Fall of the three brains theory?

 Reptilian Brain Myth Is Still Alive and Kicking

Denyse o' Leary 

Do we have a three-part brain — reptilian, mammalian, and human? Curiously, psychology textbooks teach us that we do and neuroscience studies teach us that we don’t. Who to believe? And how did that happen anyway?

In the 1960s, Yale University physiologist and psychiatrist Paul D. MacLean (1913–2007) offered the triune brain theory. On that view, the reptilian brain (brain stem) controls things like movement and breathing; the mammalian brain controls emotion (limbic system); and the human cerebral cortex controls language and reasoning (neocortex). That might have been just another theory except that it was widely promoted by celebrity astronomer Carl Sagan (1934–1996) in his book The Dragons of Eden (Random House, 1977). Praised in The Atlantic as “a rational, elegant, and witty book,” Dragons won a Pulitzer Prize in 1978, for “a distinguished book of non-fiction by an American author that is not eligible for consideration in any other category.”

Chiming Beautifully

The theory chimed beautifully with materialist thought of the day. The cool people already assumed a long slow process of evolution from mud to mind, with stops along the way for reptile, mammal, and ape. And, as we were constantly reminded, many of us may have got stuck along the way.

But, as neuroscience advanced over the years, unwelcome facts began to surface. The human brain is just not organized as if the story happened in that way. As University of Oslo psychology professor Christian Krog Tamnes puts the matter in an interview at Science Norway, “Those of us who research brain development and brain evolution have known for quite some time that this isn’t true”:

Instead, the cells that are similar to each other were found scattered throughout the brains of both species.

Emotions, such as fear and sadness, are not made in one specific place in the brain. In fact, several parts of the brain are always involved.

Which parts of the brain are active vary from time to time, and from person to person.

For example, Tamnes points to a paper on the topic last year: Despite 320 million years of separate evolution, lizards and mice share a core set of neuron types that are found all over the brain, “including in the cerebral cortex, challenging the notion that certain brain regions are more ancient than others.”

Northeastern University neuroscientist Lisa Feldman Barrett offers, “So if we absolutely need to have a metaphor, it’s much better to think of the brain as an orchestra. Even playing a simple song requires a lot of pieces to talk together effectively and in a coordinated way.”

So we can still have lots of problems but our Inner Lizard is not one of them.

What Psychology Students Are Learning

Psychology lecture rooms and textbooks have been curiously slow to let go of the reptilian brain myth, however. Is that perhaps because it is socially reassuring to think that everyone who questions our sincerely held beliefs is, neurologically maybe, a rat or reptile throwback? In 2020, Joseph Cesario and colleagues reported on a study of what psychology students are told about such matters:

This belief, although widely shared and stated as fact in psychology textbooks, lacks any foundation in evolutionary biology.

“Our experience suggests that it may surprise many readers to learn that these ideas have long been discredited among people studying nervous-system evolution. Indeed, some variant of the above story is seen throughout introductory discussions of psychology and some subareas within the discipline… 

“To investigate the scope of the problem, we sampled 20 introductory psychology textbooks published between 2009 and 2017. Of the 14 that mention brain evolution, 86% contained at least one inaccuracy along the lines described above. Said differently, only 2 of the field’s current introductory textbooks describe brain evolution in a way that represents the consensus shared among comparative neurobiologists.”

CESARIO, J., JOHNSON, D. J., & EISTHEN, H. L. (2020). YOUR BRAIN IS NOT AN ONION WITH A TINY REPTILE INSIDE. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE, 29(3), 255–260.

More information on the textbooks is offered here.

Puzzled by Sagan’s Role

Science writer and editor Ross Pomeroy seems genuinely puzzled by the role Sagan played in helping to popularize triune brain theory: “Carl Sagan was, and to this day is, generally regarded as an honest and skeptical broker of scientific information. That he presented such a disputed theory essentially as fact to the lay public is a bit surprising. What’s more, Carl Sagan continued to push the theory three years later in his far more widely read book, Cosmos.”

It’s not really so surprising if we look at the big picture. First, Sagan was a one-way skeptic. There were many things he was not skeptical about at all because they suited the popular worldview he shared and helped shape. 

For example, as Justin Gregg recounted in 2013, in 1961, he joined a semi-secret society called the Order of the Dolphin, which sought a way to communicate with intelligent extraterrestrials. He bought into the idea that dolphins had a sort of super-intelligence and a language like ours. The theory was that if we could decipher that language, we could decipher any extraterrestrial one. The Order was certainly dedicated. Gregg recounts, “As the Princeton historian D. Graham Burnett has noted, members wore insignia shaped like bottlenose dolphins and sent each other coded messages to hone their dolphinese and alien-language-decoding skills.”

Were They Nuts?

It might seem so at this distance. But club members back then included evolutionary biologist J. B. S. Haldane (1892–1964) and chemistry Nobelist Melvin Calvin — alongside SETI founder Frank Drake (1930–2022).

The lesson here is that science functions better when we follow the evidence, as the neuroscientists are doing, than when we form fan clubs for cozy ideas championed by science celebs, as the psychologists appear to be doing — at least in this area.