Neurons and glias are produced from neural stem cells. These phenomena are called "Neurogenesis." Neurogenesis largely occurs in developmental stages. However, it is now known that active neurogenesis continues throughout life in discrete regions such as the hippocampus of the adult brain of all mammals, including humans. Neurogenesis can be affected by various genetic or environmental factors. Neurogenesis is related to learning and memory and may also have a function in the vulnerability to the onset of mental illness (Neurogenesis theory). We have studied this theory by using rodents and tried to improve psychotic behavior by enhancing postnatal neurogenesis. Our results showed that administration of polyunsaturated fatty acids or breeding the animals in exciting environments improved psychotic behavior, suggesting their usefulness in preventing or curing mental illness which follows declining neurogenesis.