Inside you are three animals: a primitive crocodile, a panicky mouse, and a pragmatic human being. No, this is not related to听鈥攐ne good, one evil, the winner being the one you feed; rather, these three metaphorical animals are thought to occupy the inside of your head, fighting for control over your body.
We have all heard that we have a lizard brain. In fact, popular wandering hero Jack Reacher is said to have a highly reliable lizard brain, a sort of primitive instinct that alerts him to what is really going on. On top of this ancestral brain, we are said to have evolved a second brain, full of emotions, shared with mammals like mice and horses. Lizards are detached, but mammals? We experience fear, anger, joy, defiance, and longing. And crowning this limbic system is what makes us uniquely human: a third brain capable of speech, of contemplation, and of complex thoughts.
Even I was still carrying around this notion until last week, and it鈥檚 no wonder: it鈥檚 being taught in universities. A听听took a look at 20 introductory textbooks on psychology. The majority mentioned brain evolution鈥攚hich is a good thing鈥攂ut almost all of those made the mistake of teaching students about the lizard brain.
It will come as a shock, then, that neurobiologists have known since the 1990s that this simple and enticing idea is just not true. Its origin goes back to 1959鈥攁lthough it draws from Western cultures鈥� obsession with trinities鈥攁nd it got a boost from a well-respected science communicator: none other than Carl Sagan himself.
Welcome to Triune Park
The brain is not an easy organ to understand. The heart is a pump; but the听听inside the human skull has proven to be a more sophisticated mystery. Aristotle suspected it was there to cool the blood. In truth, it is a sort of biological computer that handles what we perceive is happening both inside and outside of our body.
Before the 1950s, scientists often thought of this organ as being sort of like two brains in one. There was the brain that produced emotions and the brain that could intellectualize, the brain that desired that second bowl of ice cream and the brain that reminded us that our stomach has a finite capacity. Paul D. MacLean would change that and put forward a new way of thinking about the brain. He called it the 鈥渢riune model of the brain.鈥� To him, there were three distinct brains inside the human skull, and their struggle for dominance helped explain why we pesky humans were capable of such good听and听such evil.听
MacLean was born in 1913 in the state of New York and his desire to experiment was manifest at an early age. According to听听of his career, he was 3 or 4 years old when he decided to put the Old Testament to the test. He went into a closet, took the Lord鈥檚 name in vain, and waited to be struck by lightning. His decades-long career illustrated that, at the very least, divine punishment was not necessarily immediate.
He had his heart set on becoming a philosopher until his mother appeared to have a heart attack. She was actually passing a gallstone, but the incident is said to have sent him to Yale University to study medicine instead. He took care of traumatized soldiers in the Pacific theatre of the Second World War and became obsessed with the duality of Man: so bright yet so dumb. How could these two brains coexist?
When he was invited to become the director of a soon-to-be brain centre at the National Institutes of Health, he dreamt big. He imagined a large, central laboratory with radiating spokes housing a Noah鈥檚-ark-like selection of animals: insects, fish, reptiles, birds, and mammals, with the entire institute fronted by gardens and fountains. I can just about hear John Hammond smiling and whispering, 鈥淲elcome to Jurassic Park.鈥�
The laboratory that was ultimately built, condensed down from his plans due to financial realities, would only open in 1971; the 12 years leading up to this, however, is when he would formulate the idea that so many people today still believe.
MacLean had come to think that the human brain could be divided into three structures that were radically different, and while they were interconnected, they also had, in a manner of speech, a mind of their own.
There was a primitive brain we had retained from lizard ancestors. For fans of anatomy and Latinate nomenclatures, he was听听the olfactostriatum, the corpus striatum, the globus pallidus, and to satellite grey matter. This lizard brain ensured our self-preservation and the survival of our species. It was responsible for the kinds of basic instincts that reptiles have. It made us set up a home site for our tribe. It pushed us to patrol it, and to forage and hunt for food. It encoded our courtship rituals and nudged us to migrate when necessary. This lizard brain was what lizards had and what we had inherited from them. It was old.
Mammals, according to MacLean, had developed another brain layer on top of this one. It was a series of structures MacLean had baptized the 鈥渓imbic system鈥� in 1952, from the Latin word for 鈥渆dge鈥� as it was seen surrounding the brainstem and creating a border alongside it. The limbic system was the seat of our emotions. Think of a frightened horse, an angry cat, a crestfallen dog. This limbic system gave us abilities that reptiles apparently did not have: it made us nurse our youngs and use our mouths to produce sounds and crudely communicate, and it introduced us to the concept of play.
But humans were even more special than that. We had a third structure, a neomammalian brain, that gave us language and intellect. Simplistic diagrams of this triadic arrangement make it look like a balloon inside of a balloon inside of another balloon. Obviously, the brain does not look like a balloon set of Russian dolls; but MacLean thought that structures in the brain could be joined together and identified in this top-middle-bottom arrangement.
This triune, which MacLean first formulated in 1959 and which he named in the early 1970s, must have听felt right. After all, Western cultures are听. Plato had divided the people into wise guardians, passionate soldiers, and greedy proles, wondering if a single person might not contain within itself the same three divisions he was seeing in society. Thinkers had imposed trinities onto the brain before MacLean, with some speaking of 鈥渃ognition, affection, and conation鈥� and Freud describing the trio of the superego, the ego, and the id. A hierarchy of three brains in one made sense.
It certainly titillated Carl Sagan, who had seen MacLean speak of this triune at a 1971 scientific conference. Sagan, a planetary scientist who would become a famous popularizer of science thanks to his writings and to his television show听Cosmos, wrote a letter to MacLean in which he drew a similarity between this 鈥渓izard brain / mammalian brain / human brain鈥� model he had proposed and the work of Plato. And then his letter took a bit of a turn: if this model was evolutionarily true, 鈥渋t should be possible to demonstrate it by introspection, or perhaps by the use of psychotropic drug.鈥� Sagan was not just a science communicator but an advocate for the decriminalization of cannabis. He wrote听听about how smoking pot helped him appreciate music, revisit childhood memories, and enhance sex. In this letter, one can imagine Sagan anticipating the movie听Altered States听and wishing to revert back to the primitive lizard living inside his brain with the right mind-altering drug.
A few years later, Sagan published his book on evolution and human intelligence entitled听The Dragons of Eden.听Years later, MacLean would听听that one of the worst things in this world was to be discovered by the popular media;听The Dragons of Eden, which won the Pulitzer Prize,听communicated the idea of the lizard brain to Sagan鈥檚 readers, and it started to seep into mainstream culture even though it had been framed as speculation. It was so appealing, this idea that evolution simply stacked a new organ on top of an old one, and the latter could be blamed for all of our ill-advised compulsions.
In 1990, MacLean plucked his research out of academia and put it in a lengthy book, the title of which might put off a few casual readers. It was called听The Triune Brain in Evolution: Role in Paleocerebral Functions. But it was too late to be taken seriously by neuroscientists. Many of the ideas it contained were already known to be scientifically wrong.
We did not, after all, have three brains fighting for supremacy.
The pancake model was too simple
In his prestigious centre, MacLean experimented on animals鈥攈amsters, lizards, dogs, rats, mice, turkeys, and squirrel monkeys鈥攔emoving one of their three putative brains and observing changes in behaviour. His theory seemed to be vindicated by this work: cutting the 鈥渓izard brain鈥� out of a male monkey鈥檚 brain stopped it from aggressively gesturing at its own reflection, which it had mistaken for a competitor. That ancestral instinct was now gone.
But his critics pointed out that MacLean鈥檚 research into the triune brain seemed motivated by听trying to prove itand not disprove it. There were alternative explanations to his findings and, more importantly, our evolving knowledge of the brain just did not add up to his layering hypothesis.
That limbic system mammals have? Reptiles, an informal category which includes lizards and snakes, have one too.
And that neomammalian brain unique to humans? It鈥檚听. Every mammal has one. It鈥檚 not even brand new; it鈥檚 a modified version of what reptiles have.
Evolution听听like it鈥檚 trying to control a radio with a flatscreen television. As听听put it, the trunk of an elephant has not been superimposed over a snout; it鈥檚听like听a snout but longer. And lest we get sucked into MacLean鈥檚 mid-century ideas about the exceptionality of Man, we know that octopuses, crows, and parrots are not mere sacs of emotions. They are听clever.
MacLean鈥檚 research was also regrettably motivated by ideas that have not aged well, such as his fear of overpopulation and how overcrowding on our planet might lead to听
Our current understanding of the brain, I am sad to report, is just not as easy to understand as three pancakes stacked on top of each other, with the lizard pancake dictating our survival, the mammalian pancake damning us with emotions, and the human pancake making us peculiarly smart and rational. Emotions are not restricted to what MacLean called the limbic system, and our so-called limbic system (a phrase that has fallen out of use) is not merely concerned with processing feelings. In fact, our entire brain communicates with different regions of itself to a profoundly intricate degree. Our brain听has听regions, sure: we have a visual centre at the back of our head, and we have areas that process language, for example. But evolution did not pile up brains like layers of rocks and sediments showing us the age of the Earth.
The lesson here may be that psychologists could benefit from talking with neurobiologists a bit more. A lizard brain, as a metaphor, may be useful to get a patient to understand their impulses and deep desires; but it perpetuates a myth, and psychology is already guilty of propagating a couple of them. Contrary to what you may read in a university-level psychology textbook, Kitty Genovese did not bleed to death because听38 witnesses failed to act, and the Hawthorne effect is not the idea that changing anything in a woman鈥檚 environment will cause her to听work harder.
Instincts and intuitions are real. Emotions can indeed be like a horse bolting out the door, carrying with it all hope at behaving rationally. Yet our brain is not neatly divided into geological layers. We have more in common with lizards than we might like to admit, and many of the animals around us are capable of cleverness.
Some scientific hypotheses get canonized despite what we now know because of how simple and satisfying they feel. The world, unfortunately, is full of complexity, and our brain does not escape that.
Take-home message:
- The idea that the human brain has, at its core, an ancestral lizard brain is not true.
- This hypothesis, called the triune model of the brain, comes from Paul MacLean, who conducted animal research in order to show that the human brain was in fact three organs in one: a lizard brain for survival, a limbic system for emotions, and a brand-new human brain for rationality.
- Evolution does not create new brains on top of old ones; our human brain has a lot in common with animal brains, and its many regions are densely interconnected.
