I wrote this book of seven and a half short, informal essays to intrigue and entertain you. It’s not a full tutorial on brains. Each essay presents a few compelling scientific nuggets about your brain and considers what they might reveal about human nature. The essays are best read in order, but you can also read them out of sequence. […] The essays don’t tell you what to think about human nature, but they do invite you to think about the kind of human you are or want to be. – Lisa Feldman Barrett

Bad behavior doesn’t come from ancient and unbridled inner beasts. Good behavior is not the result of rationality. And rationality and emotion are not at war . . . they do not even live in separate parts of the brain.

TWO THOUSAND YEARS AGO, in ancient Greece, a philosopher named Plato recounted a war. Not a war between cities or nations but inside of each human being. Your human mind, wrote Plato, is a never-ending battle between three inner forces to control your behavior. One force consists of basic survival instincts, like hunger and sex drive. The second force consists of your emotions, such as joy, anger, and fear. Together, Plato wrote, your instincts and emotions are like animals that can pull your behavior in divergent, perhaps ill-advised directions. To counteract this chaos, you have the third inner force, rational thought, to rein in both beasts and guide you on a more civilized and righteous path.

Plato’s compelling morality tale of inner conflict remains one of the most cherished narratives in Western civilization. Who among us has never felt an inner tug-of-war between desire and reason?

Perhaps it’s unsurprising, then, that scientists later mapped Plato’s battle onto the brain in an attempt to explain how the human brain evolved. Once upon a time, they said, we were lizards. Three hundred million years ago, that reptilian brain was wired for basic urges like feeding, fighting, and mating. About one hundred million years later, the brain evolved a new part that gave us emotions; then we were mammals. Finally, the brain evolved a rational part to regulate our inner beasts. We became human and lived logically ever after.

According to this evolutionary story, the human brain ended up with three layers—one for surviving, one for feeling, and one for thinking—an arrangement known as the triune brain. The deepest layer, or lizard brain, which we allegedly inherited from ancient reptiles, is said to house our survival instincts. The middle layer, dubbed the limbic system, supposedly contains ancient parts for emotion that we inherited from prehistoric mammals. The outermost layer, part of the cerebral cortex, is said to be uniquely human and the source of rational thought; it’s known as the neocortex (“new cortex”). One part of your neocortex, called the prefrontal cortex, supposedly regulates your emotional brain and your lizard brain to keep your irrational, animalistic self in check. Advocates of the triune brain note that humans have a very large cerebral cortex, which they see as evidence of our distinctly rational nature.

You might have noticed that I’ve now offered two different descriptions of the evolution of the human brain. In the earlier half-lesson, I wrote that brains evolved increasingly elaborate sensory and motor systems while budgeting the energy resources of increasingly complex bodies. But the triune brain story says the brain evolved in layers that allow rationality to conquer our animalistic urges and emotions. How can we reconcile these two scientific views?

Fortunately, we don’t have to reconcile them, because one of them is wrong.

The triune brain idea is one of the most successful and widespread errors in all of science. It’s certainly a compelling story, and at times, it captures how we feel in daily life. For example, when your taste buds are tempted by a luscious slice of velvety chocolate cake but you decline it because, honestly, you just finished breakfast, it’s easy to believe that your impulsive inner lizard and your emotional limbic system pushed you in a cake-ward direction, and your rational neocortex wrestled the pair into submission.

But human brains don’t work that way. Bad behavior doesn’t come from ancient and unbridled inner beasts. Good behavior is not the result of rationality. And rationality and emotion are not at war . . . they do not even live in separate parts of the brain.

The three-layered brain was proposed by several scientists over the years and formalized in the mid-twentieth century by a physician named Paul MacLean. He envisioned a brain that was structured like Plato’s battle and confirmed his hypothesis using the best technology available at the time: visual inspection. That meant peering through a microscope at the brains of various dead lizards and mammals, including humans, and identifying their similarities and differences by sight alone. MacLean determined that the human brain had a collection of new parts that other mammal brains didn’t, which he called the neocortex. He also concluded that mammal brains had a collection of parts that reptile brains didn’t, which he called the limbic system. And voilà, a human origin story was born.

MacLean’s tale of the triune brain gained traction in certain sectors of the scientific community. His speculations were simple, elegant, and seemingly consistent with Charles Darwin’s ideas about the evolution of human cognition. Darwin asserted, in his book The Descent of Man, that the human mind had evolved along with the body, and therefore each of us harbors an ancient inner beast that we tame through rational thought.

The astronomer Carl Sagan introduced the idea of the triune brain to the wider public in 1977 in his book The Dragons of Eden, which won a Pulitzer Prize. Today, terms like lizard brain and limbic system run rampant through popular-science books and newspaper and magazine articles. While writing this lesson, in fact, I came across a special issue of Harvard Business Review in my local supermarket that explained how to “stimulate your customer’s lizard brain to make a sale.” Beside it sat a special issue of National Geographic that listed brain regions that make up the alleged “emotional brain.”

What’s less known is that The Dragons of Eden appeared when experts in brain evolution already had strong evidence that the triune brain story was incorrect: evidence hidden from the naked eye, within the molecular makeup of brain cells called neurons. By the 1990s, experts had completely rejected the idea of a three-layered brain. It simply didn’t hold up when they analyzed neurons with more sophisticated tools.

In MacLean’s day, scientists compared one animal brain to another by injecting them with dye, slicing them paper-thin like deli meat, and squinting at the stained slices through a microscope. Neuroscientists who study brain evolution today still do this, but they also use newer methods that allow them to peer inside neurons and examine the genes within. They’ve discovered that neurons from two species of animals can look very different but still contain the same genes, suggesting that those neurons have the same evolutionary origin. If we find the same genes in certain human and rat neurons, for example, then similar neurons with those genes were most likely present in our last common ancestor.

Using these methods, scientists have learned that evolution does not add layers to brain anatomy like geological layers of sedimentary rock. But human brains are obviously different from rat brains, so how exactly did our brains come to differ if not by adding layers?

It turns out that as brains become larger over evolutionary time, they reorganize.

Let me explain with an example. Your brain has four clusters of neurons, or brain regions, that allow you to sense your body movements and help create your sense of touch. These brain regions are collectively called the primary somatosensory cortex. In a rat brain, however, the primary somatosensory cortex is just a single region that performs the same tasks. If we simply inspected human and rat brains by eye, as MacLean did, we might come to believe that rats lack three somatosensory regions found in the human brain. We might therefore conclude that these three regions are newly evolved in humans and must have new, human-specific functions.

Scientists have found, however, that your four regions and the rat’s single region contain many of the same genes. This scientific tidbit suggests something about evolution; namely, the last common ancestor of humans and rodents, which lived about sixty-six million years ago, probably had a single somatosensory region that carried out some functions that our four regions do today. The single region most likely expanded and subdivided to redistribute its responsibilities as our ancestors evolved larger brains and bodies. This arrangement among brain regions—segregating and then integrating—creates a more complex brain that can control a larger and more complex body.

It’s a tricky business to compare brains of different species to discover what is similar, because the path of evolution is twisty and unpredictable. What you see is not always what you get. Parts that look different to the naked eye can be similar genetically, and parts that differ genetically can look very similar. And even if you do find the same genes in the brains of two different animals, those genes can have different functions.

Thanks to recent research in molecular genetics, we now know that reptiles and nonhuman mammals have the same kinds of neurons that humans do, even those neurons that create the fabled human neocortex. Human brains did not emerge from reptile brains by evolving extra parts for emotion and rationality. Instead, something more interesting happened.

The brains of many animals look very different to the naked eye.

Scientists have recently discovered that the brains of all mammals are built from a single manufacturing plan, and most likely, the brains of reptiles and other vertebrates follow that same plan. Many people, including many neuroscientists, are not familiar with this work, and those who know about it are only beginning to reckon with its implications.

The common brain-manufacturing plan begins shortly after conception, when an embryo starts producing neurons. The neurons that form a mammal’s brain are created in an astonishingly predictable order. The ordering holds true for mice, rats, dogs, cats, horses, anteaters, humans, and every other mammalian species studied so far, and genetic evidence strongly suggests the order holds for reptiles, birds, and some fish. Yes, to the best of our scientific knowledge, you have the same brain plan as a bloodsucking lamprey.

If the brains of so many vertebrates develop in the same order, why do these brains look so different from one another? Because the manufacturing process runs in stages, and the stages last for shorter or longer durations in different species. The biological building blocks are the same; what differs is the timing. For example, the stage that produces neurons for the cerebral cortex in humans runs for a shorter time in rodents and a much shorter time in lizards, so your cerebral cortex is large, a mouse’s is smaller, and an iguana’s is tiny (or nonexistent—it’s debatable). If you could magically reach into a lizard embryo and force that stage to run for as long as it does in humans, it would produce something like a human cerebral cortex. (Though it wouldn’t function like a human one. Size isn’t everything, even for a brain.)

So the human brain has no new parts. The neurons in your brain can be found in the brains of other mammals and, likely, other vertebrates. This discovery undermines the evolutionary foundations of the triune brain story.

What about the rest of the story, that the human brain has an unusually large cerebral cortex that makes us the most rational animal? Well, it’s true that our cerebral cortex is big and has expanded over evolutionary time, and that allows us to do certain things a bit better than other animals, as we’ll learn in later lessons. But the real question here is whether the human cerebral cortex has gotten bigger, proportionally speaking, relative to the rest of the brain. So it’s more scientifically meaningful to ask: Is our cerebral cortex unusually large given our overall brain size?

To understand why this is a better question, let’s consider an analogy. Think for a moment about the variety of kitchens that you’ve seen in people’s homes. Some kitchens are large and some are small. Imagine that you find yourself inside a gigantic kitchen. You might think, Wow, these people must love to cook. Is this a reasonable conclusion? No, not based on the kitchen size alone. You must also consider the kitchen in proportion to the rest of the house. A big kitchen in a big house is ordinary—it’s just a scaled-up version of a typical house plan. A huge kitchen in a small house, however, is much more likely to have a special reason for its size, like the occupants are gourmet chefs.

The same principle applies to brains. A big brain with a proportionally big cerebral cortex would not be special, and, in fact, that’s exactly what we humans have. All mammals have a relatively big cortex in a brain that’s relatively large for their body size. Our cortex is just a scaled-up version of the relatively smaller cortex found in relatively smaller-brained monkeys, chimps, and many carnivores. It’s also a scaled-down version of the larger cortex found in the larger brains of elephants and whales. If a monkey’s brain could grow to human size, its cerebral cortex would be the same size as ours. Elephants have much more cerebral cortex than we do, but so would an elephant-sized human brain.

The size of our cerebral cortex, therefore, is not evolutionarily novel and does not require any special explanation. The size also says nothing about how rational a species is. (If it did, our most famous philosophers might be Horton, Babar, and Dumbo.) Western scientists and intellectuals concocted the idea of the big, rational cortex and have kept it alive for many years. The real story is that during the course of evolution, certain genes mutated to cause particular stages of brain development to run for longer or shorter times, producing a brain with proportionally bigger or smaller parts.

So you don’t have an inner lizard or an emotional beast-brain. There is no such thing as a limbic system dedicated to emotions. And your misnamed neocortex is not a new part; many other vertebrates grow the same neurons that, in some animals, organize into a cerebral cortex if key stages run for long enough. Anything you read or hear that proclaims the human neocortex, cerebral cortex, or prefrontal cortex to be the root of rationality, or says that the frontal lobe regulates so-called emotional brain areas to keep irrational behavior in check, is simply outdated or woefully incomplete. The triune brain idea and its epic battle between emotion, instinct, and rationality is a modern myth.

To be clear, I’m not saying that our big brain has no advantages. (What advantages does it provide? The answers will unfold in the lessons that follow.) And while it’s true that we’re the only animal that can build skyscrapers and invent French fries, these abilities are not due to our big brains alone, as we shall see. Moreover, other animals have evolved abilities that surpass ours in significant ways. We don’t have wings to fly. We can’t lift fifty times our own weight. We can’t regrow amputated body parts. Such abilities are superhero powers to us but business as usual for allegedly lesser creatures. Even bacteria are more talented than we are at certain tasks, like surviving in harsh, unfamiliar environments such as outer space or the insides of your intestines.

Natural selection did not aim itself toward us—we’re just an interesting sort of animal with particular adaptations that helped us survive and reproduce in particular environments. Other animals are not inferior to humans. They are uniquely and effectively adapted to their environments. Your brain is not more evolved than a rat or lizard brain, just differently evolved.

If that’s the case, why is the myth of the triune brain still popular? Why do college textbooks still depict a limbic system in the human brain and say it’s regulated by the cerebral cortex? Why do expensive executive-training courses teach CEOs to get a grip on their lizard brains if experts in brain evolution dismissed such ideas decades ago? Partly it’s because those experts need a better public relations department. But mostly it’s because the triune brain is a story that comes with its own cheering section. With our unique capacity for rational thought, the story goes, we triumphed over our animal nature and now rule the planet. To believe in the triune brain is to award ourselves a first prize trophy for Best Species.

The idea of Plato’s war, with rationality versus emotion and instinct, has long been Western culture’s best explanation for our behavior. If you restrain your instincts and emotions appropriately, then your behavior is said to be rational and responsible. If you choose not to act rationally, then your behavior may be called immoral, and if you’re unable to act rationally, you are considered mentally ill.

But what is rational behavior, anyway? Traditionally, it’s the absence of emotion. Thinking is viewed as rational, whereas emotion is supposedly irrational. But that isn’t necessarily so. Sometimes emotion is rational, like when you feel afraid because you’re in imminent danger. And sometimes thinking isn’t rational, like when you scroll through social media for hours, telling yourself you’re bound to come across something important.

Perhaps rationality is better defined in terms of the brain’s most important job: body budgeting—managing all the water, salt, glucose, and other bodily resources we use every day. In this view, rationality means spending or saving resources to succeed in your immediate environment. Let’s say you’re in a physically dangerous situation, and your brain prepares you to flee. It directs your adrenal glands, which sit atop your kidneys, to pump you full of cortisol, a hormone that provides a quick burst of energy. From a triune brain perspective, the cortisol rush is instinctual, not rational. But from a body-budgeting perspective, the cortisol rush is rational, because your brain is making a sound investment in your survival and the existence of your potential offspring.

If there was no danger and your body prepared to flee anyway, would that be irrational behavior? It depends on context. Suppose you’re a soldier in a war zone, where threats appear on a regular basis. It’s appropriate for your brain to frequently predict threat. It may sometimes guess incorrectly and flood you with cortisol when there’s no danger. In one sense, we could view this false alarm as needless spending of resources that you may need later and therefore irrational. But in a war zone, this false alarm may be rational from a body-budgeting standpoint. You might waste a bit of glucose or other resources in the moment, but over the long run, you are more likely to survive.

If you return home from war to a safer environment but your brain continues to false-alarm, as happens in post-traumatic stress disorder, that behavior could still be considered rational. Your brain is protecting you from threats it believes are present, even though the frequent withdrawals decimate your body budget. The problem is your brain’s beliefs; they are not a good fit for your new environment, and your brain hasn’t adjusted yet. What we call mental illnesses, then, may be rational body-budgeting for the short term that’s out of sync with the immediate environment, the needs of other people, or your own best interests down the road.

Rational behavior, therefore, means making a good body-budgeting investment in a given situation. When you exercise vigorously, you may have a rush of cortisol in your bloodstream and you may feel unpleasant, but we’d consider exercise rational because it’s beneficial for your future health. The surge of cortisol when you receive criticism from a coworker might also be rational because it makes more glucose available so you can learn something new.

These ideas, if taken seriously, could shake the foundations of all sorts of sacred institutions in our society. In the law, for example, attorneys plead that their clients’ emotions overwhelmed their reason in the heat of passion, and therefore they aren’t fully to blame for their actions. But feeling distressed is not evidence of being irrational or that your so-called emotional brain has hijacked your supposed rational brain. Distress can be evidence that your whole brain is expending resources toward an anticipated payoff.

Many other social institutions are steeped in the idea of a mind at war with itself. In economics, models for investor behavior assume a sharp distinction between the rational and the emotional. In politics, we have leaders with clear conflicts of interest, such as past lobbying work in industries that they now oversee, who believe they can easily set aside their emotions and make rational decisions for the good of the people. Beneath these lofty ideas lurks the myth of the triune brain.

You have one brain, not three. To move past Plato’s ancient battle, we might need to fundamentally rethink what it means to be rational, what it means to be responsible for our actions, and perhaps even what it means to be human.

  • Your Brain Is Not for Thinking
  • You Have One Brain (Not Three)
  • Your Brain Is a Network
  • Little Brains Wire Themselves to Their World
  • Your Brain Predicts (Almost) Everything You Do
  • Your Brain Secretly Works with Other Brains
  • Brains Make More than One Kind of Mind
  • Our Brains Can Create Reality