Studies were done to see the prevalence and incidence of mental illness in adult with autism is comparable to the whole intellectual disabilities adult population (Melville, Cooper & et al, 2008) Conversely, when comparison was with gender, age, ability and Down syndrome, there were no differences. (Melville & Cooper, 2008). The high prevalence was due to behavioral problems, which was not attributed the presence of autism in the individuals. (Melville, Cooper & et al, 2008). There was a trend that illustrates the low incidence of behavioral problem in adults with autism compared to the control group in the study, but is not supported by statistics. (Melville & Cooper et al, 2008). During a two year period, the recovery rate of challenging behaviors was notably lower for adult with autism (Melville, Cooper & et al, 2008)
Screening tool used in the study for psychiatric disorders was the PAS-ADD Checklist for adults with intellectual disabilities (Melville, Cooper & et al, 2008). This checklist is comprise of 29 item to help determine psychopathology frequently experience with this population (Melville, Cooper & et al, 2008). There was a physical to help rule out any health problems that contribute any psychiatric issues (Melville, Cooper & et al, 2008).
During the first study, researchers use the matched control method to identify the confounding factors that relate to mental illness and challenging behaviors (Melville, Cooper & et al, 2008). The female population in this study had higher rates of depression (Melville, Cooper & et al, 2008). They did not have psychosis (Melville, Cooper & et al, 2008). However, the male population in this study was reported to have a higher rate of challenging behaviors (Melville, Cooper & et al, 2008). Behaviors that would make psychiatric referrals for this population would be self-injurious behavior, destruction of property, verbal aggression, physical aggression towards others, etc(Melville, Cooper & et al, 2008).
A current study reports difference in the prevalence of adults with autism and intellectual disabilities and adults with intellectual disabilities without autism in experiencing mental-illness(Melville, Cooper & et al, 2008). There are several authors that have pinpointed that the experience of depression has increased with adults who have autism and intellectual disabilities (Melville, Cooper & et al, 2008). There was a similar finding in a study by Tsakanikos (Tsakanikos et al.2006) However, the solid support of evidence in the study is lacking (Melville, Cooper & et al, 2008). The link of autism and schizophrenia has been explored; however, the link has not supported in this study (Melville, Cooper & et al, 2008).