What is the key assumption of the diathesis-stress model?

1. A diathesis is an underlying predisposition for a disorder, and stress covers adverse environmental or psychological events. 2. The model integrates the two, and its key assumption is that both are necessary before a disorder develops. 3. The two factors trade off against each other: the stress needed is inversely proportional to the level of diathesis. 4. Because both are required, neither a predisposition on its own nor stress on its own is enough. 5. A diathesis need not be biological, since a pessimistic and self-defeating way of thinking also counts as one. _Source: OpenStax Psychology (1st ed., CC BY 4.0), Ch 15 "Psychological Disorders", section 15.3 Perspectives on Psychological Disorders_