Social class as risk factor for schizophrenia
There was no overall significant difference between cases and controls in the distribution of social class (Wilcoxon rank sum z=−1.3912, P=0.16). Within the 352 matched pairs, the patients and controls came from the same social class in 77 pairs; patients were from a higher social class in 149 pairs and from a lower social class in 126 pairs. The odds ratio of developing the disorder associated with social class (low v high) was 0.59 (95% confidence interval 0.40 to 0.85, P<0.0029), indicating that people from low social classes have a reduced risk of schizophrenia compared with those from high social classes.
Table shows that the risk of developing schizophrenia varies across the six social classes of origin. There was no consistent trend in odds ratios across the social classes, although people in lower classes tended to have a lower risk than those in higher classes. However, the likelihood ratio statistic from conditional logistic regression was 11.39 (df=5, P=0.044), indicating a significant overall difference in risk across social classes.
Risk of developing schizophrenia according to social class at birth
Relation between social class and age at first presentation
There was an overall significant difference in the age at first contact among the six social classes (F=2.95, df=5, 345; P=0.01). The mean age at first contact with psychiatric services was 30.1 years in the whole sample (men 28.6 years; women 32.2 years). Patients in social class I were youngest at first contact (25.3 years) and those in social class VI the oldest (34.4 years). Similarly, there was an overall significant difference in the age at first ever admission (F=2.55, df=5, 345; P=0.03). The mean age at first ever admission to a psychiatric hospital was 29.6 years (men 28.2 years; women 31.6 years). Patients in social class I were youngest at first ever admission (24.8 years) and those in social class VI oldest (33.1 years). We also found a significant difference in the age at first admission with schizophrenia (F=3.92, df=5, 345; P=0.002); the mean age at first admission with schizophrenia was 33.3 years (men 31.4 years; women 36.0 years), with patients in social class I being the youngest (28.0 years) and those in social class VI the oldest (38.8 years).
We found no evidence that sex modified the effect in any of the above measures (F=1.22, df=5, 340; P=0.30 for age at first contact; F=1.51, df=5, 340; P=0.19 for age at first admission; and F=1.84, df=5, 340; P=0.10 for age at first admission with schizophrenia), which indicates that age differences between the two sexes are identical across social classes. Sex distribution did not differ significantly across the social classes (χ2=7.50, df=5, P=0.19). Thus, sex is unlikely to confound the results. Nevertheless, we controlled for sex in the estimates of differences in age of contact, admission, and diagnosis between patients from different social classes.
Table shows that men tended to be younger than women from the same social class background at time of first contact with psychiatric services, first admission, and diagnosis of schizophrenia. Analysis of variance showed a significant main effect of sex for age at first contact (F=5.87, df=1, 345; P=0.016), age at first ever admission (F=5.41, df=1, 345; P=0.021), and age at first admission with schizophrenia (F=9.30, df=1, 345; P=0.002).
Age at first contact with psychiatric services, first admission, and first diagnosis of schizophrenia by social class