Triune Brain Theory

The nineties were declared "The Decade of the Brain," and there has been an abundance of information and research available to teachers over the past ten years. Paul MacLean, former director of the Laboratory of the Brain and Behavior at the U.S. National Institute of Mental Health, has developed a triune brain theory to explain the evolutionary development and structure of the brain. The three components are the reptilian system, the limbic system, and the neocortex.

The following metaphor, which compares these brain layers to three brothers living under one roof, can make this triune brain theory easy to understand.

The oldest brother (reptilian brain) is in charge of general security and comfort. Most of his behaviors are automatic and ritualistic, and he strongly resists change. He does not use language, though in some ways he can respond to it.

The second brother (representing the limbic system) feels deeply. He monitors emotion and plays a significant role in remembering new information and organizing events. He is concerned to some extent with self-defense through fight or flight. He also tries to maintain balance between the oldest and the youngest by keeping the oldest brother from totally dominating the family and by serving as a conscience.

The youngest and largest of the brothers is the neocortex. He is creative, can use language, compose music, and engage in very complex analysis. He is capable of formal operational or abstract thought, can anticipate and plan for the future, and is, in general, very intelligent.

Each brother is involved to some extent in the behavior and decisions of the other. They can be supportive of one another. They can also be in conflict. The older brothers are most likely to dominate when threat is perceived, because then the top priority is safety and survival. Working together, however, the three brothers can successfully deal with quite extraordinary challenges.

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Your "3-Brains-in-One" Brain


You may have thought all you had was one, but inside there are two more brains.

Actually, you already know this from your experience: for example, remember a time when you really wanted to do something, but you knew you shouldn't? The most illogical or irrational "wants" we have probably derive from older parts of our brain, while the understanding of smart versus dumb choices comes from the newest part. If that idea offends you, or seems just too "Western" or "scientific", you might take a "de-tour" for a moment and read this essay on science.

Take another example: you can be hungry, but not feel it until you pay attention to it; then when you do notice, you realize you've been getting hungrier for a long time. Hunger comes from the most basic parts of our brain, but our awareness of it is controlled by the newest part. Here are the "3 brains":

Where? / Name / Typical Animals
Brain One / Center of the Brain / "R complex" / snakes, lizards
Brain Two / Wrapped around
Brain One / "limbic system" or
"old mammalian brain" / dogs, cats
Brain Three / Outside Surface
(Wrapped around Brain Two!) / "neocortex" / primates, especially
human primates

Brain One

This is the brain we share with birds, and reptiles. Think of it as the "housekeeping brain". Just the basics: hunger, temperature control, fight-or-flight fear responses, defending territory, keeping safe -- that kind of thing. The structures that perform these functions within our brain are extremely similar to those in the brains of reptiles. Thus, this brain is called the "R complex" (R for reptilian). You can take a Tour of the R complex when you wish; and you will see parts of it in the section on obsessions.

Brain Two

As animals became more complex, other structures were added around the R complex in a shell, or "girdle". The Latin word for arc or girdle is "limbus", and this brain is called the "limbic system". We humans share this brain with older mammals like dogs, cats, and horses, and even mice (as opposed to newer mammals like chimps; we'll get to them in a moment). Their brains, and this part of our brains, are extremely similar.

Think about the difference between a mouse and a lizard, or between a cat and a snake, and you'll recognize what this mammalian brain adds to a creature's capacities. Mammals have "feelings" like ours. We'll be looking at the structures of the limbic system in the sections on mood, memory, and hormone control.The main parts of the limbic system (except the thalamus, which is generally regarded as part of Brain One) are shown below. By taking all the Brain Tours you'll see each of these parts and get a better sense of how this set of structures is positioned underneath the cortex.

Brain Three

Here is the familiar "cortex" you can see from the outside. With this brain, primates can do things that horses and cows cannot, like complex social interactions and advance planning (such as planning an attack on a neighboring troop). In humans the cortex has grown to a huge size, somehow in association with our development of language. Other primates like chimpanzees, or monkeys, have much less cortex, which is surprising since chimpanzee DNA differs from ours by only 1.6%! (stunning, really; I hope you're stunned. Recently some technical issues have arisen with this number, but for now, it's still generally regarded as, well, amazing!). If you wonder why we humans have populated the entire globe, while our chimp relatives are stuck in a shrinking rain forest with their nearly identical DNA -- read The Third Chimpanzee, by Jared Diamond.You've got a great question, and his is a great answer. (Similarly, if you wonder why white-skinned humans seem to have an unfair share of the resources and money, his other masterpiece offers a solid explanation beside skin color: Guns, Germs, and Steel).

Three Brains in One

To keep all this straight, think of the following image (ok, it's a little odd, but it seems to work; write if you have another one). The R brain is like a golf club. Let's make it a driver, one of those with a big fat wooden head. Hold the club so that the head is at the top. There's your R complex, with your spine sticking down toward the ground. The R brain is just a big swelling at the top of a spinal cord, and that's how it developed. Worms have little swellings, snakes have bigger ones. OK so far?

Next we'll add the layer that makes mammals behave so differently from reptiles. This next "layer", the old mammalian brain, evolved on top of the R complex. It was not a remodel so much as an addition, like adding on bedrooms all the way around a kitchen/bathroom. This addition covers the entire R complex, leaving the R complex deep within the brain. In our model, take the golf club, and cover the head with a sock; a big thick red one would be nice. Now you have the R brain (golf club), with the old mammalian brain wrapped around it (sock). Notice that the red sock forms a shell, or continuous border around the golf club head.

To complete the brain picture, add a bicycle or hockey helmet on top of your red-socked golf club head: that's the newest mammalian addition, the "cortex", and it is the grey squiggly stuff you can see on the outside.If you'd like a tour of the cortex itself, as you've seen it so far (like, what are those colored parts?), click here.

Brain Tours

You can go directly to the brain part of your choice, but if you're going for the entire tour, proceed in the order below. This way you can go from the outside in, which may help you keep it all straight.

Tour / Brain Structure
1: Your 3 brains in one / Reptilian Brain, Old Mammalian Brain, Cortex
2. Mood / Anterior Cingulate
3. Memory / Hippocampus
4. Fear / Amygdala
5. Obsessions / Basal ganglia (and frontal lobes)
6. Hormone Control / Hypothalamus

(#5 is being re-written -- 11/5/00)