This piece describes how clinical psychology works. It is not medical advice, and nothing here can tell you whether you or someone you know has a condition. If you are struggling, a doctor or a local mental health service is the right place to start; findahelpline.com lists free helplines by country.
Everyone has had a week when they could not sleep, felt flat, or worried so much that they could not think. Almost nobody thinks that week is an illness. Yet there are people for whom that week lasts a year, and who cannot work, eat or be with their family. Somewhere between these lies a line, and it is not a line that a blood test can draw.
That is the central puzzle of clinical psychology: how to tell ordinary suffering from a disorder, how to describe what is wrong without pretending to know more about the cause than we do, and how to help. A graduate of this course keeps three habits: treat diagnoses as useful descriptions with fuzzy edges, treat causes as many things at once, and ask of every treatment how well it was tested.
The definition in the American Psychiatric Association's manual runs roughly like this: a mental disorder is a clinically significant disturbance in thinking, emotion regulation or behaviour that reflects a problem in psychological, biological or developmental processes, and that usually comes with distress or disability. It adds exclusions: an expected response to a loss is not a disorder, and neither is behaviour that is merely socially deviant, or in conflict with society, unless it reflects a dysfunction in the person.
Philosophers have argued about this for decades. Jerome Wakefield's "harmful dysfunction" idea says a disorder needs both a failure of some mental mechanism to do what it evolved to do and harm to the person. Critics ask who decides what a mechanism is for. In practice, clinicians use three working questions: how much does it distress the person, how much does it get in the way of life, and how far is it outside what the person's culture expects?
The history shows how much the line moves. Homosexuality was listed as a mental disorder in early editions of the American manual; the American Psychiatric Association's board voted to remove it in 1973, and the World Health Organization removed it from the international list in 1990. In 1973 David Rosenhan published "On Being Sane in Insane Places", reporting that eight people who feigned hearing a voice were admitted to psychiatric hospitals; the paper was influential, and Susannah Cahalan's 2019 book The Great Pretender has since cast serious doubt on how the study was run and reported.
Most symptoms form a continuum. Plot how much sadness or worry people have and you get a bell curve with no natural gap; a diagnosis puts a line through it. Move the line and the number of cases moves with it.
An invented, normally distributed "symptom load"; real conditions are not exactly bell-shaped and real diagnosis also asks about impairment and duration. This is a picture of an idea, not a screening tool.
Without JavaScript: a cut-off 1.5 standard deviations above average labels about 6.7% of people; at 2 about 2.3%; at 1 about 15.9%.
Whoever stands just below the line and whoever stands just above it are nearly alike. That is why a diagnosis is useful for getting help and for research, and why clinicians stress that it describes a pattern, not a separate kind of person.
Two systems define what is counted. The DSM is published by the American Psychiatric Association; its fifth edition appeared in 2013 and a text revision, DSM-5-TR, in March 2022. The ICD is the World Health Organization's classification of all diseases; its eleventh revision, ICD-11, was endorsed by the World Health Assembly in May 2019 and came into effect on 1 January 2022, with a chapter on mental, behavioural and neurodevelopmental disorders. ICD is the official statistical standard for WHO member states, and many countries are still moving to ICD-11; DSM is used mainly in the United States and in much research around the world. They are mostly alike, but they differ at the edges, and the edges are revealing.
Simplified summaries. Check the manuals for exact criteria.
| DSM-5-TR (APA, 2022) | ICD-11 (WHO, in effect 2022) | |
|---|---|---|
| Who uses it | Mainly the US; widely in research | Health systems and statistics worldwide |
| Structure | Categories with symptom checklists | Clinical descriptions, with more dimensions |
| Cost | Sold as a book | Free online |
Do the manuals agree on who has what? Not as well as people assume. In the field trials for DSM-5, reported by Regier and colleagues in 2013, the agreement between clinicians interviewing the same patients on separate occasions was low for some common conditions, including major depression and generalized anxiety disorder, a result the APA itself acknowledged. Diagnoses are tools with real uncertainty, and critics have proposed more dimensional systems, such as the HiTOP model (Hierarchical Taxonomy of Psychopathology), which describes problems as positions on a few broad scales.
This is a map, not a guide to identifying anything. The two most common groups worldwide are anxiety and depressive disorders: the World Health Organization estimated in 2023 that about 1.2 billion people, roughly one in seven, were living with a mental disorder, including about 470 million with anxiety and 322 million with depression. Most conditions begin before the mid-twenties.
| Group | The core of it | Main treatments with evidence |
|---|---|---|
| Depressive | Persistent low mood or loss of interest, with sleep, energy and thinking changes | Psychotherapy, medication, or both |
| Anxiety, OCD, PTSD | Fear and worry out of proportion; obsessions and compulsions; reactions after trauma | CBT with exposure; some medicines |
| Bipolar and psychotic | Episodes of mania; losing contact with shared reality (psychosis) | Medication is central, with psychosocial care |
| Neurodevelopmental | Autism, ADHD, intellectual disability: differences present from early life | Support and adjustment, with medication for ADHD |
| Eating and substance use | Disordered eating; harmful use that is hard to stop | Specialist psychological, medical and community care |
| Personality | Long-standing patterns that cause trouble with self and others | Specialist psychotherapy (for example, DBT) |
A diagnostic test is a screen of a different kind of question, and one piece of arithmetic helps whoever reads about any test. The chance that a positive result is a true case depends heavily on how common the condition is. This is Bayes' rule at work, the subject of Bayesian Thinking for Everyday Life (17/07).
A made-up test for a made-up condition, applied to 1,000 people. It is about reasoning, not about any real instrument.
Without JavaScript: with a 2% condition and a test that is 90% sensitive and 90% specific, about 16 of every 100 positive results are true cases; at 20% prevalence, about 69.
This is why a short questionnaire is a first step, never a diagnosis: a positive result in a general population mostly leads to a conversation with a trained person, not a conclusion.
"What causes depression?" has no single answer. The biopsychosocial model, introduced by George Engel in 1977, says that biology (genes, brain, illness), psychology (thinking habits, history) and the social world (poverty, relationships, culture) all matter, and they interact. The diathesis–stress model is the simplest version: a person has some vulnerability (the diathesis), and a disorder appears when stress pushes the total over a threshold. Two people with the same stress can have different outcomes.
Twin studies suggest that genes matter, though not as destiny: heritability is high for schizophrenia (roughly 80%) and moderate for major depression (roughly 40%), and both leave plenty to the environment. Childhood adversity, loss, discrimination and poverty raise the risk of many conditions. And no single gene or chemical explains any common disorder.
The "chemical imbalance" story deserves care. The serotonin theory of depression was popular for decades, and a 2022 umbrella review by Joanna Moncrieff and colleagues concluded that the research did not show a consistent link between serotonin and depression. Critics said the review was one-sided in its method and that it did not show that antidepressants do not work, and the researchers disagreed with them. What most clinicians now say is that medicines can help, that we do not understand exactly how, and that "depression is low serotonin" is a simplification.
A toy: vulnerability varies between people (standard deviation 15), and a disorder appears when vulnerability plus stress passes 100. The units are invented; the shape is the point.
Without JavaScript: with average vulnerability 50 and stress 40, about 25% cross the threshold; at stress 60, about 75%.
Look at the curve: stress is not a switch. It moves a share of the group over the line, and the people just below the line at one level of stress are the people over it at the next. That is both why prevention matters and why no person's illness is simply their own fault.
Psychotherapy has three broad families. Cognitive-behavioural therapy (CBT), developed from the 1960s by Aaron Beck and others, works on the link between thoughts, feelings and behaviour, and on facing feared things in steps; it has the largest research base. Psychodynamic therapies, descended from Freud, look at patterns that began early in life; they have evidence for several conditions, though less of it and more argument over its quality. Humanistic approaches, from Carl Rogers, put the relationship at the centre. Newer methods include dialectical behaviour therapy for severe emotional difficulty, and EMDR for trauma, which is recommended by guidelines but debated over what part of it does the work.
How do you say a therapy "works"? You compare outcomes to a control group and report the gap in standard deviations, the effect size (Cohen's d). By convention 0.2 is small, 0.5 medium and 0.8 large. The picture below turns that into something you can feel.
Two normal curves with equal spread. "Chance a treated person does better" is the probability of superiority, Φ(d ÷ √2).
Without JavaScript: d = 0.2 means a randomly chosen treated person does better than a random control about 56% of the time; d = 0.5, about 64%; d = 0.8, about 71%.
Notice that even a "large" effect leaves wide overlap: many people in the control group do better than the average treated person. That is true of nearly everything in this field and it is not a reason for despair. A Lancet network meta-analysis in 2018 (Andrea Cipriani and colleagues; 522 trials, 116,477 people) found that all 21 antidepressants it studied beat placebo in short-term trials, with odds ratios between about 1.4 and 2.1, so the benefit is real but modest and not the same for everyone. Psychotherapy and medication have each been shown to help in depression, and for many anxiety conditions psychotherapy does as well or better, with longer-lasting benefit once treatment stops.
A caution. Published results flatter treatments. In a 2008 analysis in the New England Journal of Medicine, Erick Turner and colleagues compared FDA records with journal articles for antidepressant trials, and found the journal literature looked about 94% positive while the FDA files showed about half. Pre-registration and publishing negative results have helped, but trial size, who is excluded and who drops out still affect how well a result transfers to a real patient. Another persistent finding is that the relationship between patient and therapist predicts outcomes across many methods, which is part of why a good fit matters. Whether all therapies are equally good (the "Dodo bird verdict") is still argued.
Most of the world's research comes from rich countries, and most of the world's people with mental disorders live elsewhere. The World Health Organization reports that only about a third of people with depression receive formal care, and the gap is far wider in poor countries. Its Mental Health Atlas 2020 put the global median at 13 mental health workers per 100,000 people, with fewer than two per 100,000 in low-income countries and over 60 in high-income countries.
The one-in-seven figure is WHO's 2023 estimate; the worker figures are from the 2020 Atlas (the low and high ends as reported in secondary summaries). Not everyone with a disorder needs a specialist, so this is a measure of scale, not of waiting lists. The bar is drawn on a square-root scale.
Without JavaScript: at 13 workers per 100,000, about 1,100 people with a disorder (one in seven) per worker; at 60, about 240; at 1.4, over 10,000.
One response is to take treatment out of the clinic. The Friendship Bench, created in Harare, Zimbabwe, by the psychiatrist Dixon Chibanda, trains lay health workers, often older women known as "grandmothers", to give six sessions of problem-solving therapy on a bench in the grounds of a clinic. In a 2016 randomised trial in JAMA with 573 adults, six months later about 14% of the Friendship Bench group still had symptoms of depression on the measure used, against about 50% of those who got usual care. It spread to several other countries. The WHO's mhGAP programme and Vikram Patel's work on task-sharing follow the same logic: train more people to deliver a limited, evidence-based set of skills.
Culture shapes how distress shows. The DSM now lists "cultural concepts of distress": ataque de nervios, common in Latin America and the Caribbean; kufungisisa ("thinking too much") among the Shona of Zimbabwe, which can describe worry or depression; susto, fright that is believed to cause illness. These do not map neatly onto Western categories, and a good clinician asks about meaning before sorting a person into a diagnosis.
Disorders are patterns on a continuum, defined by distress and impairment, and described by two manuals that agree on much and not on all. Causes are biological, psychological and social at once. Several therapies help on average, with real uncertainty, and the largest problem worldwide is not choosing a treatment but getting any.
Try this: the next time a headline claims a new therapy or drug works, find three things: the size of the effect, the comparison group, and whether the result was reproduced.
Sources. American Psychiatric Association, DSM-5 (18 May 2013) and DSM-5-TR (18 March 2022; includes prolonged grief disorder); WHO, ICD-11 (endorsed by the World Health Assembly on 28 May 2019, in effect 1 January 2022; complex PTSD 6B41, gaming disorder 6C51; gender incongruence in the sexual-health chapter; personality disorder graded by severity). Wakefield, American Psychologist 1992; Rosenhan, Science 1973, and Cahalan 2019; homosexuality removed from the DSM in 1973 and from the ICD in 1990. Regier et al., "DSM-5 Field Trials in the United States and Canada, Part II," American Journal of Psychiatry 2013. WHO fact sheet on mental disorders (2023 figures: about 1.2 billion people, 470 million with anxiety, 322 million with depression; about a third of people with depression receive formal care): who.int/news-room/fact-sheets/detail/mental-disorders. WHO, Mental Health Atlas 2020 (global median 13 workers per 100,000; WHO news release of 8 October 2021); low and high ends as given in secondary summaries and not checked against the report. Engel, Science 1977; Moncrieff et al., Molecular Psychiatry 2022, and the critical comment by Jauhar and colleagues (2023); Cipriani et al., The Lancet 2018 (522 trials, 116,477 participants, odds ratios 1.37 to 2.13); Turner et al., New England Journal of Medicine 2008. Chibanda et al., "Effect of a primary care-based psychological intervention on symptoms of common mental disorders in Zimbabwe," JAMA 2016 (573 participants, 24 clinics; depression symptoms 14% against 50% at six months). Twin heritability of about 80% for schizophrenia and about 40% for major depression: Sullivan and colleagues (2000, 2003), given as round figures. Computed in node from the code on this page: prevalence above a cut-off of 1.5 SD is 6.7%; PPV 15.5% at 2% prevalence and 69.2% at 20%; probability of superiority 55.6%, 63.8% and 71.4% at d = 0.2, 0.5 and 0.8.