Inside the mind: 7.5 revelations about how your brain really works

Are you sure the brain's main job is to think? That you control your brain, and not the other way around? That everything around you is real and not just a projection like a movie playing on the screen of your mind? If so, the new book by psychologist and neuroscientist Lisa Feldman Barrett, Seven and a Half Lessons About the Brain, has a few surprises in store for you.

We often believe that thinking sets us apart from animals, but our brain does much more than that. We only think occasionally, but we’re always breathing, pumping blood, producing hormones and enzymes, renewing cells, and maintaining body temperature. Managing all this is the brain’s main job. It’s the chief manager of our body’s biochemical factory, keeping track of and balancing energy resources. It controls over 600 muscles, regulates dozens of hormones, pumps thousands of liters of blood daily, manages the energy of billions of neurons, aids digestion, and fights off illness—every second of our lives. Our memories, emotions, thoughts, hallucinations, and sensations are merely side effects of the brain’s primary mission: keeping us alive and healthy, ideally long enough to pass on our genes.

You've likely heard of the "triune brain" theory, which suggests that our brain consists of three competing parts: the "reptilian brain" for survival, the "limbic system" for emotions, and the neocortex for higher reasoning. Though popular, this theory was debunked in the 1990s by molecular genetics. We now know that reptiles' brains have neurons similar to those in mammals. The human brain doesn't have entirely new parts missing in other animals; rather, the differences lie in how long it takes these parts to develop. For example, if an iguana’s brain were to develop longer, it might grow something resembling our neocortex.

The brain is an intricate network of 128 billion neurons. Neurons are in constant communication, exchanging chemical and electrical signals through synapses, creating over 500 trillion connections. Neurons don't communicate directly, which would be too slow; instead, they use smaller and larger hubs for distribution, allowing simultaneous communication. This complexity enables our brain to be flexible, think abstractly, imagine the future, create art, and use language. But like any complex system, it can malfunction, leading to conditions like depression, schizophrenia, chronic pain, or Parkinson's disease.

An infant brain is not a miniature adult brain—it’s a brain awaiting a set of wiring instructions so that it can complete its development. Where do those wiring instructions come from? They come from the infant’s experiences, which arise largely from the infant’s caregivers. Every cuddle you give an infant, every word you speak to an infant, everything you do with an infant wires that infant’s brain. We all know that it’s important how we treat our children, but it’s remarkable the extent to which an infant’s experiences, particularly with its caregivers, set that infant’s brain on a developmental trajectory to become the person who that infant will become.

When you drink a glass of water because you’re thirsty, your thirst seems to disappear, but it takes about 20 minutes for water in your stomach to make its way into your bloodstream and to signal to your brain that you now have enough water. So your thirst is quenched approximately 20 minutes before your brain realizes that the osmolarity of your blood has changed. How can it happen so quickly? Because your brain knows from past experience that drinking water leads to quenching your thirst. Your brain is predicting what’s going to happen next and what you should do about it rather than reacting to what you see and hear or drink.

We think our brain reacts to sensory input, but that’s not quite true. The brain is trapped in a dark skull, relying solely on chemical and physical signals. To make sense of them, it uses memories and past experiences. Instead of asking, "What is this?" it asks, "What does this remind me of?" The brain often decides on a course of action before we’re even aware of it.

Humans are social creatures, relying on others for survival. This means that part of our brain’s job is outsourced to those around us. Friends, family, neighbors, and even strangers affect how our brains function. For instance, someone raising their voice at you can trigger stress hormones, speeding up your heartbeat. While other species help each other, humans uniquely use language to regulate each other’s brains, even across vast distances. For our nervous system, nothing is better than positive interactions with others, and nothing is worse than hostility.

Just as no two fingerprints are identical, no two brains are the same. Although we all start with a typical brain structure, the specific connections between neurons are shaped by random internal and external factors—our environment, lifestyle, language, diet, social interactions, and even how we spent last evening. These unique connections contribute to the individuality of each brain.

Most of our lives take place in a made-up world. We live in a city or town whose name and whose borders were made up by people. Every word in every book uses those made-up symbols. We can acquire books and other goods with something called "money," which is represented by pieces of paper, metal, and plastic and is also completely made up. We actively and willingly participate in this made-up world every day. It is real to us. The human world uniquely combines material objects with ideas. Our brain doesn’t need augmented reality glasses to merge the two. Concepts like national borders, money, and laws are as real to us as air or gravity. But how do billions of brains, each locked in its skull, live in the same world? Barrett explains this using a detective analogy: imagine you’re interviewing multiple witnesses to the same event and create a concise report by removing repeated details. Our brains compress massive amounts of information, helping us recognize things we’ve never seen before. This skill enables us to turn sounds into words, words into ideas, and ideas into our shared reality.