The notion of the reptilian brain fits more broadly into the theory of the “triune brain”, which has now been invalidated. Credits: Adobe Stock
The “reptilian brain” is a concept supposed to explain our primitive behaviors – from the most basic needs such as food or reproduction to our most violent impulses. It is not uncommon to see the term used in personal development books, by advertisers claiming to target the reptilian part of our brain to sell their products, or even by pseudo-therapists who wish to help us tame the “crocodile in us”.
The concept of the reptilian brain fits more broadly into the theory of the “triune brain” developed by the neurobiologist Paul MacLean in the 1960s.
This general theory of the organization of the brain relates to an archaic part of our evolutionary heritage a set of “primary” attitudes, for example the sexual instinct, the survival instinct, aggressiveness…
More precisely, the human brain as it is today would be, according to this theory, composed 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, the seat of primary behaviors, while the other two, developed more recently, would be dedicated on the one hand to emotions and on the other hand to cognition.
Although the theory was quickly deemed wrong by the scientific community, it nonetheless enjoyed great popularity with the public, which still persists to this day. Canal Détox takes stock of the foundations of this idea which supported the interpretation of many 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 makes it possible to simply explain human behavior considered complex and that it makes it possible to complete the theories Freudian ideas also developed and became popular during the 20th century. MacLean’s theory has long been considered valid by part of the medical community, until the end of the 1980s.
However, several problematic aspects were quickly pointed out, in particular the evolutionary biology perspective 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. On the contrary, the different groups of vertebrates diverged from each other at different points in geological time. There is therefore no basis for the idea that the evolution of vertebrates consisted in superimposing more recent cerebral structures on older cerebral structures, to account for the emergence of a complex cognition.
The human brain is not made up of complex structures superimposed on a “reptile brain” with less complex functioning: rather, it is made up 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. The knowledge acquired in the field of neurology and neurobiology, on the contrary, makes it possible to say that the cerebral functions depend of interdependent networks and not distinct brain structures, which function in isolation from each other.
Also, if the theory of the reptilian brain is very erroneous, it is nonetheless important to continue scientific research to better understand how the different regions of the brain 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 the book by Sébastien Lemerle, “The reptilian brain, on the popularity of a scientific error” (CNRS editions, 2020).
Text written with the support of Philippe Vernier, CNRS research director, director of the Institute of Life Sciences Frédéric Joliot (CEA) and Xavier Leinekugel, Inserm researcher, Laboratory U1249