Definition: diathesis (dî-âth'ĭ-sĭs)-stress model: The idea that genetic vulnerabilities toward a disorder must be triggered by environmental stressors for the disorder to actually manifest.
People have this funny idea that if they inherit the genetics for a disorder, they're "doomed," as in doomed to get it. That's not actually true.
According to the diathesis-stress model, all genetic predispositions toward a disorder must be triggered by a stressor (or stressors) in the environment.
Schizophrenia, because it's so genetic, is often used as an example. If an identical twin develops schizophrenia, the other twin has about a 50% chance of developing it as well. While that's pretty high in terms of chances, it's still only 50%, because the disorder has to be triggered.