# Biological explanation of depression

> IB Psychology · IB Psychology 2027 First Assessment
> Source: https://www.owlsprep.com/study/ib-psychology-hwb-biological-explanation-of-depression/

One or more biological explanations of one mental health disorder.

## Learning objectives

- One or more biological explanations of one mental health disorder.

## How often is this tested?

Based on OwlsPrep's analysis of official CIE IB_PSYCH P2 past papers (2019–2025): **Biological explanation of depression** appears **11 times** in the last 7 years — **3.8%** of all P2 questions (11 of 288).

Most-tested forms: Essay: Contrast (22 marks) (6), Essay: Discuss (22 marks) (5).

## Core concept

A biological explanation says that a mental health disorder like depression has physical roots — in genes and in brain chemistry. Twin and family studies suggest depression runs in families (a heritability of roughly a third), and reduced serotonin-system function is linked to low mood, which is why many antidepressants target it. Crucially, genes and chemistry set a vulnerability rather than a fixed fate: they raise risk, and whether depression appears usually depends on that vulnerability meeting life stress (the diathesis-stress idea). So a biological explanation is one probabilistic part of the picture, not the whole cause.

## Key studies

IB Psychology answers must be supported by named studies. Learn these to cite as evidence:

**Sullivan, Neale, and Kendler (2000)** — A meta-analysis of twin studies estimated the heritability of depression at about 37%. — *Quantifies the genetic contribution to depression — and, by leaving ~63% to other factors, shows genes are only part of the cause.*

**Caspi et al. (2003)** — People with the short 5-HTT (serotonin-transporter) allele showed more depressive symptoms after stressful life events, while the genotype alone did not predict depression. — *Anchors the genetic/neurochemical explanation while showing it works only in interaction with stress — biology as vulnerability, not destiny.*

## Evaluation (AO3)

**Strength — measurable and treatable** — Biological factors can be estimated (twin designs, genotyping) and give testable, treatable targets — for example drugs acting on serotonin.

**Limitation — single-factor and correlational** — Heritability is a population statistic and the gene-depression link is largely correlational; a lone gene or 'chemical imbalance' cannot on its own explain a disorder also shaped by cognition and environment.

## Scope

> **note**
>
> This card is the biological explanation of one disorder (depression). Cognitive and environmental explanations of the same disorder are HWB_2 and HWB_4; the biological treatment that follows is HWB_8. The underlying genetics/neuro-mechanisms live in the Biological approach (BIO_5/6/9).

## Common pitfalls

- **Wrong:** Depression is simply caused by a 'chemical imbalance' (too little serotonin) or by a single 'depression gene'.
  - Why it fails: A frequently penalised misconception in IB Psychology exams.
  - Correct: Biological explanations describe a vulnerability that raises risk and interacts with the environment — heritability is only ~37%, and Caspi shows the gene predicts depression only alongside stress. Present it as one interacting factor, not the sole cause.

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