Why does the same hard event lead to a disorder in one person but not another? The diathesis-stress model says a disorder appears when a built-in vulnerability (the diathesis) meets enough stress. Protective factors, such as support and coping skills, raise the bar. Try it with one person, then with a whole crowd.
One person
Blue: vulnerability · Red: stress · Green line: the threshold where symptoms are likely to appear.
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For example: genes, problems before birth, a hard early childhood
For example: a trauma, a loss, money trouble, heavy cannabis use in the teen years
For example: caring people, coping skills, good sleep, early treatment
A teaching model. The numbers here are made up to show an idea. This is not a risk test, and it cannot tell anyone whether they have, or will get, a disorder. Only a trained clinician can assess that.
Genes are not destiny. For schizophrenia, if one identical twin has it, the other has roughly a 33 to 50% chance, depending on the study. So at least half of these twin pairs do not share the diagnosis. Vulnerability loads the dice. Stress and protection help decide the roll.