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Don’t Listen to Your Lizard Brain

The evolution of the brain can help us understand human behavior.

Do you ever surprise yourself, finding that you have done something without thinking about it? Do you ever notice that you feel sad or happy, but aren't sure why?

In 1990, physician and neuroscientist Paul MacLean provided one possible explanation of this phenomenon in his book, The Triune Brain in Evolution. Although scientists now know that some of the details may be wrong, it remains a useful concept. The idea is that our human brains are really composed of three parts:

1. The reptilian brain, composed of the basal ganglia (striatum) and brainstem, is involved with primitive drives related to thirst, hunger, sexuality, and territoriality, as well as habits and procedural memory (like putting your keys in the same place every day without thinking about it or riding a bike).

2. The paleomammalian (old mammal) brain, including the hypothalamus, hippocampus, amygdala, and cingulate cortex, is the center of our motivation, emotions, and memory, including behavior such as parenting.

3. The neomammalian (new mammal) brain, consisting of the neocortex, enables language, abstraction, reasoning, and planning.

Automatic routines which, over time, we have learned do without thinking about them, such as playing tennis and even driving, are largely performed by our reptilian brain. So when we are driving and, at the same time, engrossed in a conversation with a friend, we may find that we have driven somewhere with no memory of how we did it — that’s because the reptilian brain was doing most of the driving.

Sometimes, something that we are not conscious of, such as a particular smell, can trigger a complex emotion for reasons that our conscious mind cannot understand. That can occur because the paleomammalian brain has processed the smell, retrieved a memory related to the smell, and triggered the emotion relevant to that experience. It is only once our neomammalian brain becomes conscious of the smell and the memory that we understand our emotion. For example, you may find that you are at a restaurant and suddenly feel an overwhelming sense of sadness that you cannot understand. It is only upon reflection that you realize that the woman at the next table is wearing the same perfume as your best friend, who died last year.

In my work as a cognitive behavioral neurologist and researcher, the concept of the triune brain often helps me understand what is happening to my patients. Most of my patients’ brain diseases, such as Alzheimer’s, affect the paleomammalian and neomammalian brains, leaving the reptilian brain relatively intact. This pattern of brain damage helps explain many situations:

“I know grandma has Alzheimer’s, but how can she remember how to play the piano when she cannot remember my name?”

Conscious memory and naming occur in the paleomammalian and neomammalian brains, whereas procedural activities, such as playing a musical instrument, occur in the reptilian brain.

“My father was always a polite, gentle man, but since he developed dementia, he will take other people’s food and may hit someone to get his way.”

Our neomammalian brains — and, in particular, our frontal lobes — govern our behavior, inhibiting the primitive drives from our reptilian brain when they are not socially appropriate. When Alzheimer’s dementia is in the moderate or severe stage, the neomammalian brain deteriorates to the point that it can no longer regulate the reptilian brain, such that primitive urges and drives are acted upon.

Because the neomammalian frontal lobes are the main governors of the reptilian brain, when a patient comes into my office and the family tells me that the very first problems which occurred were socially inappropriate behaviors, such as asking one’s daughter-in-law for sexual acts (a real example), I worry that a disease affecting the frontal lobes is present, such as frontotemporal dementia.

Given that there have been 10 million years of evolution developing our neocortex — our neomammalian brain — why does it seem to fail so often in normal individuals? Why do we so often hear about politicians and celebrities acting on their primitive drives and urges and committing horrendous acts?

The answer is one that any small child can give you: We all can make a choice, a choice as to whether we are going to give in to the primitive urges and desires of our reptilian brain or, instead, use our neocortex to control them.

© Andrew E. Budson, MD, 2017, all rights reserved.


MacLean, Paul D. (1990) The Triune Brain in Evolution: Role in Paleocerebral Functions. Springer.

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