Both schizophrenia and bipolar disorder have a typical onset between adolescence and mid twenties. Males become ill earlier than females by around 2.5 years in schizophrenia; a similar gender difference is also present for the first episode of mania. A male excess has often been reported in schizophrenia while most studies of bipolar patients have reported no sex difference in incidence.
The families of bipolar patients tend to be of a higher socioeconomic status than the general population, although we suspect that this finding may be confounded by general intelligence. Some studies have found an association between social deprivation at birth and schizophrenia.
One of the environmental factors that differentiate best between the disorders is urban upbringing, which has strongly been associated with the risk of schizophrenia, but not bipolar disorder.9
Further, studies focusing on the effect of neighborhood variation in city areas have demonstrated much more variation in rates of schizophrenia.
Migrants are at increased risk for schizophrenia. A meta-analysis estimated the risk for bipolar disorder in migrants also to be increased10
but this conclusion was based on only 5 studies, and the excess was attributable to African Caribbean migrants to the United Kingdom. When the contribution of the latter group was removed, the risk of bipolar disorder was no longer significantly increased in migrants. Subsequent individual studies including those from our group have reported increased rates of bipolar disorder among black and other minority ethnic groups, however, risk remains stronger for schizophrenia.
Obstetric events have been frequently reported to increase risk of schizophrenia; such factors do not seem to predispose to bipolar disorder. An excess of winter/spring births has consistently been reported in schizophrenia. While studies have observed associations of winter birth with bipolar disorder, most have not replicated this.
Recently, rekindled interest in the effects of early childhood trauma on developing psychosis led to observations that child abuse is highly prevalent in both. Particularly, strong association was observed between childhood trauma and auditory hallucinations in both disorders.
Both disorders are associated with cigarette and illicit drug use, particularly cannabis use, but the evidence is much stronger for schizophrenia. Furthermore, cannabis use is associated with earlier age of onset in both disorders.
Advanced parental age at conception is commoner than expected in schizophrenia. The fact that autism is also associated with increased paternal age and that both it and schizophrenia show an excess of CNVs has raised the possibility that such mutations may arise during the repeated mitosis in the progenitor sperm cells as men age. However, contrary to this, new evidence suggests that this may be an effect of older age at marriage rather than older age of having children, which can not be explained by de novo mutations in paternal germ cells (see Petersen et al11
). Research concerning the relationship between paternal age and the risk of bipolar disorder is sparse.
Finally, individuals with schizophrenia have drastically reduced reproductive output. Bipolar disorder, however, is associated with normal reproductive success.