Mental disorders can be caused by biological, psychological, and environmental factors. The diathesis–stress model suggests that a person may be predisposed for a psychological disorder that remains unexpressed until triggered by stress. Suppose that two identical twins (with the same genetic profile) grow up in the same household (sharing the same parents, the same basic diet, the same access to television, and so on). As a teenager, one twin but not the other develops a mental disorder such as schizophrenia. How could this be?

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