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Revenge of the Lizard Brain

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There’s a scene in Fear & Loathing in Las Vegas in which the writer, high out of his mind on hallucinogens, watches a roomful of casino patrons transform into giant lizards and lunge at each other in bloody combat. Under the veneer of civilization, the scene suggests, we’re all still reptiles, just waiting for the moment to strike.

Strange as it seems, this drug-fueled vision reflects a biological theory that, back in the ‘60s, looked like it might be gaining some traction: Paul MacLean’s infamous “Triune Brain” theory, whose basic idea is that every human brain contains three independent competing minds - the reptile, the early mammal, and the modern primate.

Like the Fear & Loathing scene, the Triune Brain idea holds a certain allegorical appeal: The primal lizard - a sort of ancestral trickster god - lurking within each of us. But today, writers and speakers are dredging up the corpse of this old theory, dressing it with some smart-sounding jargon, and parading it around as if it’s scientific fact. This isn’t just late-night-radio fringe stuff, either: it’s showing up at TED and in Forbes.

To understand what it is, exactly, that these people are claiming, it helps to know a few key points about MacLean’s - shall we say - unique personal views on neuroanatomy. Take, for instance, the basal ganglia - that bundle of neural structures near the base of the forebrain. They’re crucial for learning and reinforcing habits, like nail-biting and toothbrushing. Back in the 1960s, biologists thought the forebrains of reptiles and birds were mainly composed of basal ganglia (they aren’t), so MacLean decided to group these structures, along with the brainstem, under the label “reptilian complex.” This “R-complex,” MacLean claimed, was responsible for the “aggression, dominance, territoriality, and ritual displays” of our distant reptilian ancestors.

MacLean also noticed that some of the more complex neural structures folded around the basal ganglia - such as the amygdala, the hypothalamus and the cingulate cortex - play central roles in emotions like disgust, nervousness, doubt and so on. So he figured these brain areas must’ve arisen in the earliest mammals to cope with tasks like family bonding and child-rearing. He gathered them under a heading and slapped the label “paleomammalian complex” on it.

Finally, MacLean noted that the neocortex - the uppermost, outermost layer of the brain - is found only in mammals, and is linked with “high-level cognitive abilities” like abstract planning, tool-making, language, and self-awareness. Thus, he termed it the “neomammalian complex.”

But MacLean wasn’t done. He went on to hypothesize that these three “complexes” not only represented three distinct stages of brain evolution, but remained three separate, semi-independent brains, “[each] with its own special intelligence, its own subjectivity, its own sense of time and space and its own memory." MacLean was saying, in other words, that every human brain contains three independent subjective consciousnesses.

All in all, a truly mind-blowing trip to lay on your friends. Problem is, MacLean’s pet hypothesis doesn’t hold up under scrutiny. For example:

  1. Basal ganglia are found in the brains of the earliest jawed fish, which means MacLean’s “reptile complex” originated long before the first tetrapods wriggled onto land.
  2. The earliest mammals already had well-formed neocortices, which means at least some “high-level cognitive abilities” predate mammals altogether.
  3. Many reptiles exhibit “paleomammalian” behaviors such as familial bonding and child-rearing, and many birds exhibit “neomammalian” skills like tool-making, verbal comprehension and dialect development.
  4. In functional terms, a human brain doesn’t behave like a series of separate “complexes,” but as a unified whole. Some neural networks do inhibit others - but the shapes of those networks have nothing to do with “reptilian” or “mammalian” layers.

How is it, then, that modern authors as educated as Seth Godin and Rick Hanson (among others) are writing entire essays that present “the lizard brain” as well-documented scientific fact? How does Godin keep a straight face onstage as he tells us that “the lizard is a physical part of your brain” and that “the reason we call wild animals ‘wild’ is because they have lizard brains”?

It’s because the idea makes a weird kind of intuitive sense. We’re bundles of instincts and inhibitions and desires that don’t fit neatly together. It’d be comforting, in a way, if we could pin those conflicts on little lizard brains - just name those ancient demons and drive ‘em out, like we did in simpler times.

Whether we like it or not, though, the lizard is simply us. Every habit and hangup, every dread and desire in our minds is dependent on neural pathways that were once laid down by our personal experiences. Like every other organism on earth, we carry the history of a long, successful lineage in our genetic and biological makeup. The question of what to do with those resources, though, isn’t predetermined by the past. It’s up to you.

Image: Genesis12, modified by Ben Thomas

The views expressed are those of the author and are not necessarily those of Scientific American.

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