The reasons for this paring back of synapses is a mystery, but synaptic pruning is thought to sharpen and reinforce the “correct” synapses, while removing the weak and unnecessary ones. “It reinforces an old intuition,” a psychiatrist in Boston told me. “The secret of learning is the systematic elimination of excess. We grow, mostly, by dying.” We are hardwired not to be hardwired, and this anatomical plasticity may be the key to the plasticity of our minds.