A neuron normally grows like a branched tree, allowing it to connect to other neurons. In an enriched environment, branches grow more lavishly. In a deprived environment, branches shrivel.

      Does the same happen in humans? In the early 1990s, researchers in California realized they could take advantage of autopsies to compare the brains of those who completed high school with those who completed college. In analogy to the animal studies, they found that an area involved in language comprehension contained more elaborate dendrites in the college educated.⁷

      So the first lesson is that the fine structure of the brain reflects the environment to which it is exposed. And this is not just about dendrites. As we’ll learn shortly, world experience modulates almost every measurable detail of the brain, from the molecular scale to overall brain anatomy.

EXPERIENCE NECESSARY

      Why was Einstein Einstein? Surely genetics mattered, but he is affixed to our history books because of every experience he’d had: the exposure to cellos, the physics teacher he had in his senior year, the rejection of a girl he loved, the patent office in which he worked, the math problems he was praised for, the stories he read, and millions of further experiences—all of which shaped his nervous system into the biological machinery we distinguish as Albert Einstein. Each year, there are thousands of other children with his potential but who are exposed to cultures, economic conditions, or family structures that don’t give sufficiently positive feedback. And we don’t call them Einsteins.

      If DNA were the only thing that mattered, there would be no particular reason to build meaningful social programs to pour good experiences into children and protect them from bad experiences. But brains require the right kind of environment if they are to correctly develop. When the first draft of the Human Genome Project came to completion at the turn of the millennium, one of the great surprises was that humans have only about twenty thousand genes.⁸ This number came as a surprise to biologists: given the complexity of the brain and the body, it had been assumed that hundreds of thousands of genes would be required.

      So how does the massively complicated brain, with its eighty-six billion neurons, get built from such a small recipe book? The answer pivots on a clever strategy implemented by the genome: build incompletely and let world experience refine. Thus, for humans at birth, the brain is remarkably unfinished, and interaction with the world is necessary to complete it.

      Consider the sleep-wake cycle. This internal clock, known as the circadian rhythm, runs roughly on a twenty-four-hour cycle. However, if you descend into a cave for several days—where there are no clues to the light and dark cycles of the surface—your circadian rhythm would drift in a range between twenty-one and twenty-seven hours. This exposes the brain’s simple solution: build a non-exact clock and then calibrate it to the sun’s cycle. With this elegant trick, there is no need to genetically code a perfectly wound clock. The world does the winding.

      The flexibility of the brain allows the events in your life to stitch themselves directly into the neural fabric. It’s a great trick on the part of Mother Nature, allowing the brain to learn languages, ride bicycles, and grasp quantum physics, all from the seeds of a small collection of genes. Our DNA is not a blueprint; it is merely the first domino that kicks off the show.

      From this viewpoint, it is easy to understand why some of the most common problems of vision—such as the inability to see depth correctly—develop from imbalances in the pattern of activity delivered to the visual cortex by the two eyes. For example, when children are born cross-eyed or wall-eyed, the activity from the two eyes is not well correlated (as it would be with aligned eyes). If the problem is not addressed, the child will not develop normal stereo vision—that is, the capacity to determine depth from the small differences between what the two eyes are seeing. One eye will grow progressively weaker, often to the point of blindness. We’ll return to this later to understand why and what can be done about it. For now, the important point is that the development of normal visual circuits relies on normal visual input. It is experience-dependent.

      So genetic instructions play only a minor role in the detailed assembly of cortical connections. It couldn’t be any other way: With twenty thousand genes and 200 trillion connections between neurons, how could the details possibly be prespecified? That model could never have worked. Instead, neuronal networks require interaction with the world for their proper development.⁹

NATURE’S GREAT GAMBLE

      On September 29, 1812, a baby was born who would inherit the grand ducal throne of Baden, Germany. Unfortunately, the baby died seventeen days later. That was the end of that.

      Or was it? Sixteen years later, a young man named Kaspar Hauser showed up in Nuremberg, Germany. He carried a note that explained he had been given away as a child, and he apparently knew only a few sentences, including “I want to be a cavalryman, as my father was.” He attracted widespread attention and the audiences of powerful people; many began to suspect he was the hereditary prince of Baden, switched in those first weeks with a dying baby in a nefarious plot by those who stood to inherit the throne.

      The story grew famous beyond the royal intrigue: Kaspar became the exemplar of a feral child. According to his own telling, Kaspar had spent his entire youth alone in a dark cell. It was only a meter wide, two meters long, and one and a half meters high. It had a straw bed and a small wooden horse. Each morning he awoke to discover some bread and water, nothing more. He saw no one enter or leave. Occasionally the water he drank would taste a bit different, and then he would grow sleepy afterward—and when he awoke his hair would be cut and his nails clipped. But it was not until just before his release that he had direct contact with another human, a man who taught him how to write but always kept his face hidden from view.

      Kaspar Hauser’s story aroused international attention. He grew up to write prolifically and touchingly about his childhood. His story lives on today in plays, books, and music; it is perhaps history’s most famous story of a feral childhood.

      But Kaspar’s claim was almost certainly false. Beyond the extensive historical analysis that rules it out, there is a neurobiological reason: a child raised without human interaction does not grow up to walk, speak, write, lecture, and thrive, like the successful Kaspar. After a century of popular press about Kaspar, the psychiatrist Karl Leonhard put a fine point on it:

      If he had been living since childhood under the conditions he describes, he would not have developed beyond the condition of an idiot; indeed he would not have remained alive long. His tale is so full of absurdities that it is astonishing that it was ever believed and is even today still believed by many people.¹⁰

      After all, despite some genetic pre-specification, nature’s approach to growing a brain relies on receiving a vast set of experiences, such as social interaction, conversation, play, exposure to the world, and the rest of the landscape of normal human affairs. The strategy of interaction with the world allows the colossal machinery of the brain to take shape from a relatively small set of instructions. It’s an ingenious approach for unpacking a brain (and body) from a single microscopic egg.

      But this strategy is also a gamble. It’s a slightly risky approach—one in which the brain-shaping work is partially relegated to world experience rather than hardwiring. After all, what if a child is actually born into Kaspar’s story and has an infancy characterized by total parental neglect?

      Tragically,