Where have all the women gone?: Participant gender in epidemiological and non-epidemiological research of schizophrenia

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Abstract

Though archival literature states that schizophrenia occurs equally in males and in females, recent epidemiological studies report higher incidence of schizophrenia in men than in women. Moreover, there is longstanding evidence that women may be under-represented in non-epidemiological research literature. Our first goal was to quantify gender ratios in non-epidemiological research published in 2006. Secondly, we sought to investigate which factors contribute to high numbers of men in research studies. Our final goal was to compare gender ratios in non-epidemiological schizophrenia research to reported incidence rates. In a recent meta-analysis of incidence, there were 1.4 males for each female with schizophrenia. In non-epidemiological studies of the schizophrenia patients, there was an average of 1.94 men for every woman. Although the degree to which men outnumbered women varied according to study type and region of study, research studies included more men than women across all investigated variables. Either the incidence rates are higher for men than has previously been reported or women are less visible in research settings than in the greater community. Importantly, the discrepancy between gender ratios in epidemiological and non-epidemiological research is consistent. However, specific, identifiable factors are present when male participants are greatest, suggesting that many research environments yield a higher number of men. Thus much of our understanding of the illness and its treatment is based on research conducted disproportionately with men.

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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