If you find the question difficult to answer, you're not alone. But a new research study may finally provide a solid answer.

"Children with autism clearly are not all the same," says David G. Amaral, research director of the UC Davis M.I.N.D. Institute. "Autism is a group of disorders rather than a single disorder — several autisms versus one autism. We are determined to provide the specific biomedical and behavioral criteria that accurately define distinct subtypes."

Amaral will lead the Autism Phenome Project, a large-scale, longitudinal study which will enroll 1,800 children over several years. Participants will undergo a thorough medical evaluation in addition to analyses of their immune systems, brain structures and functions, genetics, environmental exposures and blood proteins. With so much information available, researchers hope to unravel some of the biggest riddles in the field.