Epidemiologist Irva Hertz-Picciotto has found that the seven- to eight-fold increase in the number of children born in California with autism since 1990 cannot be explained by either changes in how the condition is diagnosed or counted. The methodology eliminated migration as a potential cause of the increase. It also revealed that less than one-tenth of the increased number of reported autism cases could be attributed to the inclusion of milder cases of autism. Only 24 percent of the increase could be attributed to earlier age at diagnosis. Published in Epidemiology, study results suggest that research should shift from genetics to the host of chemicals and infectious microbes in the environment.