Articles with headlines like “Your Lizard Brain” and “Don’t Listen to Your Lizard Brain” get featured on Psychology Today, a magazine whose sales have soared to the top 10 in the nation. Clickbait articles bashing the “basic ‘lizard brain’ psychology” of an opponent political group appear on mainstream news websites. Over the last few decades, MacLean’s theory has become part of the cultural zeitgeist. The neocortex was, MacLean asserted, the crowning jewel of brain evolution – the structure, in other words, which made humans (and perhaps other intelligent mammals) unique. In other words, he thought that we never got rid of the “reptilian” brain we inherited from our reptile ancestors, but instead evolved new brain structures on top of our old reptile brain.īased on these shaky foundations, together with other loose observations regarding what he considered to be uniquely mammalian behavior, MacLean went on to develop a full-blown theory of human brain evolution. The theory held that inside our brains there is a primitive reptilian complex, which is surrounded by an “old” mammalian structure called the limbic system, which is itself surrounded by a “new” mammalian structure called the neocortex. What’s more, MacLean’s claim about the prominence of the globus pallidus in the reptilian brain is false: it forms just one part of reptiles’ brains, exactly as it does in the monkey brain.īased on these loose observations, MacLean argued that we might have a “lizard” brain inside of our brain. Also, to my knowledge, MacLean’s original observations have not been replicated. It’s unclear why cutting out this part of the monkey’s brain made the monkeys stop showing aggressive displays, but this brain area, more commonly called the globus pallidus, is known to be involved in an enormous variety of processes. This behavioral change seemed to fit MacLean’s hunch that he had taken out a “reptile”-like part of the monkey’s brain, since he thought that aggressive gesturing is a typical example of “reptilian behavior.” When MacLean took out this part of a male monkey’s brain, the monkey stopped aggressively gesturing at its own reflection (which it thought was another male monkey). MacLean proposed his (incorrect) theory after he made some curious observations about the effects of cutting out what he called the “reptilian complex” of a monkey’s brain (so named because he thought it looked similar to the tissue that made up most of a reptile’s brain). Under this hypothesis, brain evolution is an additive process: new layers of brain tissue emerge on top of old layers, leading to a tenuous but effective coexistence between the “old brain” and the “new brain.” The triune brain hypothesis, developed by the neuroscientist Paul MacLean between the 1960s and 1990s and widely popularized by the astronomer Carl Sagan, asserts that we have a “lizard brain” under our “mammal brain,” and that our “mammal brain” is itself under our primate/human brain. You may have heard of it as the proposal that we have “lizard brains.” Nowhere is that clearer than in the hugely popular – and entirely wrong – theory called the Triune Brain Hypothesis. But, our history of searching for how, precisely, we came to be exceptional has often led to bad science – and to popular acceptance of bad science. Nothing else in the known universe has produced art, science, technology, or civilization. Despite our best intentions, scientists sometimes make a very basic mistake: we look for what makes humans unique.Ĭertainly, humans are not just unique, but extraordinary.
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