This study did not find a socioeconomic gradient of risk for schizophrenia, when using either parental education or parental occupation to index social class. Instead, it demonstrated a small increase in the incidence of schizophrenia only in offspring from the lowest social class, defined by fathers’ occupation at the time of birth. This increase in risk for schizophrenia, although statistically significant, was modest. It was independent of effects of other variables, such as paternal age and low birthweight. The increased risk for schizophrenia for the lowest social class was seen across sex, paternal age, and paternal country of origin.
Advantages of this study include (1) a large, population-based cohort of approximately 89,000 subjects; (2) a sufficiently-long follow up period (33 years after birth), with large numbers of cases (3); a validated method for the diagnosis of schizophrenia through a national registry of admissions to psychiatric facilities; (4) a well-characterized and well-defined method of identifying parental social class at birth; and (5) follow-up data required for the use of appropriate statistical methods (proportional hazards regression models), taking into account varying length of follow-up and control for covariates.
Our results accord with those of other cohort studies, which also do not find a socioeconomic gradient of risk for schizophrenia [19
], as measured by parental education, occupation, income or wealth. Our data are consistent with adversity or disadvantage instead conveying increased risk for schizophrenia. In a Swedish birth cohort study, hazard for schizophrenia increased with an accumulation of adverse social factors, such as unemployment, “unclassified” employment, receipt of social welfare and single-parent households [39
]. In this same study, paternal unemployment itself yielded an adjusted hazard ratio for schizophrenia of 1.40, similar to our finding for the lowest social class [39
]. In a Danish national registry study, parents of young people with schizophrenia were more likely to be unemployed or in the lowest quartile for income in the year prior to admission of their child [5
There are a number of potential mechanisms, not mutually exclusive, which could account for this effect of the lowest social class on risk for schizophrenia. A threshold effect of social class (vs. a gradient) on risk of schizophrenia in Israel (as in Scandinavia) may reflect a different social structure in Israel [7
] than in the United Kingdom [31
]. Alternatively, as studies of socioeconomic gradients implicate the current stress of hierarchies on medical health [31
], such graded hierarchies may not be relevant to schizophrenia. One potential mechanism for increased risk of schizophrenia among the lowest social classes, as described above, is adversity, which can increase maternal stress with potential effects on the fetus [20
] and/or increase liability for later behaviors in offspring such as drug use, which may increase risk for schizophrenia [35
]. A second possibility is that members of the lowest social class had less access to medical resources. However, in Jerusalem during this time, there was universal access to prenatal care, which was free of charge. Although the poorest group had a slight excess in low birthweight babies, this did not account for the increased risk of schizophrenia in offspring from this disadvantaged group. A third alternative is that members of the lowest social class had increased early exposures to toxins, infections, or nutritional difficulties, which have been implicated in increasing schizophrenia risk [3
]. Exposures to these environmental risk factors in this cohort are not known. Finally, it is possible that parental psychopathology could account for both low social class and increased risk for schizophrenia in offspring; genetic transmission of social selection cannot be ruled out. A limitation of the current study is that parental psychopathology is not known; however, in other cohort studies already described, the effect of adversity on schizophrenia risk in offspring persisted even when adjusting for parental psychopathology [5
In this study, social class was ascertained only at birth, and hence social class over the life course is not known. Another limitation is the lack of data on parental wealth and income, which were unrelated to risk for schizophrenia in other studies [5
]. However, it is unlikely that the pattern of association would be different with multiple measures of social class over time through childhood. An additional limitation is that occupations were ascertained from birth certificates only, which increases the risk for intracategory variability. Further, among immigrants, who comprise a segment of the parents of the cohort, the nature of education and its relation to employment may vary with the country of origin. Other limitations, as described, include a lack of data on environmental exposures and parental psychopathology.
The role of social class in schizophrenia has been considered for more than a century, with debate as to the causal direction and the nature of their relationship. Accumulating evidence from several large population cohort studies, including the one presented here, now suggests that the association of social class with schizophrenia is distinct from that of other diseases and health outcomes. These studies provide no evidence of a socioeconomic gradient of risk. However, a modest increase in risk for schizophrenia is apparent for those at the bottom of the social ladder.