# Diathesis–stress model

> IB Psychology · IB Psychology 2027 First Assessment
> Source: https://www.owlsprep.com/study/ib-psychology-bio-diathesis-stress-model/

The interaction of environmental factors and genetics in human behaviour.

## Learning objectives

- The interaction of environmental factors and genetics in human behaviour.

## Core concept

The diathesis–stress model says a behaviour or disorder appears when a predisposition (the diathesis — often genetic) meets a trigger (stress from the environment). Neither alone is enough: a vulnerable person can stay well without stressors, and a resilient person can weather the same stress. It explains why the same event affects people so differently, and why genes and environment must be studied together rather than separately.

## Key studies

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

**Caspi et al. (2003)** — People with the short 5-HTT allele showed more depressive symptoms after stressful life events than long-allele carriers, while the genotype alone did not predict depression. — *The textbook demonstration of gene × environment — the vulnerability expressed only when the stressor was present.*

## Evaluation (AO3)

**Strength — interactionist fit** — Explains the data better than gene-only or environment-only accounts and predicts individual differences in response to the same event.

**Limitation — measurement & replication** — 'Diathesis' and 'stress' are hard to operationalise, and some large replications of Caspi were weaker, so the effect size is debated.

## Scope

> **note**
>
> Introduced here as a biological-approach model; it recurs in Health and well-being (e.g. explanations of depression). Depression's fuller treatment lives in that context.

## Common pitfalls

- **Wrong:** A genetic predisposition makes the outcome fixed or inevitable.
  - Why it fails: A frequently penalised misconception in IB Psychology exams.
  - Correct: The diathesis only raises risk; without the environmental trigger it may never express — that is the entire point of the model.

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