The “reptilian brain”, seat of our primitive behaviors, right? | Press room

The notion of the reptilian brain fits more broadly into the “triune brain” theory, which has now been invalidated. Credits: Adobe Stock

The “reptilian brain” is a concept that supposedly explains our primitive behaviors, from the most basic needs such as food or reproduction to our most violent impulses. It’s not uncommon to see the term used in personal development books, by advertisers who claim to target the reptilian part of our brain to sell their products, or even by pseudo-therapists who want to help us tame the “crocodile in us.”

The notion of the reptilian brain fits more broadly into the “triune brain” theory developed by neurobiologist Paul MacLean in the 1960s.

This general theory of brain organization relates a set of “primary” attitudes to an archaic part of our evolutionary heritage, for example the sexual instinct, the survival instinct, aggressiveness…

More precisely, the human brain as it is today would, according to this theory, be made up of three “layers”. Each would have developed at different times, and would correspond to a stage in the evolution of the human species. Each would control a specific aspect of our behavior. The oldest of these structures would thus correspond to a brain inherited from reptilian ancestors, home to primary behaviors, while the other two, developed more recently, would be dedicated on the one hand to emotions and on the other to cognition.

Although the theory was quickly deemed incorrect by the scientific community, it enjoyed widespread popularity with the public, which still persists to this day. Canal Détox makes a balance of the foundations of this idea, which has supported the interpretation of numerous research works in psychology developed later.

The limits of a very popular model

If the model developed by Paul MacLean and the concept of the “reptilian brain” have enjoyed such popularity, it is perhaps because it allows a simple explanation of human behavior considered complex and that it allows completing the theories that also developed Freudian ideas. and became popular during the 20th century. MacLean’s theory has long been considered valid by the medical community, until the late 1980s.

However, several problematic aspects were quickly pointed out, particularly from the point of view of evolutionary biology and neuroscience. First of all, it is incorrect to say that the brain has evolved in successive stages, with new and more complex “layers” added over time. Rather, the different groups of vertebrates diverged from each other at different points in geologic time. Thus, there is no basis for the idea that vertebrate evolution consisted of superimposing more recent brain structures on older brain structures to account for the emergence of complex cognition.

The human brain is not made up of complex structures superimposed on a less complex functioning “reptilian brain”, but of structures homologous to those of other vertebrates, but different in their relative sizes and in certain aspects of their organization. . .

Furthermore, there are no purely emotional or purely cognitive circuits in the brain. On the contrary, the knowledge acquired in the field of neurology and neurobiology allows us to affirm that brain functions depend on interdependent networks and not on different brain structures that function in isolation from each other.

Furthermore, if the reptilian brain theory is seriously wrong, it is important to continue scientific research to better understand how different brain regions are connected and to study their dysfunctions. Ultimately, this could open up new diagnostic and therapeutic perspectives in the field of psychiatry, for patients with various pathologies, from depression to schizophrenia.

To go further: Read Sébastien Lemerle’s book, “The reptilian brain, on the popularity of a scientific error” (CNRS Editions, 2020).

Text written with the support of Philippe Vernier, research director of the CNRS, director of the Frédéric Joliot Institute of Life Sciences (CEA) and Xavier Leinekugel, researcher at Inserm, Laboratory U1249

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