The Concept of Psychiatric Comorbidity is Overused and Poorly Understood

Danish psychiatrists argue that the concept of psychiatric comorbidity has been carelessly applied in psychiatry and should be re-thought.


A recently published research study, penned by a team of researchers at the University of Copenhagen and the University of Melbourne, encourages a reevaluation of the approach to psychiatric comorbidity‚ÄĒthe co-occurrence of two or more mental disorders. The article ‚ÄúPsychiatric Comorbidity: A Concept in Need of a Theory‚ÄĚ was published in Psychological Medicine.

Though the notion of psychiatric comorbidity is relatively new, it has rapidly become a fundamental concept in clinical practice and psychiatric research. However, this latest study highlights numerous challenges related to its inadequate definition, differential diagnostic problems, and the reification‚ÄĒor treating of abstract entities as if they were concrete‚ÄĒof mental disorders.

According to the researchers, these issues could significantly impact the efficacy of diagnostic assessments in current clinical and psychiatric research practices.

‚ÄúDeveloping a robust theoretical framework to approach psychiatric comorbidity could revolutionize how we understand, diagnose, and treat multiple concurrent mental disorders,‚ÄĚ said Dr. Julie Nordgaard from the University of Copenhagen, the study‚Äôs lead author. ‚ÄúWe need to shift from the broad-brush approach currently employed to a more nuanced system that captures the complex reality of patient experiences.‚ÄĚ

The concept of comorbidity was originally devised by Alvan Feinstein in 1970 for use in general medicine. In the 1980s, comorbidity was brought into psychiatry, and over time its use has become common in clinical thinking and practice. This explosion of comorbidity presents practical and ethical issues to the field and the people it treats.

Comorbidity typically refers to the co-occurring clinical presence of two or more ‚Äúdisease entities.‚ÄĚ However, Feinstein‚Äôs original formulation included qualifying criteria, such as clear origins of the disease or distinct clinical psychopathology, that psychiatry often has difficulty meeting. This has led others to add different conditions for its use within psychiatry, but not without problems.

Julie Nordgaard and co-authors state:

‚ÄúThe number of patients diagnosed with psychiatric comorbidity has increased significantly, and empirical research on comorbid mental disorders has similarly grown. This development is also reflected in the diagnostic manuals, where the term ‚Äėcomorbid‚Äô appeared 0 times in DSM-III but more than 600 times in DSM-5.
Despite the popularity of the concept of psychiatric comorbidity, it has often been criticized. In fact, there is a striking incongruence between the widespread use of psychiatric comorbidity in clinical practice and empirical research and theoretical studies that generally pose strong reservations toward psychiatric comorbidity.‚ÄĚ

The current article examines the origins and development of the concept of comorbidity. The authors propose several difficulties with its use in psychiatry, illustrated by clinical examples. They conclude by suggesting their own methods for remedying these issues.

In Feinstein‚Äôs original definition of comorbidity, a ‚Äúdistinct additional clinical entity‚ÄĚ was required. He used the example of lung cancer co-occurring with, for example, coronary artery disease, discovered independently of the cancer. This demonstrates what the authors call the necessary mutual independence of comorbid entities.

Because of Feinstein‚Äôs formulation, where comorbidity requires either clear etiology or clearly demarcated pathology, difficulties arise when comorbidity is used in psychiatry. For example, ‚Äúmost mental disorders have‚Ķunknown etiology,‚ÄĚ given the uncertainty about the exact development of many mental disorders in terms of genetics and environment.

In terms of demarcation, psychiatry also runs into problems. Many mental disorders have overlapping symptoms, which can make diagnosis tricky.

Traditionally, according to the authors, diagnostic hierarchies were put in place, suggesting that diagnosing comorbid disorders should be avoided if one diagnostic category can account for all of a person‚Äôs symptoms. Specific categories were ranked in terms of how diagnosis should proceed, such as ‚Äúorganic disorders‚ÄĚ and ‚Äúschizophrenia‚ÄĚ outranking anxiety and personality disorders.

The authors state that hierarchical diagnosis like this has ‚Äúgradually eroded‚ÄĚ and given way to the outbreak of comorbid diagnoses because of a distrust in the diagnostic hierarchy.

Furthermore, the researchers argued in favor of a more hierarchical diagnostic system with explicit exclusionary rules. Such a system could streamline clinical practice and research by reducing informational complexity and mitigating unwarranted psychiatric comorbidity‚ÄĒinstances where multiple diagnoses might not accurately represent a patient‚Äôs mental health condition.

There are several problems associated with comorbid diagnoses, such as the fact that people tend to experience adjustment difficulties with even one diagnosis as they adapt to a new understanding of themselves and make life changes to cope with the disorder.

Additionally, comorbidity can often lead to inappropriate polypharmacy or the prescription of multiple pharmaceutical drugs to deal with different symptoms. Finally, there is a risk of compartmentalizing a person’s psyche:

‚Äúthereby not seeing the patient as a whole and unified human being but instead as a person with a psyche composed of, say, one part schizophrenia, one part OCD, and one part ADHD, and where each of these parts may invite different disorder-specific treatments.‚ÄĚ

One alternative proposal in the paper is considering trait versus state issues in pathology. Trait conditions ‚Äúcan fluctuate in severity‚ÄĚ but are present for more extended periods. Trait conditions include ‚Äúautism spectrum disorder, ADHD, schizophrenia, or personality disorders.‚ÄĚ On the other hand, state conditions are typically shorter-term, like affective problems such as major depressive disorder.

When diagnosing someone who presents with both depression and personality struggles, the authors argue that both should not initially be diagnosed. Instead, the depression should be treated first to see if the personality issues recede once the depression goes away. Only then should the comorbid diagnosis be considered. This is one example of considering trait and state conditions in diagnosis to avoid rampant comorbidity and potentially losing sight of the overall clinical picture.

The authors ask, for example, whether an anxiety disorder occurring independently of schizophrenia is really the same thing as anxiety occurring alongside or as an aspect of schizophrenia.

Ultimately, however, the ‚Äúclinical judgment‚ÄĚ required for clear diagnosis requires ‚Äúvaried clinical experience, reading psychopathological literature, and ongoing discussions about psychopathological phenomena with experts.‚ÄĚ

Some researchers have suggested things like biomarkers, treatment response, and risk factors as ways to assess diagnostic validity, but the authors argue that without clear etiology/understanding of the pathogenesis of disorders, these are not strong diagnostic tools.

They argue instead for the trait/state distinction and some degree of hierarchical diagnosis, as well as clear exclusionary criteria for diagnosis. For example, if someone is diagnosed with category A, then they cannot be diagnosed with category B. Some exclusion criteria already exist, but the authors believe that it is unclear, relying on clinicians to determine whether symptoms are best explained by one diagnosis over another without strong guidelines.

Several researchers now believe that a dimensional approach would solve many of the problems the authors discuss, but the authors argue that a dimensional approach may actually be grounded in the kind of ‚Äúliberal assessment of psychiatric comorbidity‚ÄĚ they are arguing against.

Supporting the authors‚Äô argument about polypharmacy as linked to comorbidity, existing research has found that polypharmacy is on the rise and poses significant dangers. As for network or dimensional based approaches, it does seem that comorbidity is considered ‚Äúthe rule rather than the exception,‚ÄĚ which should lead researchers and clinicians to consider the nuances of comorbidity as a clinical tool.



Nordgaard, J., Nielsen, K. M., Rasmussen, A. R., & Henriksen, M. G. (2023). Psychiatric comorbidity: A concept in need of a theory. Psychological Medicine, 1-7. (Link)

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Micah Ingle, PhD
Micah is part-time faculty in psychology at Point Park University. He holds a Ph.D. in Psychology: Consciousness and Society from the University of West Georgia. His interests include humanistic, critical, and liberation psychologies. He has published work on empathy, individualism, group therapy, and critical masculinities. Micah has served on the executive boards of Division 32 of the American Psychological Association (Society for Humanistic Psychology) as well as Division 24 (Society for Theoretical and Philosophical Psychology). His current research focuses on critiques of the western individualizing medical model, as well as cultivating alternatives via humanities-oriented group and community work.


  1. My thinkboi thoughts.
    ‚ÄúPsychiatric Comorbidity: A Concept in Need of a Theory‚ÄĚ does not go together very well with the phrase “reification‚ÄĒor treating of abstract entities as if they were concrete‚ÄĒof mental disorders.” even if they are in two different sequential paragraphs.
    If I get correctly some of what the paper says, besides being very surpised as Colbert would have said, they are proposing a change in the diagnostic algorithm. I haven’t read the paper, but that seems to me to run into the search satisfaction fallacy that in psychiatry might very probably be associated with a priming effect, anchoring and a cognitive dissonance afterwards.
    Particularly when you cannot rule out a disease or disorder since there is no “gold standard” no lab or x-ray test to do so, like in the rest of medicine, to rule out anything. Particularly with psychosis or certain personality so called disorders that can “manifest” ar a later date. Even self harm, one can allways do another look and convince oneself that the “signs” were there all the time. There are no, as far as I know, ruling out algorithms in psychiatry, since there are no valid, as in anchored in reality not in belief, tests to do so. Unlike the rest of medicine.
    Following on that, the second proposal as I understand from this review seems to bring back the “personality theories” into reuse after having some 30 “theories” with it’s individual traits, that as far I understand have been throughly debunked, all of them. There are even for the lay reader several books about it, if I’m remebering correctly.
    No personality theory, in my uneducated opinion, is a theory, and given the abundance of them, probably all or most of them are actually false. If out of 30 one were somehow to be correct, that means any has at least a probability, agnostically, of being false 29 out of 30, at least. The probability of the next “theory” of being wrong is actually closer to 1, that is it will be, agnostically 30 out of 31, and so forth. Without even getting into the particulars of the proposal. After all that’s why “theories” are abandoned, because they empirically fail again and again until it is very obvious that they can’t be correct no matter how many more experiments are conducted to test them and refine them.
    So, from an agnostic data anlytical perspective, each time a “new theory” is proposed, agnostically the a priori probability of it being wrong actually increases, not decreases, which is what should not happen in scientific research. And also not in plain empiricism, I suspect. Theories, scientific ones, grow by accretion of corroborating findings, as Thomas Kuhn has suggested.
    To me it also seems to assume somehow that traits are actually causally linked to a disorder or disease, when apparently they are aware that there is a lack of causality in the whole field. Shifting one unsound construct for another unsound construct, to my mind, ostensibly in search for hypotheses from which to build, at some point a Theory.
    Maybe they dream of publishing, I know I do: “Psychiatry: A set of goals, dreams, desires, needs and aspirations in search for hypotheses that could some day lead to a theory”, like Galileo might have done some centuries ago.
    As for the state in which a patient is actually in, how could anyone tell? you can’t meassure the, I imagine, causally relevant variables in the population, also not the clinical expression of those variables, how can you meassure them in a particular individual?.

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  2. To add an euphemistic pun: I pitty the PsychoMendeleev that at some not so distant future, despondently hopefull, tries to construct the Table of Psychoelements knowing without going into the particular details of each and all studies, that at least 50%, half of the observations he or her has to fit in some “octaves” to build a Law, are actually false. And! that he or her has to explain them too in his or hers new Theory, as the paradigmatic shift requires according to Kuhn’s proposal. It has to explain them “better” than the old paradigm in order to replace it.
    Assuming, of course, that the fakes of psych research are actually to be explained also by the psychoelements. Which to me seems a fair assumption.
    Recursion can be a cruel master, him or her.

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  3. It’s starts with a fundamental misunderstanding of mental illness. The body only has one immune system that responds to both physical and psychological stress. What is being diagnosed as disease is usually just one symptom of many. The current model of treating depression is akin to endlessly prescribing apirin and never trying to find the cause of the fever.

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    • It’s to me like symptomatological arithmetic: add and substract until the complaints stop, the patient sees another practitioner, the law intervenes, a bad outcome is put forward in a way that cannot be denied, or worse. And sometimes the humble aspiring actually does provide relief for some self limited uncomplicated by aspirin disorders, in a person in whom aspirin is not damaging, at a dosage, etc.. Which, from RW take and take on on the SSRIs in an “assisted dialog” with a bot seems not to be the case for depression and/or SSRIs.

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  4. I think, the article forgets to mention that the concept of comorbidity presumes the presence of two or more diseases that function and operate independently of each other.
    This is clearly not the case with most psychiatric diagnoses. There is no good reason whatsoever to assume that depression and anxiety operate independently of each other, for example.

    The attempt by the authors to differentiate between trait- and state diagnoses does not translate into anything practically useful since the trait diagnoses are neither more ‘real’ nor are they really necessarily more permanent.
    In my opinion, psychiatric comorbidity is a convenient and sciency-sounding concept that is meant to normalize the practice of diagnosing patients with a bunch of DSM-5 insurance codes as opposed to just one. And since psychiatrists can’t agree on a diagnosis, c.f. Kappa values, the concept also allows practioners to avoid the question altogether whether or not a lack of agreement on a diagnosis may mean that the diagnostic tools themselves are flawed.

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    • And in most medicine having two different, not only with two different ethiologies, i.e. origins, but not “linked” with each other is rare. Like having two cancers, let alone two different “unrelated” ones in the same organ (the blood, linfatic organs and bone marrow would count as one for some, even sometimes gut and skin would be added to the mix)?.
      Or two such kidney diseases? Two such heart diseases? Two such brain diseases?. Even a brain and peripheral nervous would be difficult to separate since they are at least connected by the vascular beyond the nervous.
      In most medicine that would raise strong suspicion of being connected or related somehow. Particularly when it involves the brain, given that there are LOTS of publications that deal with potential new diseases that affect 5-10-20 people. That precisely links “just” the symptoms/signs, the absence of a good formal by the book of science diagnosis, and THE unknown cause.
      Like diabetes and hypertension linked to sendentarism, fatty sugary relatively cheap food and increased weight, etc. That gave rise to the concept, as I recall, of “metabolic syndrome”.
      Although even if rare, 1 in 200 people have 2 rare, a 1:100,000 people rare, GENETIC diseases, given the high number of those particular diseases.
      But I am sure mental diseases are not even genetic as MIA has “unveiled” so convincingly.
      In the heart it is more common, but they are related, valve “failure” with heart overload for whatever reason, etc. Or lung work exposure disease with tobacco disease and obesity (which overloads the heart and “restrict” lung taking air in). But, are they unrelated to “capitalism” (uuugh) and the sedentary “productive” exhausting life we are supposed to have? Wouldn’t that count as a culturally appropiate “cuasidisease state”?
      So the “unlinked” or “unrelated” or “different origin” is to me more ambiguity in disease “definition”. Partciularly when the ethiology, the cause, the origin, of the illness/disorder/disease/spectrum, is unknown, as I think this review somehow quotes being recognised by the authors. Asking for them, the diseases, not to be connected when you well know that the origin is unknown almost sounds fraudulent to me.
      Wouldn’t all mental ilnesses (uugh) be related to “our way of life”?. Do “savage” (uggh) people, cultures, civilizations, be free of them? Specially when mental illness is supposed to take into account the “culture”, the beliefs of said communities to “invent” an illness that doesn;t touch that? Like don’t make an illness out of demonic possesion when the comunity believes in that?.
      Didn’t father Amorth provide “treatment” for those, sometimes in 3 sessions per week, and no one tried to diagnose his community as psychotic or “dissociated”, let alone with an hyperfaithfull personality disorder?. Or is it that the Church is stronger than MIA? (even than the CCHR and Scientology, not making prop!).
      Aren’t we all, seen like that, victims of our “scientific progress” in mental health? In a similar manner we are victims of capitalism?. ūüôā

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  5. “Since the criteria for psychiatric diagnoses are so vague and unspecific, it is not surprising that many patients, and also healthy people, will test positive for more than one diagnosis. In psychiatry, this is called co-morbidity, but it is absolute nonsense. If you invented diagnostic criteria for spotting elephants, wildebeests and rhinoceroses that overlapped for these species, you would not say you have spotted an elephant, which is also a wildebeest and a rhinoceros.”

    From my MIA article, about healthy people: Self-test for Adult Symptom Deficiency Disorder (ASDD),

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  6. Micah,
    Conditions like autism, ADHD, Bipolar, BPD, PTSD and schizophrenia share many things. For example, they all have neuroinflammation, dysbiosis, low Vitamin D, histamine intolerance, methylation problems and more.

    So if they share the same things then it’s easy to understand why more than one is being diagnosed, don’t you think?

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    • Actually, the “comorbidity” is, in my view, not because these “conditions” share things, but because they are not actual medical “conditions” at all. What should be diagnosed is neuroinflammation, histamine intolerance, vitamin D deficiency, etc, rather than wasting time on these meaningless DSM “diagnoses” that don’t tell us anything about the actual problem at hand.

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  7. ‚ÄúDeveloping a robust theoretical framework to approach psychiatric comorbidity could revolutionize how we understand, diagnose, and treat multiple concurrent mental disorders,‚ÄĚ said Dr. Julie Nordgaard from the University of Copenhagen, the study‚Äôs lead author. ‚ÄúWe need to shift from the broad-brush approach currently employed to a more nuanced system that captures the complex reality of patient experiences.‚ÄĚ <— Please, allow me to translate
    from psychobabble gobbledygook into plain English:

    "Too many people have figured out our bullshit. We need new bullshit. People don't believe the old bullshit any more. We need more complicated & sciencey-sounding bullshit. We can't keep bullshitting people on the same old bullshit. We need new bullshit."
    Got that? You're welcome!….

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