The study is published in the Archives of General Psychiatry. Its findings add further evidence to the theory that older men are more likely to father children with genetic problems. Previous work has focussed on physical illnesses such as prostrate cancer, a particular form of dwarfism, and Marfan syndrome which causes defects of the eyes, bones, heart and blood vessels. As men get older, their sperm reproduces through division. Each successive division introduces a slight risk of error in the genetic material of the new sperm, which is then passed on to the child.
Mutations are tiny, and difficult to spot without knowing in advance what mutations to look for.
Any abnormalities in women's eggs can be picked up more easily, as almost all divisions in a woman's eggs occur before she is born. Egg abnormalities also usually involve larger chromosomal changes, and are therefore easier to test for. The study does not identify what the specific genetic cause of schizophrenia is, but its authors say their results do help to explain why the mental illness remains prevalent. As schizophrenics are less likely to form relationships and have children because of their illness, the researchers say evolution would normally decrease the number with the disease over the course of generations. And because the rate is consistent in different populations, environmental variations, which differ, cannot account for the incidence.