Research suggests that some of the core symptoms of autism abate to some extent in adolescence and young adulthood (Seltzer, Shattuck, Abbeduto, & Greenberg, 2004). Improvements in communication were most common, with social deficits tending to persist. In some cases, these social deficits are the result of a lack of social motivation, while in other cases, individuals have a high degree of social motivation but lack the social skills to form meaningful and satisfying social relationships. Some individuals continue to demonstrate restricted interests and repetitive behaviors into adulthood, while for others, these symptoms may attenuate in adulthood. It is important to note that while many individuals show a reduction in symptoms in adulthood, this is not the case for all individuals, and it is rarely the case that symptoms abate to the extent that an individual no longer meets diagnostic criteria for the disorder in adulthood.

While intellectual ability (IQ) tends to remain stable over time, some individuals show significant increases or decreases in IQ over time (Howlin, Goode, Hutton, & Rutter, 2004). The most common pattern of change seems to be small declines in Performance (nonverbal) IQ and small increases in Verbal IQ from childhood to adulthood, although some individuals demonstrate striking increases in Verbal IQ. Brereton and Tonge (2002) investigated neuropsychological profiles in adults and found persistent impairments in “subtle social cognition” marked by decreased attention to eyes and voices, as well as impairments in memory, executive functioning, attention, cognitive flexibility, psychomotor processing speed, and motor coordination.

Early communication skills and level of cognitive functioning are the strongest predictors of outcome, with those individuals having strong early language skills or higher IQ faring better in adulthood (Eaves & Ho, 2008; Howlin, 2000; Seltzer et al., 2004). Those with an IQ above 70 have the best chance for success at living independently. However, even then, outcome is variable, as some individuals with very high IQ face significant challenges in adulthood.

Identified rates of psychiatric comorbidities in adults with ASD vary greatly across studies. This is likely in part due to differing samples. Additionally, it is challenging to assess comorbid psychopathology in individuals with ASD and intellectual disability, as most traditional measures for assessing psychiatric disorders rely on self-report, interviews, and behavioral observations that are inappropriate for individuals with more severe levels of intellectual disability (Myrbakk & von Tetzchner, 2008). Howlin (2000) found rates of psychiatric comorbidities ranging from 11 to 67 %. Engström, Ekström, and Emilsson (2003) similarly found rates that ranged from 9 to 89 %. Mood disorders and anxiety are the most common diagnoses in adults with ASD, and in many cases, depression and anxiety co-occur.

Estimated rates of depression in ASD have ranged from 20 (Shtayermman, 2007) to 33 % (Howlin, 2000). Bipolar disorder, or mania, is the next most common mood disorder in adults with ASD and was found in 21 % of the sample in one study (Howlin, 2000). Depressed mood may be particularly common in high functioning adults, who have insight into their social and adaptive difficulties, and who may desire to make changes but have limited success in doing so; however, depression is also seen in individuals with comorbid autism and intellectual disability (Smith & Matson, 2010).

Anxiety disorders are also common, as for example, Shtayermman (2007) found that 30 % of individuals with ASD also met criteria for generalized anxiety disorder. Smith and Matson (2010) and Gillott and Standen (2007) compared groups of adults with autism and intellectual disability to a group of adults with intellectual disability only, and both found that the groups with autism and intellectual disability had significantly higher levels of anxiety than the groups with intellectual disability alone. The most commonly occurring types of anxiety in adults with ASD and intellectual disability were panic/agoraphobia, separation anxiety, obsessive–compulsive disorder (OCD), and generalized anxiety disorder (Gillott & Standen, 2007). The authors attributed the increased occurrence of anxiety in the ASD group to fears of change, anticipation of future events, and sensitivity to sensory stimuli. In addition, participants were found to have higher stress from all of these sources than did controls. The authors further noted that the increased anxiety in the ASD group may be secondary to social deficits, in that individuals with ASD may have less social support to help them cope with their anxiety. In the absence of appropriate coping strategies, individuals may turn to aggression and self-injury.

In addition to certain disorders commonly co-occurring with ASD, a number of other disorders are easily mistaken for ASD in adults. Oftentimes, a comprehensive evaluation by professionals with expertise in ASD is essential in making a differential diagnosis. Without this expertise as to the nature of the ASD presentation, many individuals may seem to fit the DSM diagnostic criteria for ASD when in fact their difficulties may be attributable to another diagnosis. Thus, extensive clinician training, as well as a thorough exploration of possible alternative explanations for a patient’s symptoms, is critical for making an accurate diagnosis, especially in complex cases with multiple co-occurring symptoms.

Although anxiety disorders commonly co-occur with ASD, it is also possible that an anxiety disorder could be mistaken for an ASD when in fact the anxiety alone accounts for the individual’s presentation. Many forms of anxiety (e.g., social phobia, generalized anxiety, agoraphobia) can impact social functioning, and thus careful consideration must be given to whether the individual’s social difficulties can be accounted for by the anxiety or whether a primary social deficit more indicative of an ASD is present. OCD also shares some features with ASD, particularly with regard to rigid and repetitive behaviors; a determination as to whether an impairment in social cognition is also present can aid in differentiating between these conditions. Furthermore, in OCD, the repetitive behaviors represent a compulsion that is often tied directly to a particular obsessive or anxious thought, and occurs as a means of countering that anxiety (e.g., repeated hand washing tied to a fear of germs; compulsive checking tied to an anxiety that a door was left unlocked or a stove left turned on). In ASD, the repetitive behaviors are not typically tied to a specific obsessive thought in this way, and circumscribed interests, although impairing in their intensity, are not tied to anxiety as are obsessive thoughts.

Depression may also contribute to social difficulties that can be misidentified as an ASD. The lack of energy and lethargy characteristic of depression can contribute to a failure to take initiative, and this can include the social realm (e.g., staying home, not having the motivation or energy to socialize). As a result, individuals with depression can become socially isolated, which may lead to a referral for a possible ASD. Evaluation of an individual’s social cognition and social skill, along with a psychiatric evaluation for depression, can help to determine whether the individual’s social presentation is secondary to depressed mood.

Intellectual disability has a high rate of co-occurrence with ASD, and in some individuals with intellectual disability, it can be difficult to determine whether an ASD is also present. Although this differential has typically been made prior to adulthood, at times a patient may present with a history of a diagnosis of intellectual disability when it becomes apparent that an ASD is also present; or with a diagnosis of an ASD when in fact the impairments are attributable solely to the intellectual disability. In these cases, a comprehensive evaluation can help to clarify the diagnosis. Individuals with intellectual disability (without ASD) may exhibit repetitive behaviors and motor mannerisms similar to those seen in ASD. Intellectual disability can also impact language and communication skills. Thus, assessment of social functioning is the most critical factor in differentiating these two conditions. While individuals with autism often exhibit reduced eye contact, joint attention, and seeking to share enjoyment, these skills are typically intact in individuals with intellectual disability. While not diagnostic in itself, the individuals’ cognitive profile may provide supportive information, in that individuals with ASD tend to have a high degree of scatter in their profile (i.e., significant areas of strength and weakness) whereas individuals with intellectual disability tend to have a more uniform profile. Additionally, individuals with ASD and comorbid intellectual disability and those with intellectual disability alone have deficits in adaptive skills development, but in individuals with intellectual disability alone, their adaptive skills tend to be equally impaired and generally on par, or even slightly above, their cognitive abilities. In ASD, individuals’ adaptive skills tend to be well below their cognitive skills with particular deficits in adaptive socialization.

Nonverbal learning disability (NLD) is another common differential diagnosis, particularly in higher functioning school-aged children and adults. NLD is a learning disability characterized by a specific pattern of neuropsychological strengths and weaknesses. The NLD profile is commonly seen in individuals with Asperger’s syndrome (Rourke, 1989). The assessment of NLD requires a comprehensive neuropsychological evaluation. Individuals with NLD have strengths in some neuropsychological processes, including simple motor tasks, auditory perception, verbal attention and memory, and phonological processing, and marked weaknesses in others, such as visual and tactile perception, complex motor tasks, visual attention and memory, problem solving, and social aspects of language. Although individuals with NLD have deficits in social interaction skills, and some individuals with a social disability also evidence an NLD profile based on neuropsychological assessment, not all individuals with NLD have ASD. Instead, their social deficits are a function of their learning profile. For example, they may have difficulty reading nonverbal cues in social interactions due to their impairments in nonverbal information processing. Another feature distinguishing ASD from NLD is the presence of unusual or repetitive behaviors and circumscribed interests. These behaviors are seen in ASD but not NLD. Given the overlap between ASD and NLD and the complexity of an NLD profile, a comprehensive neuropsychological and diagnostic evaluation is essential when the diagnosis of NLD is in question.

Individuals with attention deficit hyperactivity disorder (ADHD) often have impairments in executive functioning that may cause them to underperform in post-secondary education thus leading to a referral for an evaluation. In addition, ADHD often leads to secondary social impairments that can be mistaken for an ASD, leading to a referral for a differential diagnosis. For example, the impulsivity associated with ADHD can lead to socially inappropriate behaviors, which can, in turn, contribute to individuals feeling socially ostracized. Furthermore, impairments in executive function associated with ADHD can impact the ability to see the “big picture,” which may lead the individual to miss important information in social interactions. A careful evaluation of executive functioning and social presentation will help to differentiate between ADHD and ASD. Furthermore, individuals with ADHD would not be expected to display the communication deficits or restricted interests and repetitive behaviors common in ASD.

Individuals with schizophrenia or other psychotic disorders may share some characteristics seen in ASD as well. For example, psychosis can lead to socially inappropriate behaviors as well as a lack of personal and social insight and awareness. In addition, individuals with thought disorders may use nonsensical or atypical language. In making the differential between a psychotic condition and an ASD, it is critical to obtain a detailed history in order to determine onset. While ASD must be present in early development (before age 3), the onset of psychotic disorders typically occurs in late adolescence or early adulthood. (Of note, childhood forms of schizophrenia do exist, although they are rare, and individuals with an adolescent- or adult-onset psychosis may show some prodromal signs during childhood.) In addition, the presence of positive symptoms such as hallucinations and delusions would be suggestive of a psychotic condition rather than an ASD. The evaluation should also carefully assess the individual’s grounding in reality. With regard to speech patterns, it is easy to mistake scripted language for non-reality-based language if the evaluator is not familiar with the source from which the individual is scripting. For example, an individual with ASD may make an off-topic, tangential, or fantastical comment that may appear disconnected from reality, when in fact the comment was a quoted line from a favorite movie or television program. It is helpful to transcribe samples of the individual’s language, and then investigate whether it may have been scripted. Speaking with parents or siblings can be helpful, as they are often familiar with the individual’s favorite television programs and movies and can identify scripted phrases that the individual frequently uses. In some cases, an internet search with the phrasing in quotations may also identify the source from which the individual was repeating lines. If the individual seems to be making frequent nonsensical comments that cannot be tied to an identifiable source, then this may be more suggestive of a thought disorder. Some speech patterns (e.g., tangential or circumstantial speech, neologisms) may be common in both conditions. However, some patterns of speech (e.g., “word salad” in which the individual jumbles words together with no apparent meaning) are more specific to schizophrenia.