last updated:

May 02, 2021

[Brain Image]    

PSY 340 Brain and Behavior

Class 36: Approaches to Classifying and Diagnosing Mental Disorders


Classifying Mental Disorders & Psychiatric Conditions: Conflicting Models

[Not in our text]

Dog ≠ Lion ≠ Wolf ≠

Dogs       are not       Lions       are not        Wolves       are not       Tigers

Each animal is a distinct and different type or species.

How about "mental disorders"?

Can we say "Depression" is not "Schizoprhenia" is not "Anxiety" is not "Substance Abuse"?
Are the "mental disorders" each a distinct and different type or species?

The study of pathological or abnormal mental and behavioral processes is known as psychopathology. Over the last century and a half, there have been multiple approaches or models of psychopathology. The field of psychopathology is mostly researched by psychiatrists (medical doctors) and clinical psychologists.

A. The Need for Order. In science (biology, medicine, etc.) as well as other areas of life, it is crucial to bring order to all of the data, to find ways of ordering or classifying the data so that we have a deeper understanding of what we are researching or looking at/looking for. To do so, science often uses taxonomies.

Taxonony (taxis in Greek = ordering or arrangement of things; nomos = law, custom, how things are distributed or managed)

  • 1.Classification, esp. in relation to its general laws or principles; that department of science, or of a particular science or subject, which consists of or relates to classification; esp. the systematic classification of living organisms.
  • 2. A classification of something; a particular system of classification. (Oxford English Dictionary)

There are many different examples of taxonomies or classifications in science.

  • Red
                    Fox ClassificationBiology
    • In 1735, the Swedish scientist, Carl Linneaus, published his book, Systema Naturae ("The System of Nature"), in which he classified animals and species of plants using a binomial or "two-name" system. So, for example, human beings would be classified as "homo sapiens" (= "wise humans") and, later, the most famous dinosaur was given the name "Tyrannosaurus rex" (= "king [rex] of the tyrant lizard [tyrannos (tyrant) + sauros (lizard)].
    • Modern scientific classification of all living organisms has 8 levels in its taxonomy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, & Species
    • In the example on the right showing how a "red fox" is classified, it belongs to the Domain of Eukaryotes (cells have DNA in nucleus), the animal Kingdom, the Phylum of animals which had a "notochord" or "cartilage-like skeletal rod"  in their development, the Class of mammals (animals with mammary glands et al.), the Order of those animals which eat meat (carnivores), the Family of dog-like animals, and the Genus of foxes. Vulpes vulpes is the Linnean binomial name for the red fox.
    • Most General to Most Specific: Notice that taxonomies like the modern scientific classification attempt to start with the most general common feature(s) of the organism, and then at each successive level or layer, to become more and more specific about which feature(s) the organism possesses. 
  • Zoology
    • While biology is the general study of living organisms, zoology is “the scientific study of the behavior, structure, physiology, classification, and distribution of animals” (Oxford Languages). Some of the taxonomic approaches that zoology uses to classify animals includes morphological (body shapes & structures), embryological (the ways animals develop from fertilization through birth and beyond), behavioral (how different species of animals behave in different ways), genetic (looking at DNA, etc.), biochemical (what chemicals are produced by different animals) and ecological (how animals fit within different niches in the natural world).

A general presupposition in Biology and Zoology = "Nature can be carved at its joints" = the world is filled with "natural kinds"

  • In the Phaedrus, an ancient Greek philosophical treatise, Plato quotes Socrates that when we want to understand the world, we divide things by classes "where the natural joints are, and not try... to break any part, the way a bad carver would" (Phaedrus, 265e). The metaphor of a butcher carving an animal properly by finding its natural joints is meant to suggest the belief that the world is made up of distinct and different natural kinds of realities such as different species of animals, of plants, and other natural phenomena: a dog is not a lion is not a wolf is not a tiger.
  • Yes, there are some hybrids among plants and animals. Many of these are the result of human interventions in which two species are deliberately mixed together, e.g., mules are sterile hybrids between a male donkey and a female horse. But, in general, different species of animals and plants are "reproductively isolated by strong barriers to hybridization, which include genetic and morphological differences,.." (Wikipedia) Over the course of human history, we have successfully bred new species of both animals and plants (e.g., wheat). Nonetheless, once such a new species is developed, it contains its own set of individual characteristics that make it the species it is.
  • And, of course, too, evolution has produced over many hundreds of millions of years a great amount of diversity to be found in the kinds of living organisms that inhabit the earth. Life scientists continue to look at evolutionary processes as well as to identify new species of plants and animals that we have never known about before (Indeed, in 2020, zoologists discovered in East Asia two new species of monkeys [Raffles’ banded langur (Presbytis femoralis) and the East Sumatran banded langur (Presbytis percura)], very rare creatures who are in danger of going extinct].
  • So, when biologists and zoologists refer to different species, they are pointing to "natural kinds". 

B. Sciences of Health and Disease. One of the most compelling questions in medicine, psychiatry and clinical psychology is whether it is even possible to identify different kinds of mental disorders or mental diseases in a way that is parallel to classifying distinct species of animals and plants. In medical, psychiatric, and clinical psychological science, the challenge to create a classification or taxonomy is generally described as the development of a nosology.

Nosology  (nosos in Greek = disease; logos/logy in Greek = study of)

  • 1a. A treatise dealing with diseases; a classification or arrangement of diseases.
  • 1b. A list or catalogue of known diseases.
  • 1cc. A collection or combination of diseases 
  • 2.The systematic or scientific classification or investigation of diseases; the branch of medical science which deals with this. (Oxford English Dictionary)
In medicine the notion of "disease" can be a kind of slippery one if a person insists that there are very distinct boundaries of physical symptoms, etc. so that each disease is a very separate reality.
Medical Nosology
  • How many illnesses, diseases or medical disorders are there in human beings? That number is very much disputed and under constant research.
    • The World Health Organization has claimed that there are over 10,000 human diseases that are "monogenic" -- that is, caused by a single gene. And, they estimate that such diseases affect about 1% of the population of the world. Such diseases include cystic fibrosis, hemophilia, sickle cell anemia, and other. 
    • Another estimate is that there are about 9200 "orphan disease" that are very rare (fewer than 6 in 10,000 patients)
    • Yet, the ICD-10 described below has about 70,000 codes for diseases which is probably the highest number than anyone currently claims.
    • All of this is to say, that we do not have anywhere near agreement on even how many diseases there are.
  • Are there common elements or similarities that somehow link diseases together? These similarities may include
    • Symptoms: What changes in the person's body or behavior is regularly associated with the disease or disorder?
    • Causes: What are the underlying factors which are causing the disease/disorder?
    • Course: How does the disease begin and show itself? What happens to the person with the disease as time goes on? How does it resolve itself or not?
    • Treatments: What are the ways in which the disease/disorder can be treated so as to reduce or eliminate the negative effects of the illness?
    • Prognosis: What will the future be for someone with this disease? Will there be residual illness or impairment? Will the person get the illness again?

What are some of the general medical classification systems used today? There are two major ones which are most important

  • International Classification of Diseases (ICD; World Health Organization). The 10th edition (ICD-10), first published in 1992 with small updates, is the current system used most widely in the world. The 11th edition (ICD-11) will become officially effective on January 1, 2022.
  • In the United States, the ICD-9-CM (CM="Clinical Modification") was used until 2015 when US Medicare and other governmental agencies adopted the ICD-10-CM for the recording of causes of deaths.
  • The Systematized Nomenclature of Medicine (SNOMED), originated in the US in the 1960s, became SNOMED-CT (CT = Clinical Terminology) in 1999, and is now the most widely-used system in the world by which to code and classify medical data.  In 2020, SNOMED-CT includes the definition or description for more than 350,000 concepts in medicine (including diseases, symptoms, causes, etc.). In many ways, SNOMED-CT can be thought of as a huge thesaurus. It does allow physicians and other clinical personnel across the world to enter vital data about illnesses, treatments, etc. which can be recognized across multiple languages and medical approaches to treatment.

C. Classifying Mental Disorders

There are two general approaches to classifying what we call mental illnesses or mental disorders: (1) Categorical Approach and (2) Dimensional Approach

1. The Categorical Approach
  • Emil KraepelinThe traditional model for psychopathology from the late 19th and early 20th century is known as the "categorical" approach and was pioneered by the German psychiatrist, Emil Kraepelin (1856-1926). He called his effort to classify mental disorders as "clinical" and published the most widely-influential textbook in psychiatry in the early 20th century. 
  • Kraepelin argued that mental illness was primarily the result of biological and genetic processes. He classified mental illnesses by grouping their symptoms together into the notion of a syndrome, that is, a collection of symptoms that are correlated and appear together in a medical disease. It is not the symptoms of themselves but the pattern of the symptoms appearing together that defines the syndrome.
  • Kraepelin argued that there were two different types of psychoses: dementia praecox (= "early madness" and renamed as "schizophrenia" (= "split in the mind") by Eugen Bleuler later on) and manic depression (now seen as a range of disorders including bipolar disorder and major depression).
  • He worked with Alois Alzheimer and is a co-discoverer of Alzheimer's disease.
  • Thus, according to Kraepelin and his followers,  each mental disorder if basically a discrete or separate kind of "illness" (mental illness is a "disease") and that persons with these disorders function in ways that are different from how "normal" persons function. In the categorical approach, a person either has or doesn't have the illness or disorder.
  • DSM-5Kraepelin's categorical approach was eclipsed for many decades of the 20th century when Freudian theory dominated psychiatry in Europe and the United States.
  • However, since 1980 with the publication of the 3rd edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM) by the American Psychiatric Association, Kraepelin's model for classifying mental disorders has found a new prominence. The current edition, DSM-5, published in 2013, describes a large array of mental disorders grouped into general categories such as "neurodevelopmental, schizophrenia spectrum, bipolar, depressive, anxiety, obsessive-compulsive, trauma-related, and dissociative" disorders. There are approximately 160 different disorders listed in the DSM-5 (which is a reduction from the almost 300 disorders listed in the DSM's 4th edition). 
  • Criticisms of the DSM-5 have included a concern that (1) too many problems of ordinary life are now being labeled as a "mental disorder" and (2) the overall reliability of the criteria for these mental illness (that is, consistency by doctors in diagnosing) is still too low. 
2. The Dimensional Approach

Over the past decade, psychiatry and clinical psychology are increasingly challenged by emerging genetic and other data which undermine the traditional categorical approach to psychiatric conditions and serious mental disorders.

The findings from genetics research (e.g., genome-wide association studies of specific disorders) suggest that there are wide overlaps among the various disorders. Because of this, the future listings of psychiatric disorders may be quite different from the ones we are used to today. In the figure below, you see the abbreviations for individual problematic genes (e.g., DRD2, DISC1) which have been found to be common to the various psychiatric disorders.

Genetic Overlap

What else is wrong with the categorical approach? Kotov et al (2017, p. 457) make these five claims:
  • Disorders are on a continuum"The evidence to date suggests that psychopathology exists on a continuum with normal-range functioning; in fact not a single mental disorder has been established as a discrete categorical entity" (p. 457)
  • Low reliability of diagnoses: "Traditional diagnoses generally show limited reliability [= consistency of diagnosis] as can be expected when arbitrary categories are forced onto dimensional phenomena...For example, the DSM-5 Field Trials found that 40% of diagnoses did not meet even a relaxed cutoff for acceptable interrater reliability" (p. 457)
  • Multiple forms of a disorder: "Many existing diagnoses are quite heterogeneous and encompass multiple pathological processes."
  • Extensive co-morbidity: "Co-occurrence among mental disorders, often referred to as comorbidity, is very common in both clinical and community samples...high co-morbidity suggests that some unitary conditions have been split into multiple diagnoses, which co-occur frequently as a result, indicating the need to draw boundaries between disorders"
  • Overlooked patients: "Many patients fall short of the criteria for any disorder, despite manifesting significant distress or impairment that indicates the need for care"
So, as the number of distinct "disorders" described in the DSM have grown larger with many of the disorders sharing very similar or overlapping symptoms, critics have concluded that there ought to be a different approach to understanding serious mental disorders. Such efforts these days propose alternative models such as the ones here
The "dimensional" approach holds that either
  • [1] each disorder lies at the end of a spectrum ranging from "normal" to "abnormal" and that the line dividing what is normal from what is abnormal is unclear, and/or
  • [2] each disorder is composed of a set of symptoms which are shared to varying degrees across different kinds of disorders
In the simple model cited by Adam (2013) below, five major psychiatric conditions are characterized by differing mixtures of four dimensions: cognition, negative symptoms, positive symptoms, and mood swings.
Dimensions of psychopathology

A different example: Autism. As Chung (2021) argues, "Genetic research has taught us that what we commonly call autism is actually a spectrum of hundreds of conditions that vary widely among adults and children.  Across this spectrum, individuals share core symptoms and challenges with social interaction, restricted interests and/or repetitive behaviors." (emphasis added)

The "family of disorders" approach (there is no commonly accepted name for this) argues that each different diagnostic label (e.g., schizophrenia, depression, anxiety) actually represents a set of multiple different disorders which may share certain symptoms in common. Thus the diagnostic label is a kind of "family name" for a collections of related but distinct disorders much like the general notions of "cancer" or "heart disease" (which are really multiple kinds of diseases).
  •  For example, our textbook talks about two possible forms of depression: early onset depression (before age 30) which seems associated with higher levels of depression, attention deficit disorder, anxiety problems, migraine headaches, and irritable bowel disorder in family members versus
  • late onset depression (after ages 45-50) which seems associated with families in which members have circulatory problems.

The Hierarchical Taxonomy of Psychopathology (HiTOP): A Dimensional Alternative to Traditional Nosologies (Kotov, et al., 2017)

A recent comprehensive dimensional model for psychopathology has emerged for the work of a very large consortium of researchers across the world, but centered at SUNY Stony Brook on Long Island (https://renaissance.stonybrookmedicine.edu/HITOP)

Here is their latest (2020) model for understanding psychopathology.
  • At the top of their model is the notion that there is a general factor (called "p") that is present in all versions of psychopathology. Note by the way that this factor ranges in strength from minimal to very severe, that is, it lies on a continuum.
  • The p factor, in turn, is represented by a second layer consisting of a continuum of problems with (1) emotional dysfunction, or (2) psychosis, or (3) externalizing problems.
  • At each of the descending layers in the model, the various factors are expressed along a continuum from mild through extreme.
  • The research consortium is also in the process of developing highly reliable ways of measuring where on these factors individual persons may fall. These emerging instruments will allow a greater level of reliability in diagnosing and, then, treating individual patients. Notice that at the 4th level down (the grey rectangles containing diagnostic labels), the researchers term these labels as "syndromes/disorders" which serve to link this approach to psychopathology to aspects of the categorical models.



Chung, W. (2021, April 30). How Big Data are unlocking the mysteries of autism. Scientific American. https://www.scientificamerican.com/article/how-big-data-are-unlocking-the-mysteries-of-autism/