Autism explosion half explained, half still a mystery - New Scientist Why have the numbers of autism diagnoses ballooned in recent decades? Researchers have long claimed that changes to the way the condition is diagnosed are the main cause. But now a series of a studies have shown that diagnostic changes alone cannot account for the increase. They suggest that other causes, perhaps environmental factors, are also contributing to the rise in cases. "These studies give me the feeling that there must be a true increase in the number of children affected," says Tom Insel, director of the National Institute of Mental Health in Rockville, Maryland. The studies are the work of sociologist Peter Bearman at Columbia University in New York and colleagues. They have spent three years trying to disentangle the causes of the roughly sevenfold increase in autism rates seen in many developed nations over the past 20 years. They have identified three factors that are driving up autism rates, but found that these account for only half of the observed increase.