When we consider the brain’s complexity, the less-than-precise process of brain development, and the myriad factors—from SES to gut bacteria—that can influence development, we are left to marvel at how so many of us end up with brains that pass for normal. We all must have had neurons that migrated to wrong locations, made incorrect connections, were exposed to viruses or other harmful substances. If the brain were as fragile as it might seem, to end up with a normal brain would be almost impossible.

Apparently, animals have evolved a substantial capacity to repair minor abnormalities in brain development. Most people have developed in the range that we call normal because the human brain’s plasticity and regenerative powers overcome minor developmental deviations. By initially overproducing neurons and synapses, the brain gains the capacity to correct errors that might have arisen accidentally.

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These same plastic properties later allow us to cope with the ravages of aging. Neurons are dying throughout our lifetime. By age 60, investigators ought to see significant effects from all this cell loss, especially considering the cumulative results of exposure to environmental toxins, drugs, traumatic brain injuries, and other neural insults. But this is not what happens.

Although some teenagers may not believe it, relatively few 60-year-olds are demented. By most criteria, the 60-year-old who has been intellectually active throughout adulthood is likely to be much wiser than the 18-year-old whose brain has lost relatively few neurons. A 60-year-old chess player will have a record of many more chess matches from which to draw game strategies than does an 18-year-old, for example.

Clearly, some mechanism must enable us to compensate for loss and minor injury to our brain cells. This capacity for plasticity and change, for learning and adapting, is arguably the most important characteristic of the human brain during development and throughout life.