Where have all the women gone?: Participant gender in epidemiological and non-epidemiological research of schizophrenia
Introduction
Recent epidemiological literature has shown unequal incidence of schizophrenia for men and women. Multiple meta-analyses found that approximately 60% of those who develop schizophrenia are men (Aleman et al., 2003, McGrath et al., 2008, McGrath et al., 2004). This is not clearly the result of a systematic bias in diagnostic systems. Rather, it appears that more males get ill. Younger male onset is one of the explanations evoked for higher male incidence (Häfner et al., 1989). In earlier studies that relied on the third edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III; American Psychiatric Association, 1980), this could certainly be true since women who developed schizophrenia after age 45 were excluded and thus incidence – the number of new cases per year – of schizophrenia was biased towards men. Yet the recent meta-analyses used the DSM-III-R or later editions and the comparable International Statistical Classification of Diseases and Related Health (ICD) (World Health Organization, 1992), which removed the age “cutoff,” and the gender imbalance endured.
However, this disparity seems to be exaggerated in the non-epidemiological research literature (e.g., concerned with features of the illness, treatment efficacy, cognitive processes, and neuroimaging), especially in recent decades. In non-epidemiological studies published from 1974 to 1976 in the Archives of General Psychiatry, Journal of Abnormal Psychology, and Journal of Nervous and Mental Disease, − 59% of participants with schizophrenia were male (Wahl, 1977). However, in a later review of studies published in similar journals from 1985 to 1989, twice as many male as female patients were research participants (Wahl and Hunter, 1992), greatly surpassing the rates predicted by contemporary epidemiological research. Thus, although epidemiological and non-epidemiological researches show greater proportions of men than women, this disproportion has become more pronounced in non-epidemiological research. Possible explanations for an imbalance of male and female schizophrenia patients in the non-epidemiological research literature include greater numbers of men among chronic patients (Häfner, 2003, Lewine et al., 1984), gender proportions differing by country (Bhatia et al., 2006, Jablensky et al., 1992), differential mortality (Phillips et al., 2004), and effects of research agenda. To update this research and to investigate possible moderators of the effect, we analyzed the numbers of men and women with schizophrenia appearing in various research studies. Furthermore, as previous publications considered epidemiological and non-epidemiological research independently, we tested for statistical differences between these two literatures. We hypothesized that there would be fewer females in non-epidemiological schizophrenia research than expected from incidence rates. If true, this would suggest that the non-epidemiological literature is weighted toward male patients and may not generalize fully to female patients.
Section snippets
Epidemiological comparative value
As our baseline for comparison, we chose the incidence rate of 1.4 male schizophrenia patients to every female patient, or 58% males, taken from the most recent and complete schizophrenia meta-analysis (McGrath et al., 2008). We used this incidence value rather than estimated prevalence (i.e., 1.1 or 52% male [McGrath et al., 2008]) because it is a more consistent epidemiological measure (Babigian, 1980), supported by a more extensive body of literature, and because it was a more conservative
Non-epidemiological research literature
The 220 articles from the non-epidemiological research literature included 147,725 males and 104,853 females. Thus, the non-epidemiological research literature is 66% male and 34% female [male proportion = M = 0.66, SD = 0.12, N = 220], translating to an average proportion greater than the epidemiological baseline. We analyzed articles from Schizophrenia Bulletin and Schizophrenia Research separately from the other five journals in order to rule out the possibility that gender ratios in articles
Discussion
Our analysis revealed a widespread mismatch between the incidence of schizophrenia in females and their participation in research studies. While the expected epidemiological incidence rate was 58% males and 42% females, non-epidemiological study participant groups consistently included a higher proportion of males, averaging 66% of study samples. Additional points emerge from the examination of moderators of this effect. There was some variability in gender proportions as a function of study
Conclusions
In an effort to understand the gender discrepancy, we separately examined a number of broad and obvious differences across studies that might moderate the effect, including journal of publication, study type, sample, institution, continent, and diagnostic criteria. With limited exceptions, a difference in the number of male and female participants appears across the variables. These dimensions are overlapping and are not offered as independent tests of the discrepancy. Rather, as a whole, they
Role of funding source
This research was supported by the National Institute of Mental Health Intramural Research Program (NIMH IRP). The National Institute of Mental Health Institutional Research Program ethics committee and the National Institute of Mental Health Office of Science Policy, Planning, and Communications (OSPPC NIMH) have reviewed and approved this manuscript.
Contributors
JL contributed to study design and coordination, data collection and coding, and statistical analysis and drafted the paper. JGe acquired and coded the data. DD and JM were involved in the statistical analyses and interpretation. BE contributed to the statistical analyses and interpretation and draft of the paper. DW acquired the project funding and contributed to the draft. JGo was senior author, initiated the project, contributed to the study design and coordination, statistical analysis and
Conflict of interest
James Gold receives royalty payments from the BACS and has consulted for Pfizer, Astra Zenaca, and Glaxo Smith Kline. All other authors declare that they do not have any conflicts of interest. All authors had access to the data and played a role in writing this manuscript.
Acknowledgements
Thank you to the IRTAs in our Neuropsych lab for their thoughtful contributions to the development of this paper.
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