Asperger syndrome (AS), also known as Asperger's, is a developmental disorder characterised by significant difficulties in social interaction and nonverbal communication, along with restricted and repetitive patterns of behavior and interests. As a milder autism spectrum disorder (ASD), it differs from other ASDs by relatively normal language and intelligence. Although not required for diagnosis, physical clumsiness and unusual use of language are common. Signs usually begin before two years of age and typically last for a person's entire life. The exact cause of Asperger's is unknown. While it is probably partly inherited, the underlying genetics have not been determined conclusively. Environmental factors are also believed to play a role. Brain imaging has not identified a common underlying problem. In 2013, the diagnosis of Asperger's was removed from the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), and people with these symptoms are now included within the autism spectrum disorder along with autism and pervasive developmental disorder not otherwise specified (PDD-NOS). It remains within the International Classification of Diseases (ICD-11) as of 2019 but as a subtype of autism spectrum disorder. There is no single treatment, and the effectiveness of particular interventions is supported by only limited data. Treatment is aimed at improving poor communication skills, obsessive or repetitive routines and physical clumsiness. Interventions may include social skills training, cognitive behavioral therapy, physical therapy, speech therapy, parent training, and medications for associated problems, such as mood or anxiety. Most children improve as they grow up, but social and communication difficulties usually persist. Some researchers and people on the autism spectrum have advocated a shift in attitudes toward the view that autism spectrum disorder is a difference rather than a disease that must be treated or cured. In 2015, Asperger's was estimated to affect 37.2 million people globally. Autism spectrum disorder affects males more often than females and females are typically diagnosed at a later age. The syndrome is named after the Austrian pediatrician Hans Asperger, who, in 1944, described children in his practice who lacked nonverbal communication skills, had limited understanding of others' feelings, and were physically clumsy. The modern conception of Asperger syndrome came into existence in 1981 and went through a period of popularization. It became a standardized diagnosis in the early 1990s. Many questions and controversies remain. There is doubt about whether it is distinct from high-functioning autism (HFA). Partly because of this, the percentage of people affected is not firmly established. The extent of the overlap between AS and high-functioning autism (HFA – autism unaccompanied by intellectual disability) is unclear. The ASD classification is to some extent an artifact of how autism was discovered, and may not reflect the true nature of the spectrum; methodological problems have beset Asperger syndrome as a valid diagnosis from the outset. In the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), published in May 2013, AS, as a separate diagnosis, was eliminated and folded into autism spectrum disorder. Like the diagnosis of Asperger syndrome, the change was controversial and AS was not removed from the WHO's ICD-10 or ICD-11. The World Health Organization (WHO) defined Asperger syndrome (AS) as one of the autism spectrum disorders (ASD) or pervasive developmental disorders (PDD), which are a spectrum of psychological conditions that are characterized by abnormalities of social interaction and communication that pervade the individual's functioning, and by restricted and repetitive interests and behavior. Like other psychological development disorders, ASD begins in infancy or childhood, has a steady course without remission or relapse, and has impairments that result from maturation-related changes in various systems of the brain. ASD, in turn, is a subset of the broader autism phenotype, which describes individuals who may not have ASD but do have autistic-like traits, such as social deficits. Of the other four ASD forms, autism is the most similar to AS in signs and likely causes, but its diagnosis requires impaired communication and allows delay in cognitive development; Rett syndrome and childhood disintegrative disorder share several signs with autism but may have unrelated causes; and pervasive developmental disorder not otherwise specified (PDD-NOS) is diagnosed when the criteria for a more specific disorder are unmet.