David Eagleman

humans the situation is somewhat different. The human brain comes into the world with some amount of genetic hardwiring (for example, for breathing, crying, suckling, caring about faces, and having the ability to learn the details of their native language).

The detailed wiring diagram of the human brain is not preprogrammed; instead, genes give very general directions for the blueprints of neural networks, and world experience fine-tunes the rest of the wiring, allowing it to adapt to the local details.

Only a couple of decades ago it was thought that brain development was mostly complete by the end of childhood. But we now know that the process of building a human brain takes up to twenty-five years. The teen years are a period of such important neural reorganization and change that it dramatically affects who we seem to be. H

Hormones coursing around our bodies cause obvious physical changes as we take on the appearance of adults – but out of sight our brains are undergoing equally monumental changes. These changes profoundly color how we behave and react to the world around us.

Why the difference between the adults and teens? The answer involves an area of the brain called the medial prefrontal cortex (mPFC). This region becomes active when you think about your self – and especially the emotional significance of a situation to your self. Dr. Leah Somerville and her colleagues at Harvard University found that as one grows from childhood to adolescence, the mPFC becomes more active in social situations, peaking at around fifteen years old. At this point, social situations

An entire life, lavishly colored with agonies and ecstasies, took place in these three pounds.

In a sense, the process of becoming who you are is defined by carving back the possibilities that were already present. You become who you are not because of what grows in your brain, but because of what is removed.

In a newborn brain, neurons are relatively unconnected to one another. Over the first two to three years, the branches grow and the cells become increasingly connected. After that, the connections are pruned back, becoming fewer and stronger in adulthood.

If developing brains are not given the proper, “expected” environment – one in which a child is nurtured and looked after – the brain will struggle to develop normally.

Without an environment with emotional care and cognitive stimulation, the human brain cannot develop normally.