TheCourageousCadaver

Autism is a neurodevelopmental disorder characterized by impaired social interaction, verbal and non-verbal communication, and restricted and repetitive behavior. Parents usually notice signs in the first two years of their child's life.^[2] These signs often develop gradually, though some children with autism reach their developmental milestones at a normal pace and then regress.^[3] The diagnostic criteria require that symptoms become apparent in early childhood, typically before age three.^[4]

While autism is highly heritable, researchers suspect both environmental and genetic factors as causes.^[5] In rare cases, autism is strongly associated with agents that cause birth defects.^[6] Controversies surround other proposed environmental causes;^[7] for example, the vaccine hypotheses have been disproven. Autism affects information processing in the brain by altering how nerve cells and their synapses connect and organize; how this occurs is not well understood.^[8] It is one of three recognized disorders in the autism spectrum (ASDs), the other two being Asperger syndrome, which lacks delays in cognitive development and language, and pervasive developmental disorder, not otherwise specified (commonly abbreviated as PDD-NOS), which is diagnosed when the full set of criteria for autism or Asperger syndrome are not met.^[9]

Early speech or behavioral interventions can help children with autism gain self-care, social, and communication skills.^[2] Although there is no known cure,^[2] there have been reported cases of children who recovered.^[10] Not many children with autism live independently after reaching adulthood, though some become successful.^[11] An autistic culture has developed, with some individuals seeking a cure and others believing autism should be accepted as a difference and not treated as a disorder.^[12]

Globally, autism is estimated to affect 21.7 million people as of 2013.^[13] As of 2010, the number of people affected is estimated at about 1–2 per 1,000 worldwide. It occurs four to five times more often in boys than girls. About 1.5% of children in the United States (one in 68) are diagnosed with ASD as of 2014, a 30% increase from one in 88 in 2012.^[14][15][16] The rate of autism among adults aged 18 years and over in the United Kingdom is 1.1%.^[17] The number of people diagnosed has been increasing dramatically since the 1980s, partly due to changes in diagnostic practice and government-subsidized financial incentives for named diagnoses;^[16] the question of whether actual rates have increased is unresolved.^[18]

Autism is a highly variable neurodevelopmental disorder^[19] that first appears during infancy or childhood, and generally follows a steady course without remission.^[20] People with autism may be severely impaired in some respects but normal, or even superior, in others.^[21] Overt symptoms gradually begin after the age of six months, become established by age two or three years,^[22] and tend to continue through adulthood, although often in more muted form.^[23] It is distinguished not by a single symptom, but by a characteristic triad of symptoms: impairments in social interaction; impairments in communication; and restricted interests and repetitive behavior. Other aspects, such as atypical eating, are also common but are not essential for diagnosis.^[24] Autism's individual symptoms occur in the general population and appear not to associate highly, without a sharp line separating pathologically severe from common traits.^[25]

Social deficits distinguish autism and the related autism spectrum disorders (ASD; see Classification) from other developmental disorders.^[23] People with autism have social impairments and often lack the intuition about others that many people take for granted. Noted autistic Temple Grandin described her inability to understand the social communication of neurotypicals, or people with normal neural development, as leaving her feeling "like an anthropologist on Mars".^[26]

In a pair of studies, high-functioning children with autism aged 8–15 performed equally well as, and adults better than, individually matched controls at basic language tasks involving vocabulary and spelling. Both autistic groups performed worse than controls at complex language tasks such as figurative language, comprehension and inference. As people are often sized up initially from their basic language skills, these studies suggest that people speaking to autistic individuals are more likely to overestimate what their audience comprehends.^[38]

Autistic individuals display many forms of repetitive or restricted behavior, which the Repetitive Behavior Scale-Revised (RBS-R)^[39] categorizes as follows.

• Stereotypy is repetitive movement, such as hand flapping, head rolling, or body rocking.
• Compulsive behavior is intended and appears to follow rules, such as arranging objects in stacks or lines.
• Sameness is resistance to change; for example, insisting that the furniture not be moved or refusing to be interrupted.
• Ritualistic behavior involves an unvarying pattern of daily activities, such as an unchanging menu or a dressing ritual. This is closely associated with sameness and an independent validation has suggested combining the two factors.^[39]
• Restricted behavior is limited in focus, interest, or activity, such as preoccupation with a single television program, toy or game.
• Self-injury includes movements that injure or can injure the person, such as eye-poking, skin-picking, hand-biting and head-banging.^[9]

No single repetitive or self-injurious behavior seems to be specific to autism, but only autism appears to have an elevated pattern of occurrence and severity of these behaviors.^[40]

Autistic individuals may have symptoms that are independent of the diagnosis, but that can affect the individual or the family.^[24] An estimated 0.5% to 10% of individuals with ASD show unusual abilities, ranging from splinter skills such as the memorization of trivia to the extraordinarily rare talents of prodigious autistic savants.^[41] Many individuals with ASD show superior skills in perception and attention, relative to the general population.^[42] Sensory abnormalities are found in over 90% of those with autism, and are considered core features by some,^[43] although there is no good evidence that sensory symptoms differentiate autism from other developmental disorders.^[44] Differences are greater for under-responsivity (for example, walking into things) than for over-responsivity (for example, distress from loud noises) or for sensation seeking (for example, rhythmic movements).^[45] An estimated 60%–80% of autistic people have motor signs that include poor muscle tone, poor motor planning, and toe walking;^[43] deficits in motor coordination are pervasive across ASD and are greater in autism proper.^[46]

Unusual eating behavior occurs in about three-quarters of children with ASD, to the extent that it was formerly a diagnostic indicator. Selectivity is the most common problem, although eating rituals and food refusal also occur;^[47] this does not appear to result in malnutrition. Although some children with autism also have gastrointestinal symptoms, there is a lack of published rigorous data to support the theory that children with autism have more or different gastrointestinal symptoms than usual;^[48] studies report conflicting results, and the relationship between gastrointestinal problems and ASD is unclear.^[49]

Parents of children with ASD have higher levels of stress.^[27] Siblings of children with ASD report greater admiration of and less conflict with the affected sibling than siblings of unaffected children and were similar to siblings of children with Down syndrome in these aspects of the sibling relationship. However, they reported lower levels of closeness and intimacy than siblings of children with Down syndrome; siblings of individuals with ASD have greater risk of negative well-being and poorer sibling relationships as adults.^[50]

It has long been presumed that there is a common cause at the genetic, cognitive, and neural levels for autism's characteristic triad of symptoms.^[51] However, there is increasing suspicion that autism is instead a complex disorder whose core aspects have distinct causes that often co-occur.^[51][52]

Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether ASD is explained more by rare mutations with major effects, or by rare multigene interactions of common genetic variants.^[54][55] Complexity arises due to interactions among multiple genes, the environment, and epigenetic factors which do not change DNA but are heritable and influence gene expression.^[23] Studies of twins suggest that heritability is 0.7 for autism and as high as 0.9 for ASD, and siblings of those with autism are about 25 times more likely to be autistic than the general population.^[43] However, most of the mutations that increase autism risk have not been identified. Typically, autism cannot be traced to a Mendelian (single-gene) mutation or to a single chromosome abnormality, and none of the genetic syndromes associated with ASDs have been shown to selectively cause ASD.^[54] Numerous candidate genes have been located, with only small effects attributable to any particular gene.^[54] The large number of autistic individuals with unaffected family members may result from copy number variations—spontaneous deletions or duplications in genetic material during meiosis.^[56] Hence, a substantial fraction of autism cases may be traceable to genetic causes that are highly heritable but not inherited: that is, the mutation that causes the autism is not present in the parental genome.^[53]