"In the industrialised world, roughly 1 person in every 25 has severe mental disorder, and nearly half of us will experience some kind of mental illness during our lives. Many conditions, including schizophrenia and bipolar disorder, as well as developmental conditions like autism, are at least in part inherited from our parents. If they affect people's chance of survival you would expect natural selection to have eliminated them, but instead they persist at high levels. Some argue that these genes bring benefits – mental illness and genius have a long-standing link – but archaeologist Penny Spikins at the University of York, UK, goes further. She believes that mental illness and conditions such as autism persist at such high levels because in the past they were advantageous to humanity."