The DSM-IV-TR cautioned against diagnosing anxiety disorders in individuals with ASD, including GAD, social anxiety, and separation anxiety (APA 2000). The DSM-5 criteria cautions against diagnosing separation anxiety and social anxiety as well and additionally extend this caution to obsessive-compulsive disorder (OCD; APA 2013). Despite this, anxiety disorders are some of the most commonly co-occurring psychiatric disorders in individuals with ASD, with up to half of children meeting criteria for any anxiety disorder according to parent ratings, and many children meeting diagnostic criteria for multiple anxiety disorders. In fact, children and adolescents with ASD show higher levels of anxiety than typically developing controls (for a review, see MacNeil et al. 2009).

Despite the high rate of co-occurring anxiety disorders in individuals with ASD, anxiety may often be unrecognized or mislabeled. There is considerable overlap between the symptoms of ASD and those of anxiety. Behaviors commonly associated with ASD such as verbal rituals and repetitive questioning can also be interpreted as symptoms of anxiety. Core deficits of ASD, including communication deficits and difficulties in emotion recognition and labeling, complicate diagnosis even further as many children with ASD are unable to recognize or report symptoms of anxiety .

Considerable debate has arisen regarding the relationship between OCD and ASD as repetitive and ritualistic behaviors are core symptoms of both disorders. One distinction between disorders is that repetitive behaviors associated with ASD do not seem to occur against the person’s will, in contrast to OCD where compulsions are uncontrollable and unpleasant. In a comparison of repetitive behavior in typically developing children, high functioning autism (HFA), or OCD, Zandt et al. (2009) found that children with OCD exhibited more routines and rituals as well as more compulsions and obsessions than children with ASD. In addition, the obsessions and compulsions were more sophisticated in children with OCD. Matson and Nebel-Schwalm (2007) suggested the following when diagnosing OCD in ASD. First, the individual with ASD should engage in behaviors that are typical of OCD, such as frequent hand washing to kill germs. Second, the OCD symptoms exhibited should be greater than the repetitive behaviors expected in a typical case of ASD. Lastly, individuals with ASD and comorbid OCD should show reduction in OCD symptoms from interventions known to be effective for OCD .

Rates of depression seem higher in individuals with ASD compared to community control groups of typically developing children. For instance, Gadow et al. (2005) found that 6 and 11 % of their sample of 6- to 12-year-olds with ASD screened positively for MDD and dysthymic disorder, respectively. In contrast, no typically-developing children in regular education screened positively for MDD and less than 1 % did for dysthymic disorder. Some studies have shown much higher rates in both individuals with ASD and typically developing control groups. For example, Mayes et al. (2011) assessed children ages 6–16 with the Pediatric Behavior Scale, and found that 54 % of children with HFA (IQ greater than 80) and 42 % of lower functioning children with ASD (IQ less than 80) were depressed. In contrast, 19 % of typically developing children in their sample were depressed.

Similar to difficulties associated with diagnosing anxiety, communication deficits and difficulties understanding emotions can make diagnosis of depression challenging in individuals with ASD. As a result, many of the symptoms of depression are reported by caregivers or observed in changes in the child’s behavior (Ghaziuddin et al. 2002; Stewart et al. 2006). Some case studies have reported that the onset of depression can be associated with the onset or exacerbation of problem behaviors, including self-injury and aggression, as well as a decrease in self-care behaviors, such as toileting (e.g. Clarke et al. 1999; Long et al. 2000) .

Similar to depression and anxiety, the diagnosis of bipolar disorder can be challenging in individuals with ASD. Additionally, the diagnosis of childhood bipolar disorder is a controversial topic due to the dramatic increase in diagnostic rates and unclear diagnostic criteria for this age group (Leibenluft and Rich 2008). However, clinicians should take note of any cyclical changes in activity, behavior, or interests in children with ASD, as well as a family history of bipolar disorder as this may indicate comorbid bipolar disorder.

In a recent review of research on the comorbidity of ADHD and ASD, Gargaro et al. (2011) concluded that while more neuropsychological research is needed to fully understand the comorbid presentation of ASD and ADHD, current clinical opinion, research, and theoretical models suggest that these disorders are distinct, and that their comorbidity is frequent. In one study looking at the comorbidity of these disorders, Gadow et al. (2006) examined differences in associated features in two groups of children: one group with both ASD and ADHD and one with ASD only. Results indicated that children with both ASD and ADHD were significantly different from children with ASD only in terms of co-occurring psychiatric symptoms, ASD symptoms, as well as other psychosocial variables, suggesting that children with both ASD and ADHD are distinct from children with ASD only .

As seen in Table 27.1 and elsewhere (e.g., Kim et al. 2000; Sinzig et al. 2009), the prevalence of ADHD in individuals with ASD is quite high. These rates are significantly higher than what is seen in typically developing children. For example, Gadow et al. (2004) found that 41 % of preschool children with ASD screened positive for ADHD, compared to 6 % of typically developing children. This discrepancy was even larger with school aged children, where 60 % of children with ASD screened positive for ADHD compared to 6 % of typically developing children.

Little is known about the causes of the high comorbidity of ADHD and ASD. However, genetic studies are pointing at the possibility of a common genetic basis for the two disorders. Linkage studies have shown that similar areas of the genome may be involved in both ASD and ADHD , including 5p13, 9q33–34, 16p13, and 17p11–q11. However, it is unclear if these overlapping regions reflect common risk genes or only genes that are in close proximity to each other (Smalley et al. 2004). In a community-based sample of 6,771 twins, there were significant correlations (0.54 for parent data; 0.51 for teacher data) between autistic and ADHD traits , as measured by the Childhood Asperger Syndrome Test and the Conners’ Revised Parent/Teacher Rating Scales, suggesting a genetic overlap between the two disorders (Ronald et al. 2008). In another population-based twin sample, ADHD was associated with elevated rates of ASD traits as measured by the Missouri Assessment of Genetics Interview for Children, a semi-structured interview based on the DSM-IV, and the Social Responsiveness Scale, further suggesting a genetic overlap between ADHD and ASD (Reiersen et al. 2007) .

In both typically developing children and those with ASD, disruptive behavior disorders such as ODD and conduct disorder (CD) frequently co-occur with ADHD . Simonoff et al. (2008) found that 52 % of those with ASD and ADHD also met criteria for either ODD or CD. Gadow et al. (2008) compared four groups of children: ASD and ODD ( = 19), ASD and ADHD ( = 207), ASD and both ODD and ADHD ( = 113), and ASD only ( = 235). Results indicated that children with ASD and no ODD are distinct from children with ASD and ODD. Those without comorbid ODD showed less severe symptoms of GAD and MDD. Additionally, children with ASD and both ODD and ADHD showed more severe symptoms of CD, MDD, dysthymia, obsessions, compulsions, and separation anxiety disorder, as well as more severe social deficits and perseverative behaviors than children with ASD only .

Tics are sudden vocalizations or motor movements that are recurrent and stereotyped (APA 2000). At times, they can be quite difficult to differentiate from stereotyped behaviors and other repetitive behaviors that occur as part of ASD. However, tics are typically short in duration, inappropriate for the context, and often disrupt behavior and speech (Baron-Cohen et al. 1999). Few studies have looked at the prevalence of tics in children with ASD. However, available evidence seems to suggest that tic disorders occur in a number of children with ASD . For instance, Gjevik et al. (2011) found that 11 % of their sample met criteria for any tic disorder, while Mattila et al. (2010) found a rate of 26 %. Specific estimates of motor tics range from 6 to 17 % (Gadow et al. 2004; Mattila et al. 2010), and from 6 to 13 % for verbal tics (Gadow et al. 2004; Mattila et al. 2010). When looking specifically at Tourette’s Disorder, characterized by the presence of both motor and verbal tics, prevalence rates vary from 6.5 to 14 % in clinic-based samples of children with ASD (Baron-Cohen et al. 1999; Mattila et al. 2010). In their population-based sample of children with ASD, Simonoff et al. (2008) found a slightly lower prevalence rate of 4.8 %.

Rates of verbal and motor tics appear to be higher in children with ASD compared to typically developing children. For example, Gadow et al. (2005) reported that 17 % of clinic-referred preschool children with ASD had motor tics, compared to 1 % of typically developing children. Similarly, 14 % of clinic-referred school aged children with ASD showed verbal tics, compared to 2 % of typically developing children (Gadow et al. 2004). The presence of tics in children with ASD appears to be related to the presence of more severe symptoms of other psychiatric disorders, with the combination of ADHD and tics predicting the most severe symptoms of other psychiatric disorders, including ODD, CD, MDD, dysthymia, GAD, separation anxiety disorder and OCD (Gadow and DeVincent 2005) .

Historically, autism was thought to be a form of schizophrenia. It is now clear that these two conditions are distinct disorders, although they can occur together. These two disorders can be differentiated in a number of ways. Whereas autism has its onset in early childhood, schizophrenia typically does not appear until adolescence or early adulthood. Additionally, individuals with schizophrenia typically report hallucinations and delusions, which are not common symptoms associated with ASD. Family psychiatric history also differs between the two disorders as individuals with schizophrenia are more likely to have relatives diagnosed with schizophrenia spectrum disorders and individuals with ASD are more likely to have a family history of ASD. Additionally, ASD is often associated with ID and epilepsy, which is not typical in schizophrenia. Lastly, ASD is most often a chronic condition, whereas schizophrenia, while chronic, can also include periods of complete recovery. Despite these differences, autism and schizophrenia share some features and associated deficits. Both are characterized with social and language difficulties and inappropriate or constricted affect. Additionally, both autism and schizophrenia can be conceptualized as including deficits of theory of mind and mirror neuron deficits (King and Lord 2010).

Evidence points to a genetic overlap in ASD and schizophrenia. As both disorders occur in roughly 1 % of the population, the co-occurrence of schizophrenia and ASD should be quite rare at 0.01 %. Although research is currently limited, up to 10 % of individuals with ASD receive a diagnosis of a schizophrenia spectrum disorder (Howlin 2000), and 28 to 55 % of individuals with childhood-onset schizophrenia meet diagnostic criteria for ASD (Rapoport et al. 2009). Additionally, parental schizophrenia is a risk factor for ASD, further pointing to a genetic overlap between the two disorders (Daniels et al. 2008; Larsson et al. 2005). Candidate gene, linkage, and expression studies of schizophrenia and autism have shown several regions that are associated with both disorders, including 1q21.1, 1q42, 2p16.3, 2q31.1, 7q22.1, 7q35–q36.1, 15q11–q13, 15q25, 16p11.2, 16p12.1, 17p12, 22q11.21, and 22q13.3 (for reviews, see Burbach and van der Zwaag 2009; King and Lord 2010; Rapoport et al. 2009). Lastly, similar pharmacologic treatments are indicated for both disorders as the only two drugs approved for behavioral problems associated with autism are risperidone and aripiprazole, which were originally developed for schizophrenia .

Intervention research for comorbid psychiatric disorders in ASD is riddled by the difficulty in making an accurate diagnosis, as discussed above. Several interventions have been developed that target behavioral syndromes, which may or may not be diagnosable psychiatric syndromes. Approaches to treatment have focused primarily on behaviorally-based, cognitive-behavioral, or psychopharmacological interventions. In the following section, we summarize select studies in each of these areas.