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Today, diagnostic manuals refer simply to autism spectrum disorder, or ASD, which includes people previously diagnosed with Asperger's or PDD-NOS.

These include genetic factors; the ever-broadening definition of autism itself, now known as autism spectrum disorder, or ASD; and vastly improved screening programs nationwide.

The cost to scientific research generally and to households caring for those with chronic conditions such as ASD is incalculable.

Scientists at the University of North Carolina, Stanford and UC Davis have found unusual prenatal growth patterns in the brain that appear to correlate with ASD diagnoses in early childhood, though it’s unclear what triggers that growth.

Some have found evidence of environmental factors, chiefly experienced by women in pregnancy, though some question whether any such factors can be primary determinants of ASD.