Blame the Lizard! The Myth of the Reptile Brain
In the 1960s, American neuroscientist Paul MacLean postulated that the human brain is divided into three distinct evolutionary layers:
The Reptilian Complex. This is the monster within, the beginning of our evolutionary journey. Pleasure, cruelty, unbridled instinct, aggression, submission, and domination reside here.
The Limbic system. This is the next step in our evolution, represented by mammals, conferring to the brain the ability to process and regulate emotions, to learn and remember, and to develop social bonds.
And finally, the Neocortex. The evolutionary climax that gives homo sapiens the capacity for abstract thought, language, and strategy.
MacLean’s Triune Brain theory is beautifully linear and almost pleasing for its elegant simplicity. It almost immediately gained popularity in medicine and psychology. Unfortunately, despite its lasting popularity, it has been scientifically disproven for decades. All evidence clearly shows that the brain, just like the rest of the human organism, is not organized in rigid hierarchies or sequential layers. Although it is true that different areas of the brain preferentially perform certain functions, they are also part of a vast network of recursive loops involving multiple regions of the nervous system, other organs, and biological processes.
Recent peer-reviewed literature suggests that cognition, abstract thought, memory, behaviors, and emotional regulation occur, at least partly, outside the brain, involving organs such as the gut, the heart, and the microbiome.
Disproving the validity of MacLean’s theory based on new data does not diminish his significant contributions to neuroscience. Scientists are expected to revise theories as new evidence emerges. Progress depends upon constant adjustments and change—it is through trial and error that we deepen our understanding of the world.
What is remarkable and more concerning is that despite being debunked decades ago, the reptilian brain concept remains deeply entrenched in popular culture and persists even among scientists. It is often referenced casually to explain aggressive or impulsive behaviors, and in popular culture, it has become a convenient way to rationalize toxic actions and evade personal responsibility.
I find this interesting because it is a perfect example of the critical flaw of scientific reductionist determinism. While it is true that subcortical structures such as the basal ganglia and amygdala are involved in instinctive behaviors and emotions, they also participate significantly in higher cognitive processes, including intuition, pattern recognition, and planning. Similarly, cortical areas contribute heavily to pleasure/reward, fear, and aggression.
By narrowly focusing on isolated details and neglecting broader contexts and empirical experience, selective data reinforces existing beliefs, creating a false appearance of coherence and thus preventing those beliefs from being challenged.
The idea of a reptilian brain representing our instincts being kept in check by higher functions acquired through evolution or intelligent design speaks to the human quest to emancipate from nature and it reinforces that belief.
It fits with 18th-century reductionism, a philosophy rooted in Cartesian mechanistic dualism (mind and body are separate and divided), Newtonian physics (the universe is predictable and mechanistic), and Individualism. It is founded on the belief that any system or process can be fully understood by breaking it down into its smallest parts.
In addition, figures such as Isaac Newton and Pierre-Simon Laplace went a step further, layering the reductionist framework with the belief that if you know a system's initial conditions and its rules, you can predict the outcomes or effects of that system with certainty. This belief system is defined as “determinism” and believes that once you know the smallest part of a system, you also understand its totality.
Breaking complex problems into manageable pieces is indeed practical and foundational to the scientific method. However, at some point, we must integrate these parts into the whole picture. Determinism becomes problematic when it is used to reject this crucial step, or to confute contrary empirical evidence.
It dogmatically assumes that knowing only part of a system can accurately extrapolate our knowledge to the whole without needing validation. It is a gamble, a guess made with limited information and often flawed assumptions.
The validity of determinism as a source of certainty is challenged by data and empirical evidence alike. The more complex a system is, the less it is possible to predict an outcome accurately. In the best-case scenario, we can calculate the statistical probability within a margin of error.
Clinical trials show both the advantages and the limitations of a deterministic, reductionist approach to research. A pharmaceutical substance identified through reductionism is then tested deterministically by selecting specific endpoints. Then outcomes are based on statistical probabilities typically within a 5-10% margin of error only for those chosen endpoints. This is one of the main reasons why, regardless of the outcomes of any number of studies, it is impossible to predict precisely how a given medicine will affect an individual patient.
A deterministic worldview tends to become dogmatic because it cannot easily tolerate uncertainty or complexity; to persist, it must deny its limitations.
Reductionist determinism is becoming prevalent, particularly in Western culture. This explains our proficiency in breaking things down yet our difficulty in reassembling them into a coherent whole. It is why we discount empirical and personal experiences over “objective” data, even when we know that data alone rarely captures reality. As we do this our worldview becomes increasingly fragmented and chaotic.
“It wasn’t me; it was the lizard brain.”
Yet reptiles and lizards demonstrate remarkable adaptability, contribute to stable ecosystems, and exhibit nuanced emotional and social behaviors.
Maybe, instead of blaming something that does not exist, an imaginary foe from a distant past, we should consider the possibility that traits such as cruelty, aggression, dominance, rigidity, hierarchy, manipulation, control, reactivity, or amorality are not the evolutionary baggage of a primordial predator brain, but the result of a systemic dysfunction which is exacerbated in trauma, chronic stress, and other neuropsychiatric conditions.
It seems to me that the lizard brain is the Shadow of Jungian psychology. It is the place where we lock away repressed and unacceptable memories, emotions, and behaviors—those parts of ourselves that we do not wish to acknowledge or accept. The more we suppress and deny them, the more distorted and harmful they become.
If we stop and reflect for a moment, we realize that no reptilian predator lurks in darkness, waiting to take over. There is only our own Shadow, longing to be acknowledged and healed.