Why do so many people keep saying that we have three brains?

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I recently received an email from a student -from a university other than the one where I work- in which he asked for my collaboration to validate an instrument for his degree project. The student wanted to “describe the degree of influence and persuasion that can be generated in a certain group of civilians, based on advertisements constructed to capture a certain stimulus in the mind of the consumer. Basically, to create responses and behaviors from an audiovisual product that addresses one of the three brains of the human being (cortex, limbic and reptilian)” (my italics).

I diplomatically declined, but his conception of neuromarketing seemed so distorted that I felt compelled to write about the triune brain, a concept that has become terribly contagious in the minds of many people, even in academia. And it is not the first time I have done so: I had already questioned it in an article for the journal Areté (Carvajal, 2018) where I analysed the myths propagated by the triune brain model in education.

The Dragons of Eden by scientist Carl Sagan (1980)

I first read about the triune brain in The Dragons of Eden, a book by the famous science writer Carl Sagan (1980), who recounts the experiments carried out by the physician and neuroscientist Paul MacLean with various animals, including marmoset monkeys, from which certain brain regions had been removed with the consequent behavioral change, This led MacLean to suppose the existence of encephalic structures with functional specificities in terms of their behaviour, in particular those that had to do with a sexual boasting that did not seek procreation but territorial domination, behaviour that was associated with deep structures located in nuclei of the midbrain, what MacLean would call the R-complex or reptilian.

Los dragones del edén

The expression “triune brain” was introduced by Paul MacLean in 1969 in a series of three lectures, but these were not published until four years later (MacLean, 1973). Starting from the emotional theory of James Papez (1937) MacLean developed the concept of the “visceral” brain (MacLean, 1949) which he later renamed the limbic system in 1952, suggesting new structures (hippocampus, amygdala and cingulate gyrus) to those suggested by Papez. The expression “limbic” he inherited from Paul Broca, who used it in 1850 to refer to the rim-shaped part of the cerebral cortex (from the Latin limbus).

The theory of MacLean (1990) was based on extensive studies of comparative anatomy of the brains of animals as diverse as alligators and monkeys. The conceptual and intuitive appeal of this theory has made it continue to survive in many textbooks, courses and conferences on biological psychology (Farley, 2008).

The triune brain model.

MacLean’s model of the triune brain on the structure and evolution of the brain suggested examining ourselves and the world in general through “three very different mentalities”, in two of which the faculty of speech does not intervene. The three brains could be distinguished, both anatomically and functionally, and would contain “very disparate proportions of dopamine and cholinesterase” (Sagan, 1980: 74).

Cerebro triuno

Displaying the skepticism that characterized him all his life as a popularizer, scientist, Sagan (1980) already warned in his book that it would be simplistic to propose that the three brains have a strict separation of their functions, since “all brain structures are densely interconnected”, so that the “intelligence” attributed to the reptilian complex as for example, when the marmoset monkey flaunted its genitals to dominate its territory, also involves neural circuits activated in other brain structures, for example, the limbic system and the cerebral cortices of both hemispheres.

Sagan warned, when referring to the brain functions attributed to the triune brain model, that “there can be no talk of a strict separation of functions on pain of oversimplifying the issue”. It is indisputable, Sagan said, that in man both ritual and emotional behavior are strongly influenced by abstract reasoning of neocortical origin. (Sagan, 1980: 101).

Why do we like models so much?

Models have been routinely used to help explain scientific concepts (Chittleborough and Treagust, 2009). They have been adjuvants in the understanding of the world: since ancient times models have tried to explain reality. Examples of models are: in psychology, the tripartite soul of Plato and Aristotle and the Freudian model of the I/it/superego; in chemistry, the atomic models of Democritus, Dalton, Bohr, Rutherford; in astrophysics, the models of the origin of the universe and a long etcetera.

In any field of knowledge, the mind seeks the simplest available interpretation of observations or, more precisely, balances a bias towards simplicity with a somewhat opposite constraint to choose models consistent with perceptual or cognitive observations (Feldman, 2016). 

Cerebro 1

Some reasons why the triune brain model is inaccurate

  • Emotions are not processed exclusively by the limbic system. The neocortex also fulfills functions of emotional modulation in a feedback system with the amygdala (Rempel-Clower, 2007).
  • The amygdala is not only involved in emotional processing; it has also been involved in higher cognitive activities (Schaefer and Gray, 2007) as well as in other cognitive processes such as attention, the representation of values and decision-making (Pessoa, 2010).
  • The brain stem is not only concerned with survival activities (“reptilian territoriality”); it also fulfills functions related to perception, cognition and emotion (Nishijo et al., 2018) and with the maintenance of cognitive activities during old age (Mather and Harley, 2016).

In conclusion, it is important to understand that the triune brain model is a very simplified version of the brain and is more accurately understood as an intricate interconnected network of neurons – be they from the cortex, the limbic system or the brainstem – that communicate, modulate and feed back to each other, directly or indirectly. The only praiseworthy thing about the triune brain model is that it has allowed people without any notion of neuroscience to approach this discipline and become interested in further exploring its secrets. To them goes my invitation to continue studying the latest discoveries about brain functioning, in particular those models that revolve around the most recent findings of the Human Connectome Project.

Programas relevantes:

Master in Neuromarketing

Máster in Neuroleadership

Máster in NeuroSales


   Carvajal, R. (2018). Viabilidad del modelo del cerebro triuno en educación. Areté. Revista Digital del Doctorado en Educación de la Universidad Central de Venezuela. 4(8): 11-35. http://saber.ucv.ve/ojs/index.php/rev_arete/article/view/15792

   Chittleborough GD, Treagust DF. (2009). Why Models are Advantageous to Learning Science, Educación Química, 20(1):12-17. https://doi.org/10.1016/S0187-893X(18)30003-X

   Farley P. (2008). A theory abandoned but still compelling. Yale Medicine Magazine. Disponible en: https://medicine.yale.edu/news/yale-medicine-magazine/a-theory-abandoned-but-still-compelling/

   Feldman J. (2016). The simplicity principle in perception and cognition. Wiley interdisciplinary reviews. Cognitive science7(5), 330–340. https://doi.org/10.1002/wcs.1406.

   MacLean PD, Kral VA. (1973). A Triune Concept of the Brain and Behaviour. Including Psychology of memory and Sleep and dreaming; papers presented at Queen’s University, Kingston, Ontario, February, 1969.

   MacLean PD. (1949). Psychosomatic disease and the visceral brain; recent developments bearing on the Papez theory of emotion. Psychosom Med. 1949 Nov-Dec;11(6):338-53.

   MacLean, PD. (1990). The triune brain in evolution: role in paleocerebral functions. New York: Plenum.

   Mather M, Harley CW. (2016). The Locus Coeruleus: Essential for Maintaining Cognitive Function and the Aging Brain. Trends in cognitive sciences20(3), 214–226. https://doi.org/10.1016/j.tics.2016.01.001  

   Nishijo H, Rafal R, Tamietto M. (2018). Editorial: Limbic-Brainstem Roles in Perception, Cognition, Emotion, and Behavior. Front Neurosci. Jun 12; 12:395.  doi: 10.3389/fnins.2018.00395.

   Papez JW (1937). A Proposed Mechanism Of Emotion. Arch NeurPsych. 38(4):725–743. doi:10.1001/archneurpsyc.1937.02260220069003

   Pessoa L. (2010). Emotion and cognition and the amygdala: from “what is it?” to “what’s to be done?”. Neuropsychologia, 48(12), 3416–3429. https://doi.org/10.1016/j.neuropsychologia.2010.06.038

   Rempel-Clower NL. (2007). Role of orbitofrontal cortex connections in emotion. Ann N Y Acad Sci. Dec; 1121: 72-86. doi: 10.1196/annals.1401.026.

   Sagan, C. (1980). Los Dragones del Edén. Barcelona: Grijalbo.

   Schaefer A, Gray JR. (2007). A role for the human amygdala in higher cognition. Rev Neurosci.18(5):355-63. doi: 10.1515/revneuro.2007.18.5.355.

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