Autism is a highly variable neurodevelopmental disorder that first appears during infancy or childhood, and generally follows a steady course without remission. People with autism may be severely impaired in some respects but normal, or even superior, in others. Overt symptoms gradually begin after the age of six months, become established by age two or three years, and tend to continue through adulthood, although often in more muted form. 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. 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.

As a pervasive developmental disorder, Asperger syndrome is distinguished by a pattern of symptoms rather than a single symptom. It is characterized by qualitative impairment in social interaction, by stereotyped and restricted patterns of behavior, activities and interests, and by no clinically significant delay in cognitive development or general delay in language. Intense preoccupation with a narrow subject, one-sided verbosity, restricted prosody, and physical clumsiness are typical of the condition, but are not required for diagnosis. Suicidal behavior appears to occur at rates similar to those without ASD.

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

- Stereotyped behaviors: Repetitive movements, such as hand flapping, head rolling, or body rocking.

- Compulsive behaviors: Time-consuming behaviors intended to reduce anxiety that an individual feels compelled to perform repeatedly or according to rigid rules, such as placing objects in a specific order, checking things, or hand washing.

- Sameness: Resistance to change; for example, insisting that the furniture not be moved or refusing to be interrupted.

- Ritualistic behavior: 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.

- Restricted interests: Interests or fixations that are abnormal in theme or intensity of focus, such as preoccupation with a single television program, toy, or game.

- Self-injury: Behaviors such as eye-poking, skin-picking, hand-biting and head-banging.

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

A lack of demonstrated empathy affects aspects of communal living for persons with Asperger syndrome. Individuals with AS experience difficulties in basic elements of social interaction, which may include a failure to develop friendships or to seek shared enjoyments or achievements with others (for example, showing others objects of interest), a lack of social or emotional reciprocity (social "games" give-and-take mechanic), and impaired nonverbal behaviors in areas such as eye contact, facial expression, posture, and gesture.

People with AS may not be as withdrawn around others, compared with those with other, more debilitating forms of autism; they approach others, even if awkwardly. For example, a person with AS may engage in a one-sided, long-winded speech about a favorite topic, while misunderstanding or not recognizing the listener's feelings or reactions, such as a wish to change the topic of talk or end the interaction. This social awkwardness has been called "active but odd". This failure to react appropriately to social interaction may appear as disregard for other people's feelings, and may come across as insensitive. However, not all individuals with AS will approach others. Some of them may even display selective mutism, speaking not at all to most people and excessively to specific people. Some may choose only to talk to people they like.

The cognitive ability of children with AS often allows them to articulate social norms in a laboratory context, where they may be able to show a theoretical understanding of other people's emotions; however, they typically have difficulty acting on this knowledge in fluid, real-life situations. People with AS may analyze and distill their observations of social interaction into rigid behavioral guidelines, and apply these rules in awkward ways, such as forced eye contact, resulting in a demeanor that appears rigid or socially naive. Childhood desire for companionship can become numbed through a history of failed social encounters.

High-functioning autism is characterized by features very similar to those of Asperger syndrome. The defining characteristic most widely recognized by psychologists is a significant delay in the development of early speech and language skills, before the age of three years. The diagnostic criteria of Asperger syndrome exclude a general language delay.

Further differences in features between people with high-functioning autism and those with Asperger syndrome, include the following:

- People with HFA have a lower verbal reasoning ability

- Better visual/spatial skills (higher performance IQ) than people with Asperger syndrome

- Less deviating locomotion than people with Asperger syndrome

- People with HFA more often have problems functioning independently

- Curiosity and interest for many different things, in contrast to people with Asperger syndrome

- People with Asperger syndrome are better at empathizing with another

- The male to female ratio of 4:1 for HFA is much smaller than that of Asperger syndrome

Individuals with autism spectrum disorders, including high-functioning autism, risk developing symptoms of anxiety. While anxiety is one of the most commonly occurring mental health symptoms, children and adolescents with high functioning autism are at an even greater risk of developing symptoms.

There are other comorbidities, the presence of one or more disorders in addition to the primary disorder, associated with high-functioning autism. Some of these include depression, bipolar disorder, and obsessive compulsive disorder (OCD). In particular the link between HFA and OCD, has been studied; both have abnormalities associated with serotonin.

Observable comorbidities associated with HFA include ADHD, Tourette syndrome, and possibly criminal behavior. While the association between HFA and criminal behavior is not completely characterized, several studies have shown that the features associated with HFA may increase the probability of engaging in criminal behavior. While there is still a great deal of research that needs to be done in this area, recent studies on the correlation between HFA and criminal actions suggest that there is a need to understand the attributes of HFA that may lead to violent behavior. There have been several case studies that link the lack of empathy and social naïveté associated with HFA to criminal actions.

HFA does not cause nor include intellectual disabilities. This characteristic distinguishes HFA from the rest of the autism spectrum; between 40 and 55% of individuals with autism also have an intellectual disability.

High-functioning autism (HFA) is a term applied to people with autism who are deemed to be cognitively "higher functioning" (with an IQ of 70 or greater) than other people with autism. Individuals with HFA or Asperger syndrome may exhibit deficits in areas of communication, emotion recognition and expression, and social interaction. HFA is not a recognized diagnosis in the DSM-5 or the ICD-10.

The amount of overlap between HFA and Asperger syndrome is disputed.

A high-functioning alcoholic (HFA) is a person who maintains jobs and relationships while exhibiting alcoholism.

Statistics from the Harvard School of Public Health indicated that 31 percent of college students show signs of alcohol abuse and 6 percent are dependent on alcohol. Doctors hope that the new definition will help identify severe cases of alcoholism early, rather than when the problem is fully developed.

Many HFAs are not viewed as alcoholics by society because they do not fit the common alcoholic stereotype. Unlike the stereotypical alcoholic, HFAs have either succeeded or over-achieved throughout their lifetimes. This can lead to denial of alcoholism by the HFA, co-workers, family members, and friends. Functional alcoholics account for 19.5 percent of total U.S. alcoholics, with 50 percent also being smokers and 33 percent having a multigenerational family history of alcoholism.