Apart from heredity, there are other mechanisms by which genes can constitute risk factors for schizophrenia. For example, advanced paternal age may be a risk factor for schizophrenia. There is an increased risk among children (especially females) of older fathers perhaps through X-lined de novo mutations. Pseudoautosomal abnormality has also been considered.
In sum, while genetic risk factors play an important role in the etiology of schizophrenia and can be responsible for disruptions of fetal brain development, the potential contribution of environmental factors is important. For example, in a landmark MRI study of monozygotic (MZ) twins, nearly every affected twin, compared with his or her unaffected cotwin, showed structural brain abnormalities, including enlarged ventricles and diminished amygdala–hippocampus volume. Given that MZ twins share 100% of their genes, these findings suggest that the differences in brain morphology may be attributable to environmental factors. We now go one to examine these.