Starting at around age two, the brain begins this two-decade-long pruning process, getting rid of synaptic connections that aren’t being used. By age ten, the brain will have pruned out 50 percent of the connections it had at age two, and this pruning continues into your twenties. Some adult, late-onset mental disorders, such as schizophrenia, may result from incomplete pruning of the prefrontal cortex during adolescence. You might ask, “Why don’t all neurons connect to every other neuron and just stay that way?” For one thing, the brain would be gigantic if it did this—twenty kilometers
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