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9990 · 1.2.2

Explanations of mood (affective) disorders: depressive disorder (unipolar) — practice questions

Practice and worked examples for 9990 Explanations of mood (affective) disorders: depressive disorder (unipolar). Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A student who failed important exams now believes they are 'useless', expects all future attempts to fail, and sees the world as unfair.

(a) Explain this depression using Beck's cognitive theory. [4 marks] (b) Compare Beck's explanation with a biological explanation of depression. Evaluate which is more useful. [6 marks]

Show solution outline

(a) AO1 — Beck's cognitive theory applied:

  • Negative schema activated by exam failure — global, stable attribution ('useless').
  • Cognitive triad: Negative self (inadequacy), world (unfair), future (expects failure).
  • Cognitive biases: Catastrophising and overgeneralisation from one event to all domains.
  • Maintaining cycle: negative thoughts → low mood → withdrawal → fewer positive experiences → confirms schema.

(b) AO1 — Biological comparison:

  • Genetic vulnerability + low serotonin/noradrenaline disrupt mood regulation.
  • SSRIs raise serotonin, relieving symptoms — suggests biochemical component.

AO3 — Evaluation:

  • Beck — strengths: Explains content of depressive thinking; basis for effective CBT; good face validity for this scenario.
  • Beck — limitations: Does not explain why schemas form; chicken-and-egg — mood may cause negative thoughts (biological first).
  • Biological — strengths: Drug response supports role of neurotransmitters; twin studies show heritability.
  • Biological — limitations: Reductionist — ignores life events; not all depressed patients respond to medication.

Judgement: Most useful is interactionist — exam failure (environment) triggers Beck's cognitive patterns in biologically vulnerable individuals; neither alone is sufficient.

Worked example 2

A study by McGuffin et al. (1996) investigated the genetic basis of unipolar depression. In their sample, they found a concordance rate of 46% for monozygotic (MZ) twins. In a hypothetical follow-up study with a new sample, researchers identify 250 MZ twin pairs where at least one twin has diagnosed depression. They find that in 105 of these pairs, the other twin also has depression.

(a) Calculate the concordance rate for MZ twins in this hypothetical study. [2 marks] (b) If the concordance rate for dizygotic (DZ) twins in the same study was found to be 20%, explain what these results suggest about the biological explanation for depression. [4 marks]

Show solution outline

(a) AO2 — Calculation of Concordance Rate:

  • Formula: Concordance Rate (%) = (Number of concordant pairs / Total number of pairs) × 100
  • Step 1: Substitute values: (105 / 250) × 100
  • Step 2: Calculate the result: 0.42 × 100 = 42%
  • Final Answer: The concordance rate for MZ twins in this study is 42%.

(b) AO1/AO3 — Explanation and Interpretation:

  • Comparison: The calculated concordance rate for MZ twins (42%) is more than double the rate for DZ twins (20%).
  • Genetic Argument: This supports a genetic explanation for depression. MZ twins share 100% of their genes, while DZ twins share approximately 50%. The higher rate in MZ twins suggests that shared genes contribute to the risk of developing depression.
  • Environmental Argument: However, the concordance rate for MZ twins is not 100%. If depression were purely determined by genetics, we would expect that if one identical twin has it, the other would always have it. The fact that the rate is 42% (and not 100%) demonstrates that environmental factors must also play a crucial role. This supports an interactionist approach, like the diathesis-stress model, where a genetic vulnerability (diathesis) is triggered by environmental stressors.
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