General discussion 281 studies often only included males in their study sample, did not report the ratio of males to females in their sample, or did not report whether the found associations differed between males and females in the sample. Furthermore, several studies only included females using oral contraceptives, which has implications for the generalizability of the conclusions to naturally cycling groups. Most studies did not report whether they asked their participants' sex (e.g. biological traits, such as genetic sex or gonadal status) or gender (e.g. gender identity or gender role). This is a growing problem since there is an increasing group of persons whose sex assigned at birth does not match their gender identity. To counter the historical lack of inclusion of females in research, funding agencies have worked on mandates to include both sexes in research: the National Institute of Health mandated the inclusion of females in studies in 1993 in the NIH Revitalization Act (Mastroianni et al., 1994), and the European Commission has worked on the integration of sex and gender in studies since 2002 (White et al., 2021). Effects of these initiatives are showing: in 2009, 38% of the high-impact studies in neuroscience and psychiatry included study subjects of both sexes, whereas in 2019, 68% of studies included subjects of both sexes. However, of all studies in 2019, 27% still used male-only samples and 6% used female-only samples, and maleonly samples were regularly used for mechanisms or diseases that are equally relevant for females (Rechlin et al., 2022). Although the number of studies that include both males and females is increasing, few studies in psychiatry and neuroscience also actually address the role of sex in their research questions, study design, or study results. This lack of accounting for sex in health research can have real-life consequences for public health: women are nearly twice as likely to report side effects of medication use (Tharpe, 2011; Zucker & Prendergast, 2020), possibly due to exposure to relatively higher drug dosages, and they are more likely to be hospitalized due to side effects of medications (Rodenburg et al., 2011). A growing number of studies adjusts for sex in statistical analysis, but the incorporation of sex in analyses is often done by addition of a covariate for sex instead of explicitly addressing the role of sex as a discovery variable (e.g. testing as main effect, stratified analyses per sex, addition of sex as
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