Autism is a brain development disorder that is characterized by impaired social interaction and communication, and restricted and repetitive behavior, all starting before a child is three years old. This set of signs distinguishes autism from milder autism spectrum disorders (ASD) such as Asperger syndrome.^[2]

Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether ASD is explained more by multigene interactions or by rare mutations.^[3] In rare cases, autism is strongly associated with agents that cause birth defects.^[4] Other proposed causes, such as childhood vaccines, are controversial; the vaccine hypotheses lack convincing scientific evidence.^[5] Most recent reviews estimate a prevalence of one to two cases per 1,000 people for autism, and about six per 1,000 for ASD, with ASD averaging a 4.3:1 male-to-female ratio. The number of people known to have autism has increased dramatically since the 1980s, at least partly as a result of changes in diagnostic practice; the question of whether actual prevalence has increased is unresolved.^[6]

Autism affects many parts of the brain; how this occurs is not understood. Parents usually notice signs in the first two years of their child's life. Early behavioral or cognitive intervention can help children gain self-care, social, and communication skills. There is no known cure.^[7] Few children with autism live independently after reaching adulthood, but some become successful,^[8] and an autistic culture has developed, with some seeking a cure and others believing that autism is a condition rather than a disorder.^[9]

Main article: Epidemiology of autism

Reports of autism cases grew dramatically in the U.S. from 1996 to 2005. It is unknown how much, if any, growth came from changes in autism's prevalence.

Most recent reviews tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000 for ASD;^[6] because of inadequate data, these numbers may underestimate ASD's true prevalence.^[15] PDD-NOS cases are the vast majority of ASD, Asperger's prevalence is about 0.3 per 1,000, and the remaining ASD forms are much rarer.^[119] The number of reported cases of autism increased dramatically in the 1990s and early 2000s. This increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness,^[120] though unidentified contributing environmental risk factors cannot be ruled out.^[5] It is unknown whether autism's prevalence increased during the same period; a real increase would suggest directing more attention and funding toward changing environmental factors instead of continuing to focus on genetics.^[50]

The risk of autism is associated with several prenatal and perinatal risk factors. A 2007 review of risk factors found associated parental characteristics that included advanced maternal age, advanced paternal age, and maternal place of birth outside Europe or North America, and also found associated obstetric conditions that included low birth weight and gestation duration, and hypoxia during childbirth.^[121]

Autism is associated with several other conditions:

• Genetic disorders. About 10–15% of autism cases have an identifiable Mendelianchromosome abnormality, or other genetic syndrome,^[122] and ASD is associated with several genetic disorders.^[123]

  (single-gene) condition,

• Mental retardation. The fraction of autistic individuals who also meet criteria for mental retardation has been reported as anywhere from 25% to 70%, a wide variation illustrating the difficulty of assessing autistic intelligence.^[124] For ASD other than autism, the association with mental retardation is much weaker.^[125]

• Maleness. Boys are at higher risk for autism than girls. The ASD sex ratio averages 4.3:1 and is greatly modified by cognitive impairment: it may be close to 2:1 with mental retardation and more than 5.5:1 without.^[6]

• Epilepsy, with variations in risk of epilepsy due to age, cognitive level, and type of language disorder.^[126]

• Several metabolic defects, such as phenylketonuria, are associated with autistic symptoms.^[63]

• Minor physical anomalies are significantly increased in the autistic population.^[127]

• Preempted diagnoses. Although the DSM-IV rules out concurrent diagnosis of many other conditions along with autism, the full criteria for ADHD, Tourette syndrome, and other of these conditions are often present and these comorbid diagnoses are increasingly accepted.^[128]