By the age of two, the human brain approaches its adult size, weight, and number of nerve cells. Most neurons continue to multiply through the second year of life. (In some parts of the brain, such as the cerebellum, nerve cells continue to multiply and divide into adulthood). The greatest number of synapses present in the neocortex also seems to be at two years old. By this age, the circuits of the frontal lobe begin to develop. (However, the frontal lobe does not finish developing under the genetic program until our mid-twenties!) The selective pruning of synapses that began before the age
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