Final answer:
Heredity affects the risk of developing schizophrenia through genetic vulnerability that interacts with environmental factors. High genetic risk increases the likelihood of schizophrenia, especially when combined with a disturbed family environment, as shown by Tienari's research.
Step-by-step explanation:
One way in which heredity affects the risk of developing schizophrenia is through genetic vulnerability. A study by Tienari and colleagues in 2004 indicated that adoptees with a high genetic risk for schizophrenia (biological mothers with schizophrenia) were more likely to develop the disorder when raised in a disturbed family environment.
This suggests that genetic factors may interact with environmental stressors to increase the likelihood of developing schizophrenia. This genotype-environment interaction theory supports the idea that both hereditary and external factors are necessary for schizophrenia to manifest, rather than heredity alone determining the outcome.