Explain the stress-diathesis model of mental illness?
The diathesis-stress model of mental illness states that someone must have both a biological vulnerability and a stressful situation to develop a mental illness.
If someone is only biologically predisposed to a condition, e.g. schizophrenia, they will not get it just because they are predisposed.
If two schizophrenic people have a child, the child has a high susceptibility to schizophrenia, because there is at least a partial genetic basis for the disorder. However, it is not necessarily true that the child will develop schizophrenia, because they may not encounter any environmental stressors or stressful situations.
Similarly if someone is not predisposed to a condition but encounters a stressful situation, their response will not be to develop schizophrenia or depression, because they are not so inclined genetically or biologically.
You must have both vulnerability and stress to develop a disorder.