Autism is a highly variable, neurodevelopmental disorder[28] whose symptoms first appear during infancy or childhood, and generally follows a steady course without remission.[29] Autistic people may be severely impaired in some respects but average, or even superior, in others.[30] Overt symptoms gradually begin after the age of six months, become established by age two or three years[31] and tend to continue through adulthood, although often in more muted form.[32] It is distinguished by a characteristic triad of symptoms: impairments in social interaction, impairments in communication, and repetitive behavior. Other aspects, such as atypical eating, are also common but are not essential for diagnosis.[33] Individual symptoms of autism occur in the general population and appear not to associate highly, without a sharp line separating pathologically severe from common traits.[34]