NADD Bulletin Volume X Number 5 Article 2

Complete listing

The Other Dual Diagnosis: Intellectual Disability & Mental Illness

Daniel Werges

This paper won first prize in a graduate category for the 14th Annual Paul LeClerc Competition for best research papers. This paper was initially written for course credit at the Hunter College School of Social Work, City University of New York. For further information, please contact Mr. Werges at:

In my daily work in a day habilitation program for people with intellectual disabilities, I have come to notice problem areas in relation to the dual diagnosis of intellectual disability and mental illness. First, there appears to be a general lack of awareness and knowledge in relation to mental health issues, and second, problem behaviors are often handled without considering potential causes relating to mental illness. These observations motivated me to learn more about the issues relating to dual diagnosis of people with intellectual disabilities, the current debates, treatment options, and cases of best practice. I will begin by defining dual diagnosis and a brief historical overview.

Definition & Prevalence: The Other Dual Diagnosis

Dual diagnosis refers to two diagnoses in one person and is most commonly used in relation to the concurrence of a psychiatric disorder along with substance abuse / dependence (Fellin, 1996). The term also can be used to describe the coexistence of an intellectual disability and a diagnosis of mental illness (Graziano, 2002). Intellectual disability is defined as below average intellectual functioning concurrent with deficits in adaptive functioning. The presence of an intellectual disability would be coded on Axis II of a DSM-IV based diagnosis, concurrent mental disorders are coded on Axis I (American Psychiatric Association, 2000; Austrian, 2005).

The prevalence of dual diagnosis is contested. Graziano (2002) cites a study which suggests that 20-35 percent of non-institutionalized persons with intellectual disabilities also have a mental illness, compared to 15-19 percent of the general population. Rojahn & Tasse (1996) reviewed research studies where the prevalence of psychopathology in people with intellectual disabilities varied from 10-40 percent. According to Campbell & Malone (1991), estimates of the prevalence of mental illness in people with intellectual disabilities range from 14-70%. Other literature cites sources which put the prevalence of psychiatric disorders among people with intellectual disabilities 4 to 5 times higher than in the general population (Rush, Bowman, Eidman, Toole, & Mortenson, 2004). The National Association for the Dually Diagnosed reports that many professionals have adopted an estimate of 30-35% prevalence of psychiatric disorders among people with intellectual and developmental disabilities (Fletcher, 2004).

Despite these discrepancies in findings, it is obvious that dual diagnosis is an important issue to be addressed. It can be said that individuals with intellectual disabilities are more vulnerable to mental illness than the general population. Harris (2006) lists a number of reasons for this vulnerability and higher prevalence, including reduced capacity to manage social and cognitive demands, problem-solving difficulties, poor social judgment, and communication limitations. Kerker, Owens, Zigler, & Horwitz (2004) list biological risk factors for mental illness among this population, such as higher rates of genetic abnormalities, brain damage, and biochemical abnormalities, psychological factors such as low self-esteem and damaged self-concept due to the realization of limitations, and social factors such as parental and peer rejection, limited support, and stigmatization. In summary it can be said that there is a higher prevalence of mental illness among people with intellectual disabilities and that the biopsychosocial approach of social work theory and practice (e.g. Austrian, 2005) is essential in understanding causes, assessment, and treatment of mental illness in this population.

Historical Background

A chapter titled ‘psychological evaluation and differential diagnosis’ in a 1964 book on mental retardation (Stevens & Heber) begins with the following paragraph:

“Psychological evaluations are made for a variety of purposes in the field of mental retardation. Perhaps the most prominent of these purposes are: (1) to determine whether or not an individual should be classified in the broad category of the mentally retarded; (2) if it is concluded that he does belong in the category of mentally retarded, to determine the type or subgroup of the broad category in which he should be placed; (3) to make a prognostic evaluation in respect to course of development and response to special educational, psychotherapeutic or physiotherapeutic measures; (4) if it is concluded that he does not belong in the category of mentally retarded, to determine whether he should be classified in a diagnostic category other than that of ‘normality’ (i.e., problem of differential diagnosis).”


This statement does not consider the importance of assessing mental illness in psychological evaluations, and thus demonstrates the historical phenomena of seeing mental retardation separate from mental illness. Harris (2006) notes that prior to the 1960s, intellectual disability and mental illness were seen as mutually exclusive conditions. At that point in history, affective and behavioral issues were regarded as relating to maladaptive learning and adverse psychosocial experiences rather than psychiatric disorders.

Stereotypes about people with intellectual disabilities also added to this perspective. For example, common attitudes regarded the ‘mildly retarded’ as worry-free and thus mentally healthy, and the ‘severely retarded’ as not able to express feelings and thus not able to experience emotional stress (VanderSchie-Bezyak, 2003). The fact that people with intellectual disabilities faced ‘appalling treatment’ (Menolascino & Fleisher, 1993) including segregation and sterilization during the first part of the 20th century adds to the explanation that mental illness was not considered a factor in the treatment of this population. If a person with an intellectual disability became aggressive or self-injurious due to mental illness, these kinds of behaviors were regarded as the result of the intellectual disability and were therefore untreatable; solutions that were applied revolved around punishment and physical or chemical restraint (Menolascino & Fleisher, 1993).

It was not until the 1960s, with the introduction of the Community Mental Health and Mental Retardation Facilities Construction Act, that the foundations for far-reaching changes occurred which led to the integration of people with intellectual disabilities into the mainstream of society (ibid.). However, it also required the media exposures of 1970s which revealed atrocious and abusive conditions in state institutions and subsequent law suits to add momentum to the deinstitutionalization movement (e.g. Rothman & Rothman, 2005; Covert, MacIntosh, & Shumway, 1994).

The mandate for community inclusion fueled the diagnosis and treatment of mental illness in people with intellectual disabilities, as maladaptive behavior was regarded as less tolerable in the community. Some authors also see an increased risk of mental illness if community inclusion is attempted without adequate supports and programs (Menolascino & Fleisher, 1993). Even today many service providers are not adequately prepared to deal with severe behavior problems related to dual diagnosis (Yoder, 1996).

Current Perspectives and Debates

There are countless issues in the current debates surrounding dual diagnosis. In the following I will summarize and discuss some of the most prevalent perspectives.

Underserved and Underdiagnosed 

Reoccurring debates in the literature see people with dual diagnosis as underserved (Reiss, Levitan, & McNally, 1982) and underdiagnosed (Rush et al., 2004). The most commonly cited reason for the connected issues of lack of service and diagnosis was first described by Reiss, Levitan, & Szyszko (1982) as “diagnostic overshadowing,” which refers to diagnosticians overlooking or minimizing the signs of psychiatric disturbances in a person with an intellectual disability. The psychiatric disorder might be considered as less important than the intellectual disability, or it is attributed to the disability rather than the psychosocial history.

Another issue relating to underdiagnosis is the lack of appropriate diagnostic criteria (Rush et al., 2004). Applying standard diagnostic criteria to people with intellectual disabilities can be difficult, due to receptive and expressive language skills, cognitive limitations, and other distortions, e.g. the diagnosed person might want to hide their disability or please the evaluator (Harris, 2006). Furthermore, sometimes diagnostic groupings usually applied in child psychiatry are used for adults with intellectual disabilities (Dosen, 1993) which raises further questions about the applicability of standard diagnostic criteria. The discussion of four cases involving intellectual disabilities in the DSM-IV Casebook (Spitzer, Gibbon, Skodol, Williams, & First, 1994) confirms the difficulties surrounding appropriate diagnosis. Lack of communicative language is cited in relation to difficulties determining obsessive-compulsive disorder; difficulties in determining whether a client suffered from major depressive disorder, psychotic depression, or bipolar I disorder are explained; an example of autistic disorder is discussed which initially had been diagnosed as childhood schizophrenia, a  diagnostic description which is no longer used today; and a case involving aggression which could not be related to the diagnosis of conduct disorder is analyzed.

In relation to diagnosis of mental illness in people with intellectual disabilities, it should be stressed, in particular when considering the recent historical viewpoints, that the full spectrum of recognized psychiatric disorders can be identified among people with intellectual disabilities (Harris, 2006). However, Harris (2006) also points out that anxiety, affective, and personality disorders are under recognized and that there is a tendency to overdiagnose psychotic illnesses.

Treatment Options: Pharmacology, Psychotherapy, and Beyond

The previously mentioned tendency to overdiagnose psychotic illnesses in people with intellectual disabilities and the cited case of the incorrect diagnosis of psychotic depression are indications of the overuse and misuse of antipsychotic medications among people with intellectual disabilities (Levitas & Hurley, 2006). The field of intellectual disabilities has been challenged to decrease the use of psychotropic medications when used as chemical restraints for singular behaviors or as substitutes for better staffing and/or programming (Menolascino & Fleisher, 1993). Current debates on treatment of people with dual diagnosis stress the importance of interdisciplinary teams and psychotropic medications being used side by side other treatment options, such as positive behavior support, which focuses on relationship and instruction rather than consequence and punishment, which relates to behavior modification (Bongiorno, 1996).

While in New York State aversive behavior therapy is not usually applied, the debate about the benefits and problems of aversive behavior intervention in extreme cases of aggression or self-injurious behavior continues among professionals (Menolascino & Fleisher, 1993).

Psychotherapy, another aspect of treatment, which is prescribed as a matter of fact in the general public, remains a matter of debate in relation to the treatment of dual diagnosis. Menolascino and Fleisher (1993) assert that individual and group therapy is still denied to many people with intellectual disabilities. This denial is often based in believes and attitudes which consider limitations in intelligence, cognition, and verbal abilities as obstacles to successful psychotherapy. Hurley, Tomasulo, & Pfadt (1998), however, showed that those who used standard adaptations in their practice with people with intellectual disabilities have reported productive and successful treatment. Psychotherapy is now recognized as an effective intervention strategy for the dually diagnosed (Harris, 2006; Fletcher, 2004; Lynch, 2000), but according to Harris (2006) remains underutilized due to the previously mentioned misconceptions. Other treatment services which are utilized for people with dual diagnosis are social skills training, day treatment, residential services, and crisis intervention services (Fletcher, 2004).

Falling Through the Cracks?

One of the most significant debates is whether people with the dual diagnosis of mental illnesses and intellectual disabilities “fall through the cracks” as Fletcher (1993) claims. VanderSchie-Bezyak (2003) explored this issue in depth, demonstrating the problems of clear separation between mental health and intellectual disabilities services by using a case study of a young 28-year-old male with mild mental retardation and several psychiatric disorders who was passed from one service to the next despite showing continuous signs of deterioration. It is often the case that service providers are unwilling to agree on joint care plans and instead determine that the primary responsibility should rest with other service elements. In the case of the 28-year-old male, these problems resulted in his incarceration and hence denial of much needed services. VanderShie-Bezyak (2003) lists a number of problem areas, including inaccessible and non-existing services for the dually diagnosed, discontinuity of care (e.g. passing from one service element to the other), separate support systems unwilling to collaborate, persons with challenging behaviors being regarded as undesirable, confusion of primary vs. secondary disorders, and lack of professional training. Despite these problems it should be pointed out that individuals with intellectual disabilities are often more likely to get some services than individuals in the mental health service delivery system, which is chronically under-funded, under-staffed, and often not able to offer adequate housing options (Castellani, 2005; Fellin, 1996).

Lack of Professional Training

Many authors have pointed to the lack of professional training as a major obstacle to providing adequate services to people with dual diagnosis (Fletcher, 1993; Hurley et al., 1998; VanderShie-Bezyak, 2003; Rush et al., 2004). While professionals in the field of intellectual disabilities are rarely trained outside of their specific field of expertise and therefore are unprepared to recognize mental health problems, mental health professionals are often unaware of issues facing people with intellectual disabilities (VanderShie-Bezyak, 2003). A number of studies have shown that mental health professionals are not trained in issues of intellectual disabilities, for instance, 75% of clinical and 67% of counseling programs did not include intellectual disabilities in the curriculum (Hurley et al., 1998).

The Larger Picture: Stigma, Culture, and other Psychosocial Issues

A less frequently discussed, but nevertheless important issue, relates to the standing of people with intellectual disabilities and mental health issues in society. Austrian (2005) points out that many mental illnesses may have a variety of causes, including psychosocial factors. This might be an issue of greater import for people with intellectual disabilities due to their status in society. Dosen (1993) pointed out that intellectual disability in general is seen as a negative, painful, and frightening phenomenon. Harris (2006) stressed that in particular people with milder cognitive impairments are vulnerable to experience the negative effects of social stigma. Other psychosocial factors include social rejection, frequent losses, and dependency on others (Harris, 2006). Further environmental stressors such as lack of privacy, denial of independence, and low socio-economic status also could be listed. All these psychosocial factors can lead to potential stress and trauma, feelings of inferiority, anxiety, anger, depression etc. and thus can potentially contribute to mental illness.

A final feature contributing to the psychosocial causes of mental illness in people with intellectual disabilities is the high risk for exploitation and abuse (Harris, 2006). Several studies found 50-85% of women and 25-50% of men with intellectual disabilities were sexually assaulted before the age of 18. Of those assaulted, 49 percent had been abused 10 times or more (Gross, 2006). It also needs to be stated that individuals with intellectual disabilities are at risk of abuse by providers (Alliance to Prevent Restraint Aversive Interventions and Seclusion, 2005; Oktay & Tompkins, 2004). While it is important to provide appropriate treatment and services to people with dual diagnosis, it is also crucial to advocate for change in cultural attitudes in order to work against stigmatization, discrimination, and oppression of people with intellectual disabilities (Dosen, 1993).

Best Practices and Solutions

While the presented problems appear to be immense, there are many suggested solutions and best practices. Recent advances in a number of disciplines such as neuroscience, genetics, psychopharmacology, and behavioral psychology promise improvements in the treatment of people with dual diagnosis. However, intellectual disability is often a criterion for exclusion from research studies. In recent years there have been attempts to define ways to include people with intellectual disability in research in order to promote evidenced-based treatment for this population, such as the Surgeon General’s recommendation for a research agenda for persons with intellectual disabilities (Harris, 2006). Substantive work has addressed the difficulties in applying diagnostic criteria for psychiatric disorders to persons with intellectual disabilities. The National Association for the Dually Diagnosed (NADD) together with the American Psychiatric Association (APA) have developed more suitable diagnostic criteria to use for the classification of disorders in persons with intellectual disabilities (Fletcher, Loschen, Stavrakaki, & First, 2007).

There are also a number of effective service models, which all have an interdisciplinary team approach. Regular team meetings of psychiatrists, behavior specialists, case managers, and vocational specialists can foster effective services sustained over time (VanderSchie-Bezyak, 2003). One of the first projects developed to serve the dually diagnosed was the Boston START Model, which provides emergency assessment, respite care, education for care providers, and coordination of outpatient services for people with developmental disabilities who are experiencing acute behavioral and/or emotional crises (ibid). The most important principle of the ENCOR Model, a service in Nebraska, is a zero rejection policy. Nobody will be denied services due to the severity of the mental illness or intellectual disability. A further model pioneered in Ulster County, NY, focuses on substantial efforts to coordinate between the county’s mental health department and intellectual disabilities agencies (ibid). The intensive behavior management program of the Western Region of Maryland was developed to help the community integration of people with dual diagnosis. The program is based on behavioral consultation, case management, behavioral training, behavioral support, behavioral respite, an alternative living unit, and crisis intervention. One of the core principles is case preparation and the philosophy of prevention is the best method of intervention (Yoder, 1996).

VanderSchie-Bezyak (2003) summarizes a number of recommendations for best practice service provision for people with dual diagnosis, including further development of special programs based on values of community integration and system collaboration. Furthermore, she suggests the imperative of serving all individuals regardless of the level of psychiatric disorder or intellectual disability. In addition she appeals for interdisciplinary and cross-system approaches to service delivery. Finally, she suggests that professionals from both the mental health and intellectual disabilities field need to train and educate each other in order to provide effective treatment. Another recommendation goes beyond the focus of mental health professionals and focuses on the needs of the people who work most closely with people with dual diagnoses. “Care for the caretaker” is a concept which attempts to prevent burnout of direct care staff, as burnout in turn can be linked to chronic challenging behaviors and emotional problems; additional work is necessary to train and support caregivers. (Kormann & Petronko, 2004).

This approach relates to the previously mentioned issue of abuse by service providers. A number of behavior and mental health issues can be related to basic services such as residential and day services being underfunded and run by underqualified, underpaid staff. In many organizations, a culture of staff dominance over the wishes and interests of individuals with intellectual disabilities prevails which can add to feelings of anger or depression and thus relate to the psychosocial aspects of mental illness.

Considering these elements, it is important to go beyond the short-term treatment of mental illness in people with intellectual disabilities and think about long-term mental health care. Dosen (1993), quoting Menolascino (1988), suggests that support for dually diagnosed people should not primarily focus on eliminating disruptive or destructive behaviors, but on teaching new sets of humanizing behaviors that focus on solidarity, bonding, and human need for mutual love. Basic human needs like social acceptance, social relationships, and positive affection are essential elements in working towards long term mental health for people with intellectual disabilities. A balanced, developed personality, a place of one’s own and role in the one’s surroundings in addition to functioning according to one’s own ability belong to the prerequisites of mental health of people with intellectual disabilities.

In the light of this broader picture of mental health and intellectual disability, social workers, with the focus on the person in the environment and the biopsychosocial approach, can be on the forefront of promoting and providing comprehensive services which foster mental health while simultaneously treating mental illnesses in the most effective ways. On a cultural change level, it is encouraging that the field has started to adopt the term intellectual disabilities, which bears less stigma than “mental retardation,” and that the United Nations passed a Convention on the Rights of Persons with Disabilities, which promises to improve the lives of people with disabilities worldwide (United Nations, 2006). 



This article started out by pointing to the lack of awareness of mental illness in my place of work. I also stated that challenging behaviors are often responded to without the necessary consideration of causes which might include mental illness. Therefore, I will conclude with a number of suggestions for the improvement of the program I am employed at. Two brief case examples help to illustrate the basic recommendations to improve knowledge about dual diagnoses among direct support staff.

A person diagnosed with selective mutism (American Psychiatric Association, 2000, p.78) is often pressured by staff to respond verbally to certain requests. A better understanding of this disorder would allow for more patience on behalf of staff members who regard the silence as a behavior problem. More accepting attitudes on behalf of staff would allow this person to feel more comfortable and in turn provide a better environment for him to talk on his own terms.

A person with a mood disorder and related aggressive behavior is often pressured to participate in program activities even when he appears depressed and unwell. His aggression is treated purely as a behavior problem without consideration to the underlying mood disorder. While a person with higher cognitive functioning might request a mental health day or call in sick, this person is pressured to participate in the day habilitation program even when he suffers from depression.

While the provider agency where I am employed is in general considered as a model of excellence, and while it applies most of the above recommended treatments and services through an interdisciplinary team approach, I would suggest four basic strategies based on the above recommendations in order to improve service provision in the program I serve.

First, training needs to be amended to include more knowledge and skills in relation to mental illness. While the agency is providing excellent trainings to its staff, issues relating to dual diagnosis fall short. Considering that one third of persons are estimated to have a dual diagnosis, it would allow staff to develop a better understanding of the background of mental illness related behavior and thus deal with these behaviors in more positive and supportive ways.

Second, the interdisciplinary team approach would benefit from improvement in terms of interagency work. Often services are delivered by a variety of agencies, and communication in relation to service delivery needs to be improved in this respect. Third, additional support strategies for direct service staff as suggested by Kormann & Petronko (2004) should be applied in order to improve service delivery. Finally, while many day and residential programs for people with intellectual disabilities employ psychologists for program support, persons with disabilities often receive therapeutic services from these same professionals, who are also responsible for behavior modification and/or support planning. This programmatic design is contrary to basic ethical and good practice standards of trusting and non-judgmental therapeutic relationships.








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