5.1.3 - Unipolar depression explanations
Learn the features of unipolar depression and how biological vulnerability and Beck's cognitive model explain its development and persistence. Compare what their evidence supports without treating either account as a complete explanation for every person.
Recognising the pattern of depression
Unipolar depression involves a sustained depressive episode without the manic or hypomanic episodes that characterise bipolar disorders.
“Unipolar” refers to the depressive pole of mood. It does not mean the person is always sad or can never recover. A central feature is persistent low mood and/or anhedonia, a marked loss of interest or pleasure. A major depressive episode typically involves a pattern lasting at least two weeks, present much of the time, with significant distress or interference in everyday functioning. Duration, context and the cluster of features matter; one bad day or one symptom is insufficient to establish the disorder. NIMH description.
Features can be grouped to understand their impact:
| Area | Possible features | Consequence for daily life |
|---|---|---|
| Emotional | Low mood, hopelessness, reduced pleasure | Activities that previously felt rewarding may seem empty. |
| Cognitive | Excessive guilt, worthlessness, difficulty concentrating or deciding | Reading, planning and judging oneself can become difficult. |
| Behavioural | Withdrawal, reduced activity, slowed behaviour or agitation | Social and occupational routines may be disrupted. |
| Physical | Fatigue, altered sleep, appetite or weight | Reduced energy can make ordinary tasks demanding. |
People differ: sleep or appetite may increase or decrease, and not everyone shows every feature. Thoughts about death or suicide can occur and require professional attention; they are not present in every depressive episode. Distinguishing depression from physical illness, substance effects or bipolar disorder requires assessment, rather than diagnosing from a short description. These categories describe features, not four separate disorders.
Anhedonia
A marked reduction in interest or pleasure in activities that would usually be enjoyable.
Describing a feature answers “what is experienced?” An explanation instead answers “what processes could produce or sustain it?” Reduced activity, for example, is a behavioural feature; an account of why withdrawal feeds back into mood is an explanation.
Genetic and neurochemical vulnerability
A biological explanation proposes that inherited differences and the functioning of mood-related brain systems contribute to susceptibility to depression.
Genes influence proteins involved in nervous-system development and functioning. Many genetic variants may each contribute a small amount to polygenic vulnerability; there is no single gene that determines ordinary unipolar depression. An inherited predisposition may affect how a person responds to stress or regulates mood, while later experiences influence whether an episode develops. This is a diathesis–stress account: vulnerability and stress interact. A family history increases risk, but it does not make depression inevitable.
Twin evidence helps separate possible genetic and environmental contributions. Monozygotic twins share almost all their genetic variation; dizygotic twins share on average about half their segregating genetic variation. Higher similarity for depression in monozygotic pairs is consistent with genetic influence. It is not decisive alone, because the pairs may also differ in how similarly they are treated. Sullivan, Neale and Kendler's 2000 meta-analysis estimated heritability of liability to major depression at about 37% in the twin evidence reviewed. Heritability concerns variation in a population under particular conditions; it does not mean that 37% of one person's depression is genetic. The substantial remaining variation also matters. Original meta-analysis.
The monoamine hypothesis proposes that reduced or dysregulated neurotransmission involving chemicals such as serotonin and noradrenaline contributes to depression. At a synapse, neurotransmitter release, receptor response and removal by reuptake determine how signalling works. These systems influence mood, sleep and arousal through interacting circuits. Reduced effective signalling could therefore contribute to depressive features. This proposed account is more cautious than saying depression is always a measurable shortage of serotonin.
Antidepressants that alter monoamine signalling can improve symptoms, providing evidence that these systems can be useful treatment targets. However, changing a system and obtaining benefit does not establish that a pre-existing deficiency caused the disorder. Clinical improvement can take weeks although the direct synaptic drug action starts sooner, suggesting that further adaptations matter. Biological vulnerability also includes interacting systems and life experiences, so the simple deficiency version is incomplete. Current NIMH information likewise describes genetic, biological, environmental and psychological contributions. NIMH on genetic risk.
Beck's schemas and negative triad
Beck's cognitive model explains depression through patterns in how information is interpreted, rather than through events alone.
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