Dual Diagnosis and Diagnostic Overshadowing in Ontario: A Family Guide to Mental Health and Developmental Disability

If you’re supporting an adult with a developmental disability in Ontario, one of the most important things to understand is that a mental health condition can sit alongside the disability — and that it is often missed. This guide explains what “dual diagnosis” means in this field, why depression, anxiety, trauma, and even physical pain so often get written off as “just the disability,” how mental illness can look different in this population, and where to turn for specialised help in Ontario. It is general information for families and providers, not medical advice, and it uses the Ontario legal and funding term “developmental disability” throughout.

The short version

• “Dual diagnosis” in this field means a developmental disability co-occurring with a mental health condition — not the addictions meaning (in Canada that combination is called “concurrent disorders”). Mental health conditions are markedly more common in this population: whole-population data from Scotland (Hughes-McCormack et al., 2017) found 23.4% of adults with intellectual disabilities had a mental health condition versus 5.3% of the general population, and Ontario administrative data (Lin et al., reported in Volpe & Lin, 2018) found 44% of adults with developmental disabilities had a mental illness diagnosis over two years.
• Diagnostic overshadowing is the centrepiece problem: clinicians and support workers attribute a person’s symptoms to “just the disability” and miss a separate, treatable illness — depression, anxiety, trauma, or physical pain. The term was coined by Reiss, Levitan & Szyszko in 1982.
• Mental illness often presents differently — as changes in behaviour, sleep, function, or mood rather than self-reported feelings — and communication barriers make it harder to detect.
• “Behaviours that challenge” are increasingly understood as communication of pain, distress, mental illness, or unmet need; positive behaviour support and trauma-informed care are the mainstream response.
• Psychotropic medication is over-used: in Ontario, 39.2% of adults with developmental disabilities were dispensed an antipsychotic over six years, and 29% of those users had no documented psychiatric diagnosis (Lunsky et al., 2018). The UK’s STOMP initiative is the best-known reduction effort; Canada has no formal national equivalent.

What every family should know

1. Mental illness is both more common and more often missed in adults with developmental disabilities than in the general population.
2. Diagnostic overshadowing is a documented, measurable bias, not just a theory — vignette studies show clinicians recognise fewer symptoms when told a person has an intellectual disability.
3. Presentation is different: depression may show as irritability, tearfulness, withdrawal, sleep change, appetite change, or increased self-injury rather than a verbal report of low mood.
4. Ontario has a specialised dual-diagnosis service landscape — CAMH’s Azrieli Adult Neurodevelopmental Centre and Adult Neurodevelopmental Services, the Community Networks of Specialized Care, hospital dual-diagnosis programs, and the Surrey Place primary-care guidelines — but access gaps and long waits are well documented.
5. Neither the mental-health system nor the developmental-services system fully “owns” dual diagnosis, which creates a structural gap where people fall between sectors — so families and providers often have to actively coordinate care themselves.

What “dual diagnosis” means (and what it doesn’t)

In the developmental-disabilities field, “dual diagnosis” refers to the co-occurrence of a developmental disability and a mental health condition in the same person. CAMH puts it plainly: “a dual diagnosis is given when a person has a developmental disability … and a mental health problem” (CAMH, Dual Diagnosis health-info page). This is distinct from the more common general usage — especially in the United States — where “dual diagnosis” means a mental illness plus a substance-use disorder; in Canada that combination is usually called “concurrent disorders” (CMHA Ontario). The terminology confusion is real, and it’s worth flagging if you’re searching online and landing on addictions pages that don’t fit your situation.

“Developmental disability” is the Ontario legal and funding term, defined in the Services and Supports to Promote the Social Inclusion of Persons with Developmental Disabilities Act, 2008 (SIPDDA) as significant limitations in cognitive and adaptive functioning that originate before age 18 and are likely lifelong. Per CMHA Ontario’s Dual Diagnosis fact sheet, “approximately 1 to 3 percent of Canadians have a developmental disability.”

How common mental illness really is

The evidence that mental illness is more common in this population is strong, even if the numbers vary:

• Scotland (whole population, 2017): Hughes-McCormack et al. (BJPsych Open, 2017; n = 26,349 people with intellectual disabilities out of 5.3 million) found 12.8% of children, 23.4% of adults, and 27.2% of older adults with intellectual disabilities had a mental health condition, compared with 0.3%, 5.3%, and 4.5% of the general population respectively.
• Scotland (Cooper et al., 2007): in a population-based study of 1,023 adults, “point prevalence of mental ill-health was 40.9% (clinical diagnoses), 35.2% (DC-LD), 16.6% (ICD-10-DCR) and 15.7% (DSM-IV-TR). The most prevalent type was problem behaviours” (British Journal of Psychiatry). This study is a cornerstone but is nearly two decades old.
• Ontario administrative data: over a two-year period, 44% of adults with developmental disabilities had a mental illness diagnosis and 6% an addiction diagnosis (Lin et al., reported in Volpe & Lin, Healthcare Quarterly, 2018).
• CMHA Ontario summarises the consensus: people with developmental disabilities are “three to four times more likely to develop emotional, behavioural and psychiatric difficulties than the general population.”

Prevalence estimates vary widely — reported ranges run from 7% to 97% historically — because studies use different definitions and diagnostic tools. A 2025 umbrella review (medRxiv/BJPsych Open) found that compared with the general population, schizophrenia (3.55–4.80%), anxiety (5.4–5.5%), and OCD (2.4%) appeared higher, while mood, personality, and PTSD diagnoses appeared lower — a pattern that likely reflects under-detection rather than truly lower rates.

Diagnostic overshadowing — the centrepiece problem

Diagnostic overshadowing is the tendency to attribute a person’s symptoms to their developmental disability and thereby miss a separate, treatable mental or physical illness. The term was coined by psychologist Steven Reiss and colleagues in 1982 (Reiss, Levitan & Szyszko, American Journal of Mental Deficiency, 1982), who hypothesised that “intellectual subnormality is such a salient and obvious feature of the patient’s presentation that accompanying emotional disturbances are overshadowed.” In their experiments, the same debilitating phobia was less likely to be judged a neurosis or emotional disturbance when the person was described as having an intellectual disability. The founding of the National Association for the Dually Diagnosed (NADD) in 1983 grew directly out of this recognition.

Why it happens:

• The disability is the most visible feature and dominates the clinical picture.
• Symptoms are misread as “part of the disability” or as the person’s baseline.
• Communication barriers make it harder for the person to describe internal states.
• Time pressure, clinician inexperience with the population, and stigma all contribute (per the Mason & Scior 2004 UK vignette study of psychiatrists and clinical psychologists, and subsequent literature).
• The “cloak of competence” — where relatively good surface skills mask real difficulty — can compound the error.

How it leads to missed illness (concrete examples):

• New or increased self-injury or aggression is treated as a “behaviour problem” when it may signal untreated depression, an anxiety disorder, or pain (for example a dental abscess, constipation, or reflux).
• Social withdrawal and loss of interest are read as “how they are” rather than depression.
• Repetitive movements or agitation are attributed to autism or the disability rather than assessed as anxiety, akathisia (a medication side-effect), or a movement disorder.
• Because people with intellectual disabilities are more susceptible to neurological side-effects of antipsychotics (tardive dyskinesia, neuroleptic malignant syndrome), overshadowing can also mask iatrogenic harm — harm caused by treatment itself.

The consequences are serious: overshadowing delays treatment, contributes to premature mortality, and the World Health Organization attributes part of the reduced life expectancy in this population to it. Ontario’s H-CARDD program has documented that autistic adults and adults with other developmental disabilities in Ontario are “dying too soon.” This is a large part of why families are encouraged to advocate actively when Navigating Healthcare for an Adult with a Developmental Disability in Ontario — bringing documented observations to appointments and asking explicitly whether a treatable illness has been ruled out.

How mental illness can look different

Mental illness in this population frequently shows up as observable changes rather than self-reported symptoms — changes in behaviour, function, sleep, appetite, energy, or mood. A systematic review of depression in people with severe-to-profound intellectual disability (Eaton et al., 2021) found the most commonly reported indicators were depressed affect, anhedonia, crying/tearfulness, psychomotor agitation, reduced appetite, fatigue/lethargy, irritability, and sleep disturbance. Established clinical wisdom (Sovner & Hurley, 1983) described behaviours such as aggression, self-injury, and screaming as possible “depressive equivalents.”

An important nuance: the “depressive equivalents” idea is debated. Tsiouris et al. (2003) and a 2010 replication found no evidence that challenging behaviours like self-injury or aggression are reliable equivalents of depression, arguing that assessment should focus on core observable symptoms (loss of interest, energy change, sleep and appetite change). The safest reading is that a change in behaviour is a signal that warrants assessment, not proof of a specific diagnosis.

Communication barriers are central to all of this. Many people communicate in ways other than speech, and internal states — mood, worthlessness, suicidal thoughts — are the hardest of all to convey. This is why baseline knowledge of the person, together with informant reports from family and support staff, is essential. The detail on augmentative and alternative communication (AAC) and other communication methods is covered in Communication and AAC for Adults with Developmental Disabilities in Ontario.

“Behaviours that challenge” as communication

The field has shifted away from treating distress-driven behaviour as merely a “behaviour problem” to be suppressed, and toward asking what the behaviour communicates — pain, mental illness, trauma, sensory overload, or an unmet need. In a total-population study of adults with intellectual disability known to services, Bowring, Totsika, Hastings, Toogood & Griffith (British Journal of Clinical Psychology, 2017; n = 265) found “the prevalence of overall CB [challenging behaviour] was 18.1% (95% CI: 13.94–23.19%),” broken down as self-injurious 7.5%, aggressive-destructive 8.3%, and stereotyped 10.9%.

Positive behaviour support (PBS) is the mainstream, values-based framework. It uses functional assessment to understand what a behaviour achieves for the person and what environmental factors drive it, then changes the environment and teaches skills rather than simply reacting to the behaviour. PBS takes a lifespan perspective, emphasises quality of life, and is person-centred — you can read more in Positive Behaviour Support in Ontario: A Plain-Language Guide for Families. A cluster randomised controlled trial of staff training in PBS in community intellectual-disability services tested whether it reduces challenging behaviour; the broader evidence base is generally supportive, though some newer results are mixed (for example, Bruinsma et al., 2024, found improvements in lethargy but no overall significant change in irritability). The depth on restrictive practices and safeguarding is handled in AIM’s separate dignity-of-risk and safeguarding guide.

Trauma

Adults with developmental disabilities have elevated exposure to adverse experiences, abuse, neglect, and victimisation compared with the general population. The literature commonly cites that people with intellectual/developmental disabilities are three to four times (some sources say three to six times) more likely to experience abuse or neglect (Hulbert-Williams et al., 2013; AAIDD/PMC review, 2023). One clinical study of autistic adults with intellectual disabilities referred for mental-health assessment found experiences of violence (34.1%) and sexual abuse (17.0%) were common, yet only 3.4% were diagnosed with PTSD — a striking illustration of under-recognition.

Trauma-informed care — building cultures, policies, and practices that recognise the prevalence and impact of trauma and prioritise physical and emotional safety — is increasingly recommended (McNally et al., 2021 scoping review; Keesler’s work on trauma-informed disability services and day programs). Trauma can present as changes in behaviour, and trauma symptoms may be overlooked or misinterpreted, again feeding the overshadowing cycle. The depth on abuse, exploitation, and safeguarding is handed off to AIM’s separate safeguarding document.

Getting help in Ontario

Ontario has a specialised dual-diagnosis landscape (verified current as of 2025–2026):

• CAMH Adult Neurodevelopmental Services (ANS) and the Azrieli Adult Neurodevelopmental Centre (directed by Dr. Yona Lunsky) — the first Canadian research and education centre dedicated to adults with neurodevelopmental disabilities and mental health, created through a $10.4-million gift from the Azrieli Foundation. ANS provides psychiatry, psychology, and social-work consultations. CAMH also publishes the plain-language Dual Diagnosis: An Information Guide (Lunsky).
• H-CARDD (Health Care Access Research and Developmental Disabilities) — an Ontario research program based at the Azrieli Centre/CAMH, in partnership with ICES. It produced the Atlas on the Primary Care of Adults with Developmental Disabilities in Ontario (2013) and Addressing Gaps in the Health Care Services Used by Adults with Developmental Disabilities in Ontario (2019), and (with CAMH’s Provincial System Support Program) practice guidance on transitioning “alternate level of care” dual-diagnosis patients from hospital to home.
• Surrey Place Developmental Disabilities Primary Care Program (DDPCP) — publishes the Primary care of adults with intellectual and developmental disabilities: 2018 Canadian consensus guidelines (Sullivan et al., Canadian Family Physician, 2018), an update of the 2011 guidelines developed by roughly 45 clinicians across Canada, with clinical tools including health checks and psychotropic-medication tools. The mental-health intervention guidance was updated in January 2024.
• Community Networks of Specialized Care (CNSC) — a province-wide network funded by the Ministry of Children, Community and Social Services, organised into regional networks that provide complex support coordination, dual-diagnosis justice case management, and health-care facilitation for adults with high support and complex care needs. Access is via Developmental Services Ontario (DSO). The relevant network for AIM’s regions (Waterloo, Hamilton, Halton) is the Southern Network of Specialized Care.
• Hospital dual-diagnosis programs — for example, the Dual Diagnosis Program at Parkwood Institute (St. Joseph’s Health Care London), a 12-bed inpatient plus outpatient/outreach service for Southwestern Ontario.
• ECHO Ontario Adult Intellectual & Developmental Disabilities (AIDD) — a CAMH telementoring program including streams on mental health, mental health and autism, and brain health/aging (relevant as an adult grows older; see Aging with a Developmental Disability in Ontario: A Guide for Families).
• Primary care and psychiatry remain the front line: the 2018 guidelines are built for family physicians, and annual comprehensive health checks are a core recommendation.

Access gaps are well documented. A national survey (Lunsky et al., 2007) found generic mental-health providers were poorly equipped, waitlists for specialised services were typically four months or longer, and fewer than half of respondents reported specialised expertise in inpatient or emergency settings. Aggression/challenging behaviour was the main reason for hospital admission and a barrier to discharge.

Medication and its over-use

Psychotropic — especially antipsychotic — medication is over-used in this population, frequently for behaviour control rather than a diagnosed illness.

• Ontario (Lunsky et al., 2018, Canadian Journal of Psychiatry): in a population-based study of 51,881 adults with developmental disabilities, 39.2% (20,316 people) were dispensed an antipsychotic over six years; this rose to 56.4% among those in group homes. Critically, 29% of antipsychotic users had no documented psychiatric diagnosis (rising to 43% in the group-home subgroup). Antipsychotics are the most common medication class prescribed to adults with developmental disabilities in Ontario, “often in the absence of a serious mental illness diagnosis.” Among these adults, roughly 1 in 6 had diabetes and 1 in 5 had hypertension — underscoring the metabolic risk.
• Ontario initiation study (Gomes et al., BMJ Open, 2019): of antipsychotic-naïve adults with IDD, 17.6% initiated an antipsychotic, of whom about 26.9% had no psychiatric diagnosis in the prior two years.
• UK context: Public Health England (Glover et al., 2015) estimated that “at any time, between 30,000 and 35,000 people with learning disabilities are prescribed an antipsychotic, an antidepressant or both by their GP without having the conditions for which the drugs were designed to treat … (this is 1 in every 6 people known to their GP as having a learning disability).” NHS England’s STOMP/STAMP page states that “in 2025, people with a learning disability are thought to be 15 times more likely and autistic people 7 times more likely to be prescribed an antipsychotic than the general population.”

The reduction movement. The UK’s STOMP (Stopping Over-Medication of People with a learning disability, autism or both) was launched by NHS England in 2015/2016 as part of the Transforming Care programme — a shared commitment across the Royal Colleges (Psychiatrists, Pharmacists, GPs, Nurses, Psychologists) and NHS to review and, where appropriate, reduce inappropriate psychotropic prescribing; a paediatric counterpart is STAMP. Deprescribing — planned, supervised dose reduction — is a core component. Evidence of national impact is modest but real: per NHS England Digital’s Health and Care of People with Learning Disabilities statistics, antipsychotic prescribing fell from 15.7% at STOMP’s launch (2016/17) to 14.4% in 2022–23 and 13.8% in 2024–25 (versus about 0.9% in the general population). Canada has no formal national STOMP-equivalent program; researchers (including Lunsky) have explicitly called for one, and CAMH points clinicians to STOMP resources and to the Surrey Place guidelines and tools. Training resources such as the UK’s SPECTROM (for support staff) are emerging in the research literature.

This section is general information, not a recommendation to start, stop, or change any medication — those decisions must be made with a prescriber, and abrupt discontinuation carries real risks.

What good support looks like

In a day-program or community setting, good support tends to share the same habits:

• Know the person’s baseline so that changes are noticed early.
• Treat behaviour change as a signal and rule out physical causes (pain, infection, constipation, dental problems, medication effects) and mental-health causes before assuming it is “the disability.”
• Respond in a person-centred, trauma-informed, communication-aware way: safety, predictability, choice, and respect.
• Use positive behaviour support plans built on functional assessment, not just reaction.
• Collaborate across sectors — family, primary care, psychiatry, developmental services, and specialised dual-diagnosis resources — and keep good records to share with clinicians.
• Advocate against overshadowing: bring documented observations to appointments and ask explicitly whether a treatable illness has been ruled out.

Where families get stuck: the systemic gap

Dual diagnosis sits at the boundary of two systems — the developmental-services sector (in Ontario, MCCSS-funded and accessed through DSO) and the mental-health sector (health-funded). Neither fully owns it. Ontario’s own Joint Policy Guideline for the Provision of Community Mental Health and Developmental Services acknowledges that health, mental health, and developmental services “all have a role” and that cross-sector linkages are needed — an admission that responsibility is shared and therefore diffuse. In practice this produces exclusion (mental-health services sometimes explicitly excluding people with intellectual disabilities), long waits, and people cycling through emergency departments. The CNSC exists partly to bridge this gap, but genuine authority remains distributed rather than clearly located — which is precisely why families and providers often have to actively coordinate care themselves.

Grey areas and points of confusion

• Prevalence data are thin and inconsistent. Estimates range enormously (historically 7–97%) depending on definitions and instruments. The robust whole-population figures (Scotland; Ontario administrative data) are the most reliable, but they are not Canadian community-program data and are several years old. Notably, standard DSM/ICD research criteria tend to undercount mental ill-health in this population relative to disability-specific criteria (DC-LD), as Cooper et al. (2007) demonstrated.
• “Depressive equivalents” are debated. Whether challenging behaviours are genuine markers of underlying depression, or non-specific distress signals, is unresolved (Sovner & Hurley vs. Tsiouris et al.). Treat behaviour change as prompting assessment, not as diagnostic on its own.
• Prescribing debates. There is genuine tension between reducing inappropriate antipsychotic use and the reality that, when a psychiatric disorder is uncertain, medication is sometimes used carefully for severe behaviour — CAMH itself notes medication “may be part of a treatment plan for challenging behaviour when the underlying psychiatric disorder is uncertain or unknown.” Deprescribing also carries withdrawal and relapse risks. The debate is about appropriateness, monitoring, and review — not a simple “medication bad” message.
• Terminology confusion. “Dual diagnosis” means different things in different countries; “concurrent disorders” is the Canadian term for mental illness plus substance use. A minority of scholars (for example McLennan, 2018) argue the “dual diagnosis” construct itself is problematic and should be replaced by a dimensional approach.
• The term “diagnostic overshadowing” itself has broadened. Originally about psychiatric symptoms in people with intellectual disability, it is now also used for missed physical illness and applied to mental illness generally.

How current is this, and what to double-check

• This is general information, not medical advice. Nothing here should be used to diagnose, or to start, stop, or change medication or treatment; those decisions require a qualified clinician who knows the person.
• Recency: key prevalence anchors (Cooper 2007; Hughes-McCormack 2017) and Ontario prescribing data (Lunsky 2018; Gomes 2019) are several years old; the Ontario administrative mental-illness figure (44%) traces to Lin et al. reported in 2018. Service descriptions are current to 2025–2026, but Ontario’s developmental-services and mental-health system structures change, so verify local services — especially CNSC/DSO contacts and hospital programs — before relying on them.
• Source quality: this guide prioritises peer-reviewed research and specialist/authoritative sources (CAMH, Surrey Place, H-CARDD/ICES, NADD, NHS England, and the Ontario government). Some prevalence and prescribing figures are drawn from UK/Scottish and international data because comparable Canadian community data are limited; these are flagged where used.
• Scope: this document covers mental health, dual diagnosis, diagnostic overshadowing, and behaviour-as-communication. Aging and dementia, general physical health and active living, AAC/communication, and abuse/safeguarding are covered in separate AIM documents.

Related: Navigating Healthcare for an Adult with a Developmental Disability in Ontario · Positive Behaviour Support in Ontario: A Plain-Language Guide for Families · Communication and AAC for Adults with Developmental Disabilities in Ontario · Aging with a Developmental Disability in Ontario: A Guide for Families