain made up of different strata – is now considered deeply misleading by neurobiologists. It’s easy to grasp, sure, and tells a compelling story. But evolution just doesn’t bolt-on neatly divisible mental capacities in this way.

MacLean conducted dissections of living creatures to try to prove his theory, but the results, and his interpretations of them, have since been largely discredited. [Continue reading…]