Family studies indicate that the closer a person's genetic relatedness to a person with schizophrenia, the greater the likelihood of developing the disorder. The paternal age is a factor in schizophrenia because of the increased likelihood of mutations in the chromosomes of cells that produce sperm. In contrast, women's oocytes divide twenty-three times before the time of birth and only once after that. The chance of a copying error in DNA replication during cell division increases with the number of cell divisions, and an increase in copying errors may cause an accumulation of mutations that are responsible for an increased incidence of schizophrenia. The average concordance rates are higher for identical twins than for fraternal twins and evidence also suggests that the prenatal and perinatal environments may also affect concordance rates in identical twins.