Autism is a range of neurodevelopmental conditions generally characterized by difficulties in social interactions and communication, repetitive behaviors, intense interests, and unusual responses to sensory stimuli. It is commonly referred to as autism or, in the context of a professional diagnosis, autism spectrum disorder (ASD), but the latter term remains controversial among neurodiversity advocates, neurodiversity researchers, and many autistic people due to the use of the word disorder and due to questions about its utility outside of diagnostic contexts.

Given concerns about the appropriateness of the term disorder,many sources prefer to use the word “autism” without any additional words, on the basis that this is the least controversial term among people with different perspectives or (in the United Kingdom) autism spectrum conditions (ASC)rather than ASD. While psychiatry traditionally classifies autism as a neurodevelopmental disorder, many autistic people, most autistic advocates and a rapidly increasing number of researchers see autism as part of neurodiversity, the natural diversity in human thinking, and experience, with strengths, differences, and weaknesses. On this view, promoted by the autism rights movement, autism is not pathological, but this does not preclude autistic individuals from being disabled and potentially having high support needs due to co-occurring conditions and lack of person-environment fit. This relatively positive and holistic view of autism has led to a certain degree of friction between autistic individuals, advocates, charities, researchers and practitioners.

The causes of autism are not well understood, but are likely linked to altered structures of the brain at birth. There is no official cure for autism, so interventions focus on, for example, finding and learning other modes of communication in a non-verbal autist, or applied behavior analysis interventions, which are highly controversial.
Other controversies in autism are scientific, sociological, political, or philosophical, and some have aspects of all four. First, it is controversial and uncertain if social-communication difficulties of autistic people are inherent core deficits (see empathizing-systemizing theory developed by Simon Baron-Cohen), or due to mismatch in social communication styles, cognition, and experiences resulting in bidirectional misunderstanding between autistic people and non-autistic people (see double empathy problem theory developed by autistic researcher Damien Milton and recent growing evidence that found that autistic people empathize, communicate and socialize well with autistic people), or a combination of both factors. A 2018 study has shown that autistic people are more prone to object personification, suggesting that autistic empathy may be not only more complex but also more all-encompassing, contrary to the popular belief that autistic people lack empathy.
Moreover, scientists are still trying to determine what causes autism; it is highly heritable and believed to be mainly genetic, but there are many genes involved, and environmental factors may also be relevant. It is unclear why autism commonly co-occurs with ADHD, intellectual disabilities, epilepsy and a range of other conditions. There are ongoing disagreements about what should be included as part of the autism spectrum, whether meaningful sub-types of autism exist, and the significance of autism-associated traits in the wider population. The combination of broader criteria and increased awareness has led to a trend of steadily increasing estimates of autism prevalence, causing a common misconception that there is an autism epidemic and perpetuating the myth that it is caused by vaccines.

For many autistic individuals, characteristics first appear during infancy or childhood and generally follow a steady course without remission (different developmental timelines described in more detail below). Autistic people may be severely impaired in some respects but average, or even superior, in others. Clinicians consider assessment for ASD when a patient shows:
-> regular difficulties in social interaction or communication
-> restricted or repetitive behaviors (often called “stimming“)
-> resistance to changes or restricted interests
These features are typically assessed with the following, when appropriate:
* problems in obtaining or sustaining employment or education
* difficulties in initiating or sustaining social relationships
* connections with mental health or learning disability services
* a history of neurodevelopmental conditions (including learning disabilities and ADHD) or mental health conditions.

Common and early signs for autistic spectrum disorder to look for are-
avoidance of eye-contact
little or no babbling as an infant
difficulty pointing to show interest
not showing interest in indicated objects
delayed language skills (e.g. having a smaller vocabulary than peers or difficulty expressing themselves in words)
reduced interest in other children or caretakers, possibly with more interest in objects
difficulty playing reciprocal games (e.g. peek-a-boo)
increased sensitivity or unusual response to the smell, texture, sound, taste, or appearance of things
resistance to changes in routine
repetitive, limited, or otherwise unusual usage of toys
repetition of words or phrases (echolalia)
repetitive motions or movements, including stimming
self-harming

Autistic individuals may have symptoms that do not contribute to the official diagnosis, but that can affect the individual or the family. Some individuals with ASD show unusual abilities, ranging from splinter skills (such as the memorization of trivia) to the rare talents of autistic savants.One study describes how some individuals with ASD show superior skills in perception and attention, relative to the general population. Sensory abnormalities are found in over 90% of autistic people, and are considered core features by some. Differences between the previously recognized disorders under the autism spectrum 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). An estimated 60–80% of autistic people have motor signs that include poor muscle tone, poor motor planning, and toe walking; deficits in motor coordination are pervasive across ASD and are greater in autism proper. 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.
There is tentative evidence that gender dysphoria occurs more frequently in autistic people (see Autism and LGBT identities). As well as that, a 2021 anonymized online survey of 16–90 year-olds revealed that autistic males are more likely to identify as bisexual, while autistic females are more likely to identify as homosexual.
Gastrointestinal problems are one of the most commonly co-occurring medical conditions in autistic people. These are linked to greater social impairment, irritability, language impairments, mood changes, and behavior and sleep problems.

It had long been presumed that there is a common cause at the genetic, cognitive, and neural levels for the social and non-social components of autism’s symptoms, described as a triad in the classic autism criteria.However, there is increasing suspicion that autism is instead a complex disorder whose core aspects have distinct causes that often co-occur. While it is unlikely that a single cause for ASD exists, many risk factors identified in the research literature may contribute to ASD development. These risk factors include genetics, prenatal and perinatal factors (meaning factors during pregnancy or very early infancy), neuroanatomical abnormalities, and environmental factors. It is possible to identify general risk factors, but much more difficult to pinpoint specific factors. Given the current state of knowledge, prediction can only be of a global nature and therefore requires the use of general markers.

Autism Spectrum Disorder a clinical diagnosis that is typically made by a physician based off reported and directly observed behavior in the affected individual. According to the updated diagnostic criteria in the DSM-5-TR, in order to receive a diagnosis of Autism Spectrum Disorder, one must present with “persistent deficits in social communication and social interaction” and “restricted, repetitive patterns of behavior, interests, or activities.” These behaviors must begin in early childhood and affect one’s ability to perform everyday tasks. Furthermore, the symptoms must not be fully explainable by intellectual developmental disorder or global developmental delay. There are several factors that make Autism Spectrum Disorder difficult to diagnose. First off, there are no standardized imaging, molecular or genetic tests that can be used to diagnose ASD. Additionally, there is a lot of variety in how ASD affects individuals. The behavioral manifestations of ASD depend on one’s developmental stage, age of presentation, current support, and individual variability. Lastly, there are multiple conditions that may present similarly to Autism Spectrum Disorder, including intellectual disability, hearing impairment, a specific language impairment such as Landau–Kleffner syndrome. ADHD, anxiety disorder, and psychotic disorders. Furthermore, the presence of autism can make it harder to diagnose coexisting psychiatric disorders such as depression.
Ideally the diagnosis of ASD should be given by a team of clinicians (e.g. pediatricians, child psychiatrists, child neurologists) based on information provided from the affected individual, caregivers, other medical professionals and from direct observation. Evaluation of a child or adult for Autism Spectrum Disorder typically starts with a pediatrician or primary care physician taking a developmental history and performing a physical exam. If warranted, the physician may refer the individual to an ASD specialist who will observe and assess cognitive, communication, family, and other factors using standardized tools, and taking into account any associated medical conditions.  A pediatric neuropsychologist is often asked to assess behavior and cognitive skills, both to aid diagnosis and to help recommend educational interventions.  Further workup may be performed after someone is diagnosed with ASD. This may include a clinical genetics evaluation particularly when other symptoms already suggest a genetic cause. Although up to 40% of ASD cases may be linked to genetic causes, it is not currently recommended to perform complete genetic testing on every individual who is diagnosed with ASD. Consensus guidelines for genetic testing in patients with ASD in the US and UK are limited to high-resolution chromosome and fragile X testing. Metabolic and neuroimaging tests are also not routinely performed for diagnosis of ASD.he age at which ASD is diagnosed varies. Sometimes ASD can be diagnosed as early as 14 months, however, a reliable diagnosis of ASD is usually made at the age of two years. Diagnosis becomes increasingly stable over the first three years of life. For example, a one-year-old who meets diagnostic criteria for ASD is less likely than a three-year-old to continue to do so a few years later.Additionally, age of diagnosis may depend on the severity of ASD, with more severe forms of ASD more likely to be diagnosed at an earlier age. Issues with access to healthcare such as cost of appointments or delays in making appointments often lead to delays in the diagnosis of ASD. In the UK the National Autism Plan for Children recommends at most 30 weeks from first concern to completed diagnosis and assessment, though few cases are handled that quickly in practice. Lack of access to appropriate medical care, broadening diagnostic criteria and increased awareness surrounding ASD in recent years has resulted in an increased number of individuals receiving a diagnosis of ASD as adults. Diagnosis of ASD in adults poses unique challenges because it still relies on an accurate developmental history and because autistic adults sometimes learn coping strategies (known as ‘camouflaging’) which may make it more difficult to obtain a diagnosis.
The presentation and diagnosis of Autism Spectrum Disorder may vary based on sex and gender identity. Most studies that have investigated the impact of gender on presentation and diagnosis of Autism Spectrum Disorder have not differentiated between the impact of sex versus gender. There is some evidence that autistic women and girls tend to show less repetitive behavior and may engage in more camouflaging than autistic males. Camouflaging may include making oneself perform normative facial expressions and eye contact. Differences in behavioral presentation and gender-stereotypes may make it more challenging to diagnose autism spectrum disorder in a timely manner in females. A notable percentage of autistic females may be misdiagnosed, diagnosed after a considerable delay, or not diagnosed at all.

Considering the unique challenges in diagnosing ASD using behavioral and observational assessment, specific US practice parameters for its assessment were published by the American Academy of Neurology in the year 2000, the American Academy of Child and Adolescent Psychiatry in 1999, and a consensus panel with representation from various professional societies in 1999. The practice parameters outlined by these societies include an initial screening of children by general practitioners (i.e., “Level 1 screening”) and for children who fail the initial screening, a comprehensive diagnostic assessment by experienced clinicians (i.e. “Level 2 evaluation”). Furthermore, it has been suggested that assessments of children with suspected ASD be evaluated within a developmental framework, include multiple informants (e.g., parents and teachers) from diverse contexts (e.g., home and school), and employ a multidisciplinary team of professionals (e.g., clinical psychologists, neuropsychologists, and psychiatrists).
As of 2019, psychologists wait until a child showed initial evidence of ASD tendencies, then administer various psychological assessment tools to assess for ASD. Among these measurements, the Autism Diagnostic Interview-Revised (ADI-R) and the Autism Diagnostic Observation Schedule (uADOS) are considered the “gold standards” for assessing autistic children.  The ADI-R is a semi-structured parent interview that probes for symptoms of autism by evaluating a child’s current behavior and developmental history. The ADOS is a semistructured interactive evaluation of ASD symptoms that is used to measure social and communication abilities by eliciting several opportunities for spontaneous behaviors (e.g., eye contact) in standardized context. Various other questionnaires (e.g., The Childhood Autism Rating Scale, Autism Treatment Evaluation Checklist) and tests of cognitive functioning (e.g., The Peabody Picture Vocabulary Test) are typically included in an ASD assessment battery. The diagnostic interview for social and communication disorders (DISCO) may also be used.

About half of parents of children with ASD notice their child’s atypical behaviors by age 18 months, and about four-fifths notice by age 24 months. If a child does not meet any of the following milestones, it “is an absolute indication to proceed with further evaluations. Delay in referral for such testing may delay early diagnosis and treatment and affect the [child’s] long-term outcome.”
No response to name (or gazing with direct eye contact) by 6 months.
No babbling by 12 months.
No gesturing (pointing, waving, etc.) by 12 months.
No single words by 16 months.
No two-word (spontaneous, not just echolalic) phrases by 24 months.
Loss of any language or social skills, at any age.
The Japanese practice is to screen all children for ASD at 18 and 24 months, using autism-specific formal screening tests. In contrast, in the UK, children whose families or doctors recognize possible signs of autism are screened. It is not known which approach is more effective. The UK National Screening Committee does not recommend universal ASD screening in young children. Their main concerns includes higher chances of misdiagnosis at younger ages and lack of evidence of effectiveness of early interventions. There is no consensus between professional and expert bodies in the US on screening for autism in children younger than 3 years.
Screening tools include the Modified Checklist for Autism in Toddlers (M-CHAT), the Early Screening of Autistic Traits Questionnaire, and the First Year Inventory; initial data on M-CHAT and its predecessor, the Checklist for Autism in Toddlers (CHAT), on children aged 18–30 months suggests that it is best used in a clinical setting and that it has low sensitivity (many false-negatives) but good specificity (few false-positives). It may be more accurate to precede these tests with a broadband screener that does not distinguish ASD from other developmental disorders. Screening tools designed for one culture’s norms for behaviors like eye contact may be inappropriate for a different culture. Although genetic screening for autism is generally still impractical, it can be considered in some cases, such as children with neurological symptoms and dysmorphic features

DSM is the American Psychiatric Association‘s Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR), released in 2022, is the current version of the DSM. The fifth edition, DSM-5, released in May 2013, was the first to define ASD as a single diagnosis,which is continued in DSM-5-TR.ASD encompasses previous diagnoses which included Asperger disorder, childhood disintegrative disorder, PDD-NOS, and the range of diagnoses which included the word autism.Rather than distinguishing between these diagnoses, the DSM-5 and DSM-5-TR adopt a dimensional approach to diagnosing disorders that fall underneath the autistic spectrum umbrella in one diagnostic category. Within this category, the DSM-5 and the DSM includes a framework that differentiates each individual by dimensions of symptom severity, as well as by associated features (i.e., the presence of other disorders or factors which likely contribute to the symptoms, other neurodevelopmental or mental disorders, intellectual disability, or language impairment).The symptom domains are social communication and restricted, repetitive behaviors, with the option of a separate severity – the negative impact of the symptoms on the individual – being specified for each domain, rather than an overall severity. Prior to the DSM-5, the DSM separated social deficits and communication deficits into two domains.Further, the DSM-5 changed to an onset age in the early developmental period, with a note that symptoms may manifest later when social demands exceed capabilities, rather than the previous, more restricted 3 years of age.These changes continue in the DSM-5-TR.