Autism spectrum disorder and intellectual disability frequently co-occur, although many adults with autism spectrum disorder have normal intelligence. Communication impairments are also frequently diagnosed to make a common comorbid diagnosis of autism spectrum disorder and intellectual disability or communication impairment. Autism spectrum disorder is not a degenerative disorder, and it is typical for learning and compensation to continue throughout life. Symptoms are often marked in early childhood and early school years, with developmental gains typical in later childhood in at least some areas (e.g., increased interest in social interaction). Only a minority of those with autism spectrum disorder live and work independently in adulthood; those who do tend to have average to high average language and intellectual abilities and are able to find a niche that matches their special skills and interests. In general, individuals with lower levels of impairment may be better able to function independently. However, even these individuals may remain socially naïve and vulnerable, have difficulties organizing practical demands without aid, and are prone to anxiety and depression. Many adults report using compensation strategies and coping mechanisms to mask their difficulties in public but suffer from the stress and effort of maintaining a socially acceptable façade. Scarcely anything is known about old age in autism spectrum disorder.
Autism spectrum disorder appears to have both environmental and genetic components. A variety of nonspecific risk factors, such as advanced parental age, low birth weight, or fetal exposure to valproate, may contribute to risk of autism spectrum disorder. Heritability estimates for the disorder have ranged from 37% to higher than 90%. In some cases, parents and other relatives of a child with ASD show mild impairments in social communication skills or engage in repetitive behaviors. Evidence also suggests that emotional disorders such as bipolar disorder and schizophrenia occur more frequently than average in the families of people with ASD. Recent studies have shown that people with ASD tend to have more copy number de novo gene mutations than those without the disorder, suggesting that for some the risk of developing ASD is not the result of mutations in individual genes but rather spontaneous coding mutations across many genes. As many as 15% of cases with autism spectrum disorder appear to be associated with the de novo genetic variation.
There has been debate regarding the possibility of a link between childhood vaccinations and the subsequent development of autism. Recent cohort studies involving over 1 million children and five case-control studies involving over 9,000 children suggested that vaccinations are not associated with the development of autism or autism spectrum disorder. In addition, the components of the vaccines (thimerosal or mercury) or multiple vaccines (MMR) are not associated with the development of autism spectrum disorder.