Some brain regions are replenished with new cells throughout life, but researchers aren9t sure what the newcomers contribute to the behaviors those brain regions control, which include singing, smelling, and learning. A number of experiments, including one on page 117 that identifies prolactin, a hormone that increases during pregnancy, as a neurogenesis trigger, have suggested that the new neurons might boost learning. The evidence has been correlational, however, and it doesn9t all agree.