The diathesis-stress model assumes that mental illnesses occur due to stressful conditions in the environment interacting with the biological and psychological characteristics of the individual. The model assumes that mental disorders can require a predisposition towards the disease, and it provides a general explanation for why individuals with a predisposition for a disorder, but who live in a healthy environment, may not develop the disorder, and why people who live in a stressful environment without a predisposition may not develop certain disorders.

The term “diathesis” is a medical term which refers to a predisposition towards a given state or disorder.