Final answer:
Individuals with a sibling or parent with schizophrenia have increased chances of developing the disorder, highly influenced by both genetic factors and the quality of their environment, with percentages varying across different scenarios.
Step-by-step explanation:
People who have a sibling or parent with schizophrenia have varying chances of developing the disorder based on genetic and environmental factors. A study by Tienari and colleagues indicates that of adoptees with high genetic risk (biological mothers with schizophrenia) in disturbed family environments, approximately 36.8% were likely to develop schizophrenia. However, those with high genetic risk in healthy family environments showed a significantly reduced likelihood at 5.8%. Conversely, adoptees with low genetic risk in both disturbed and healthy family environments had about a 5.3% and 4.8% chance, respectively, to develop schizophrenia. This suggests that both genetic vulnerability and environmental stress are influential in the development of schizophrenia, with environmental factors playing a crucial role in mitigating or exacerbating the risk associated with genetics.