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Alongside schizophrenia, the Edexcel specification requires the study of a second disorder in the same depth: its symptoms and features, at least one explanation, and its treatment. This lesson covers two disorders that serve this purpose and that between them showcase two contrasting approaches: unipolar depression (major depressive disorder), used as the worked example for the cognitive approach (Beck's negative triad and Ellis's ABC model, treated with CBT), and obsessive-compulsive disorder (OCD), used for the biological and behavioural account (genes, neural circuitry and serotonin, treated with drugs; plus the behavioural obsession–compulsion cycle). The core skill is to describe symptoms, explanations and treatments precisely (AO1) and to evaluate them rigorously (AO3), including the recurring issues of cause-versus-symptom, reductionism and the diathesis-stress interaction. As with all clinical content, both disorders are discussed in a measured, academic register — as conditions to be explained and treated — never described in a way that could distress or instruct.
Key Definition: Unipolar depression (major depressive disorder) is a mood disorder characterised by persistent low mood, loss of interest and pleasure (anhedonia), and associated cognitive and physical changes. Obsessive-compulsive disorder (OCD) is characterised by obsessions (recurrent, intrusive, unwanted thoughts, images or urges that cause anxiety) and compulsions (repetitive behaviours or mental acts performed to reduce that anxiety).
This lesson addresses the Edexcel 9PS0 — Paper 2, Topic 5: Clinical Psychology content on a second disorder studied alongside schizophrenia, covered here through unipolar depression and OCD: the symptoms and features of each; explanations — the cognitive explanation of depression (Beck's negative triad; Ellis's ABC model) and the biological and behavioural explanation of OCD (genetic and neural factors, serotonin; the operant obsession–compulsion cycle); and treatments — CBT for depression and drug therapy (SSRIs) and CBT with ERP for OCD. It parallels the schizophrenia sequence (symptoms → explanation → treatment) and reuses the diagnostic framework of the diagnosis-and-classification lesson. In assessment-objective terms, you should be able to describe the symptoms, explanations and treatments of the second disorder (AO1), apply these to scenarios such as a described low-mood presentation or a checking compulsion (AO2), and evaluate the explanations and treatments — the cause-versus-symptom problem, reductionism, effectiveness and the diathesis-stress synthesis (AO3).
Connects to…
Depression (DSM-5: major depressive disorder) is one of the most common conditions. Its features can be described across three response systems, and being able to give characteristics under each is useful for the symptoms part of an answer.
| Component | Characteristics |
|---|---|
| Behavioural | Reduced activity and energy; social withdrawal; disruption to sleep and appetite (increased or decreased); psychomotor change (agitation or slowing) |
| Emotional | Persistent low mood; feelings of worthlessness and guilt; loss of interest and pleasure (anhedonia); sometimes irritability |
| Cognitive | Negative self-concept; poor concentration and indecisiveness; absolutist "black-and-white" thinking; attention to, and dwelling on, the negative |
A diagnosis typically requires a cluster of these features to be present for a minimum period (around two weeks) and to cause clinically significant distress or impairment — the distress-or-impairment requirement discussed in the diagnosis lesson, which distinguishes a disorder from ordinary, transient low mood.
The cognitive approach locates the cause of depression not in events themselves but in the way events are processed — in faulty, negative thinking. Two accounts are examinable.
Beck (1967) argued that depression-prone people hold negative schemas acquired in childhood that, once activated by a negative event, produce systematic cognitive biases and crystallise into the negative triad: automatic negative views of the self, the world and the future.
graph TD
SELF["Negative view of the SELF<br/>'I am worthless'"]
WORLD["Negative view of the WORLD<br/>'everything works against me'"]
FUTURE["Negative view of the FUTURE<br/>'nothing will improve'"]
SELF --> WORLD
WORLD --> FUTURE
FUTURE --> SELF
style SELF fill:#7c3aed,color:#fff
These schemas drive cognitive biases that maintain low mood:
| Cognitive bias | Description | Example |
|---|---|---|
| Catastrophising | Treating a minor setback as a disaster | "I failed one test — my whole future is ruined" |
| Overgeneralisation | Drawing a sweeping conclusion from one event | "One rejection means no one will ever value me" |
| Selective abstraction | Attending only to the negative detail | Dwelling on one critical remark amid many positive ones |
Ellis (1962) proposed that emotional disorder follows not from events themselves but from irrational beliefs about them, formalised in the ABC model.
| Stage | Meaning | Example |
|---|---|---|
| A — Activating event | The triggering situation | An unsuccessful job application |
| B — Belief | The interpretation, rational or irrational | Irrational: "I must always succeed; failing makes me worthless" |
| C — Consequence | The emotional/behavioural outcome | Low mood, withdrawal, loss of motivation |
Ellis catalogued recurring irrational beliefs, including "musturbation" (the demand that one must succeed), awfulising (treating setbacks as catastrophic) and low frustration tolerance ("I can't stand it when things are hard"). The key point common to both accounts is that the same activating event can produce very different emotional outcomes depending on the beliefs a person brings to it — which is why the cognitive approach locates the disorder in cognition rather than circumstance.
CBT operationalises the cognitive model into therapy, combining cognitive restructuring with behavioural change:
For comparison, the biological treatment for depression is antidepressant medication, principally SSRIs (e.g. fluoxetine), which increase serotonin availability at the synapse (described in the OCD section below). Evidence suggests that for moderate-to-severe depression, CBT combined with medication can be more effective than either alone — medication can lift mood and energy enough for a severely depressed person to engage with CBT, which then equips them with durable, relapse-preventing skills. This is the rationale for stepped-care approaches.
OCD is defined by two linked features — obsessions and compulsions — described across three response systems.
| Component | Characteristics |
|---|---|
| Behavioural | Compulsions — repetitive acts (e.g. washing, checking, counting, ordering) performed to reduce anxiety; these are excessive and not realistically connected to the feared outcome. Avoidance of triggering situations is also common. |
| Emotional | Persistent anxiety and distress; accompanying feelings of guilt, disgust or shame; marked discomfort if a compulsion is resisted |
| Cognitive | Obsessions — recurrent, intrusive, unwanted thoughts the person recognises as excessive but cannot dismiss; catastrophic appraisals of what will happen if compulsions are not performed; selective attention to the perceived threat |
Key Definition: Obsessions are recurrent, intrusive thoughts, images or urges experienced as unwanted and distressing. Compulsions are repetitive behaviours or mental acts the person feels driven to perform in response, to reduce anxiety or prevent a dreaded event.
A central point is the obsession–compulsion cycle: an intrusive obsession generates anxiety; performing a compulsion produces temporary relief; that relief negatively reinforces the compulsion; but the underlying obsession returns, so the cycle repeats and intensifies over time. This behavioural (operant) account of maintenance sits alongside the biological account of vulnerability below.
The biological approach explains OCD as arising from genetic vulnerability expressed through neural and neurochemical abnormalities — in short, OCD runs in families because inherited genes shape brain systems that, when dysfunctional, produce obsessions and compulsions.
OCD shows clear familial clustering. Early family research in the tradition of Lewis (1936), who observed that a substantial proportion of his OCD patients had a parent or sibling with the disorder, established that OCD runs in families, and later twin studies show higher concordance in monozygotic (identical) than dizygotic (non-identical) twins, consistent with a heritable component. Crucially, OCD is polygenic — many genes each contribute a small amount of risk rather than a single "OCD gene" — and may be aetiologically heterogeneous (different gene combinations producing OCD in different people). Two well-studied candidate genes are the COMT gene (coding for an enzyme that breaks down dopamine) and the SERT gene (coding for the serotonin transporter responsible for serotonin reuptake).
The genetic vulnerability is thought to be expressed through abnormalities in a specific frontal-striatal circuit and in the serotonin system.
| Brain region | Normal function | Proposed abnormality in OCD |
|---|---|---|
| Orbitofrontal cortex (OFC) | Sends "worry"/error signals when something seems wrong | Heightened activity, generating excessive signals that something is wrong, experienced as obsessions |
| Basal ganglia (incl. caudate nucleus) | Filters and suppresses minor "worry" signals before they reach the cortex | Dysfunction, failing to filter the signals so they reach the cortex repeatedly, driving the obsession–compulsion loop |
The serotonin hypothesis proposes that low serotonin activity disrupts transmission in this circuit and contributes to OCD; its main support is that SSRIs, which raise synaptic serotonin, reduce OCD symptoms. A useful supporting study is Menzies et al. (2008), who found that OCD patients and their unaffected first-degree relatives showed reduced grey matter in regions including the OFC compared with controls, implying an inherited neural vulnerability rather than a consequence of the illness alone.
graph TD
GENES["Genetic vulnerability<br/>polygenic: COMT, SERT, others"] --> NEURO["Atypical neural development"]
NEURO --> OFC["Orbitofrontal cortex:<br/>heightened worry/error signals"]
NEURO --> BG["Basal ganglia:<br/>fails to filter the signals"]
NEURO --> SERO["Low serotonin in the circuit"]
OFC --> OBS["Obsessions: persistent<br/>sense something is wrong"]
BG --> OBS
SERO --> OBS
OBS --> COMP["Compulsions to<br/>reduce anxiety"]
COMP --> RELIEF["Temporary relief<br/>reinforces compulsion"]
RELIEF -.-> OBS
style OBS fill:#2563eb,color:#fff
The first-line biological treatment for OCD is drug therapy, principally Selective Serotonin Reuptake Inhibitors (SSRIs). SSRIs block the reuptake of serotonin from the synaptic cleft back into the presynaptic neuron, so serotonin remains in the synapse for longer and stimulates the postsynaptic receptor more; because low serotonin activity is implicated in OCD, prolonging serotonin's action is thought to normalise transmission in the OFC–basal-ganglia circuit and reduce symptoms. Common examples include fluoxetine and sertraline; doses for OCD are often higher than for depression, and full effects typically take several weeks. A standard supporting reference is Soomro et al. (2008), a meta-analysis of randomised controlled trials, which found SSRIs significantly more effective than placebo in reducing OCD symptoms.
The associated psychological treatment is CBT with Exposure and Response Prevention (ERP), which follows from a behavioural/cognitive analysis of OCD: the person is exposed to the trigger (the obsession-provoking situation) while the compulsion is prevented, so anxiety habituates and the person learns that the feared outcome does not occur even without the ritual. By blocking the compulsion, ERP removes the negative reinforcement that maintains the obsession–compulsion cycle. As with depression, SSRIs are frequently combined with CBT-ERP, and the combination is often more effective than either alone.
The cognitive explanation of depression is strongly supported by the effectiveness of CBT, but this evidence is open to a cause-versus-symptom challenge. CBT is one of the most effective and widely recommended treatments for depression, and its success is indirect support for the cognitive model, since changing thinking changes mood as the theory predicts. The standing difficulty, however, is cause or consequence: research consistently finds that depressed people show negative biases and a negative triad, but correlational designs cannot establish whether faulty cognition causes depression or is a symptom of it. The implication is that the cognitive model may describe the maintenance of depression more securely than its origin, and that therapeutic success, while supportive, does not settle the direction of causation.
The cognitive explanation of depression risks being reductionist and ignoring biological and social factors. By locating depression in the individual's thinking, Beck's and Ellis's accounts say little about the well-evidenced biological contribution (heritability, serotonin and other neurotransmitter systems) or the social contribution (poverty, isolation, life events). This matters because purely cognitive treatment may be insufficient for some patients — for example, those with severe depression too cognitively and behaviourally impaired to engage with therapy. The implication is that a diathesis-stress or biopsychosocial framework, in which cognitive vulnerability interacts with biological predisposition and environmental stress, is more complete than a stand-alone cognitive model, and explains why combined CBT-plus-medication often outperforms either alone.
CBT's effectiveness for depression, though strong, is qualified by the type and severity of the disorder, which has practical implications. CBT performs especially well for mild-to-moderate depression and confers durable, relapse-reducing benefits because the client retains the skills, but it depends on the client having sufficient cognitive and motivational capacity to engage — a condition that may not be met in severe depression, where psychomotor slowing and hopelessness can make structured cognitive work very difficult. This matters because it means CBT is not a universal first-line solution and that, for some patients, medication may be needed to enable engagement. The implication is that effectiveness is conditional rather than absolute, which is why stepped-care models match treatment intensity to severity — a more sophisticated conclusion than "CBT works".
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