The diathesis-stress model proposes that a psychological disorder or behaviour may develop when an underlying vulnerability interacts with sufficient environmental stress. Neither the vulnerability nor the stressor necessarily produces the outcome independently.
A diathesis is a predisposition or vulnerability. It may be biological, such as genetic variation or atypical neurochemistry, but it can also involve cognitive or psychological vulnerability. Stress refers to environmental pressures, such as trauma, chronic adversity, bereavement, or stressful life events.
The model is interactionist because the effect of one factor depends on the presence of another:
| Diathesis | Environmental stress | Predicted outcome |
|---|---|---|
| Low vulnerability | Low stress | Disorder is relatively unlikely |
| High vulnerability | Low stress | Vulnerability may remain unexpressed |
| Low vulnerability | High stress | The person may remain resilient |
| High vulnerability | High stress | Risk of developing the disorder increases |
Caspi et al. (2003) provide relevant evidence for this model. The researchers investigated variation in the serotonin transporter gene, stressful life events, and symptoms of depression in a longitudinal sample from New Zealand. Individuals with one or two short alleles of the 5-HTT gene showed more depressive symptoms following stressful life events than individuals with two long alleles.
This supports a gene-environment interaction: genetic variation appeared to moderate the effect of environmental stress. However, the findings do not show that a single gene causes depression. The study was largely correlational, depression has multiple possible causes, and later replication findings have been mixed.
A common misconception is that the diathesis-stress model is purely biological. It actually combines vulnerability and environmental experience, challenging both strict biological determinism and explanations based only on life events.
Exam technique: Define both components, explain their interaction, and use Caspi et al. as evidence. For an essay, evaluate the correlational evidence, mixed replication, and the model's value as a less reductionist explanation of behaviour.