Autism is a neurodevelopmental disorder characterized by difficulties with social interaction and communication, and by restricted and repetitive behavior. Parents often notice signs during the first three years of their child's life. These signs often develop gradually, though some autistic children experience regression in their communication and social skills after reaching developmental milestones at a normal pace.


Globally, autism is estimated to affect 24.8 million people as of 2015. In the 2000s, the number of autistic people worldwide was estimated at 1–2 per 1,000 people. In the developed countries, about 1.5% of children are diagnosed with ASD as of 2017, from 0.7% in 2000 in the United States. It is diagnosed four to five times more often in males than females. The number of people diagnosed has increased considerably since the 1990s, which may be partly due to increased recognition of the condition.