Fallacious yet widespread and documented beliefs courtesy of Wikipedia.
Until very recently medical experts believed that humans were born with all of the brain cells they would ever have. However, we now know that new neurons can be created in the postnatal brain. Researchers have observed adult neurogenesis in avians, Old World Primates, and humans. Adults of these species retain multipotent (see cell potency) neural stem cells in the subventricular zone of the lateral ventricles and subgranular zone of the dentate gyrus. The newborn neurons generated in these areas migrate to the olfactory bulb and the dentate gyrus, respectively, and are believed to integrate into existing neural circuits. However, the function and physiological significance of adult-born neurons remains unclear. Some studies have suggested that post-natal neurogenesis also occurs in the neocortex, an idea that is disputed.