Sex inclusion or “sex as a biological variable” (SABV) mandates have recently been instituted by many funders, journals, and pharmaceutical companies, responding to calls for biomedical research to address a history of androcentrism in preclinical science and accelerate the study of health disparities between women and men (White 2021; Woitowich 2020; Haverfield 2021). These policies, developed with the aim of advancing women’s health and increasing rigor and replicability in preclinical science (Clayton and Collins 2014; Klein 2015; Heidari 2016), ask preclinical model organism researchers to include “both” sexes in research designs and to report and analyze disaggregated, sex-specific results for males and females.

The US National Institutes of Health (NIH) Office of Research on Women’s Health defines sex in the following way: “Sex is a multidimensional biological construct based on anatomy, physiology, genetics, and hormones. (These components are sometimes referred to together as ‘sex traits.’) All animals (including humans) have a sex. As is common across health research communities, NIH usually categorizes sex as male or female, although variations do occur” (“What are Sex & Gender?” 2024, np). The NIH style guide advises researchers that, “Sex is a biological descriptor based on reproductive, hormonal, anatomical, and genetic characteristics. Typical sex categories include male, female, and intersex. Sex is used when describing anatomical, gonadal, chromosomal, hormonal, cellular, and basic biological phenomena” (“NIH Style Guide” 2024, np).

Sex inclusion research policies like that of the NIH seek to render features of male and female biology retrievable across experimental designs and research programs by including and comparing male and female organisms. This operationalization of sex conceptualizes maleness and femaleness as essential kinds about which strong generalizations can be made through the comparison of males and females across contexts, taxa, and levels of biological analysis. Critics (e.g., Richardson et al. 2015; Ritz 2016; Garcia-Sifuentes and Maney 2021; Maney and Rich-Edwards 2023; Massa, Aghi, and Hill 2023; Pape et al. 2024) warn that mandating this binary sex essentialist approach to the study of sex-related variation across research programs ignores the pragmatics of scientific practice, disregards the complexity, context-specificity, and contingency of sex-related variables, and will lead to a proliferation of low quality, uninterpretable, and potentially politically and socially harmful claims about male and female differences.

This essay extends these critiques through an analysis of a current scientific debate over whether researchers need to test for estrous effects when designing, analyzing, and interpreting sex-split studies using rodent models. Analysis of this debate reveals what I term a “riddle of variability” for sex difference research—to search for differences produced by sex, laboratory animal model researchers must partition some forms of variation as non-sex-related and assert strong generalizing claims about homogeneity between males and females as research subjects. These judgments about what counts as sex systematically background many sources of sex-associated variation, resulting in a context-specific construct of sex.

The debate over whether it is necessary to account for estrous-related variation in sex difference research designs is a valuable site for philosophical analysis of scientific operationalizations of sex. Specifically, it supports the contention that sex is a pragmatic, contextual construct (Richardson 2022) in lab-based experimental science and demonstrates the value of a sex contextualist framework for guiding appropriate interpretations of the findings of biological research on sex-related variation.

Researchers’ negotiations over whether to test for estrous cycle effects when designing, analyzing, and interpreting sex-split studies reveal how sex is plurally, partially, and pragmatically operationalized in laboratory model organism research design. To meet mandates for sex-inclusive rodent research design, the researcher is faced with the question of whether to define estrous cycle as a component of the variable of sex (which therefore requires measurement in order to account for sex as a biological variable) or as a trait contributing to interindividual variability that is not, in the words of researchers, “sex itself” or “intrinsic sex.”

Levy et al., Prendergast et al., and others (e.g., Mogil and Chanda 2005; Becker, Prendergast, and Liang 2016; Dayton et al. 2016) argue that compliance with sex inclusion mandates can be accomplished without requiring research protocols to control for potential sex differences in variability. That is, they conclude from reports of overall similarity in variability in males and females that it is unnecessary to track vaginal cytology at each of the four stages of the rodent estrous cycle when including females in biomedical research. Accepting this premise of male-female homogeneity of “intrinsic variability” (Prendergast, Onishi, and Zucker 2014), however, requires a scaffold of assumptions about sex differences, similarities, and variability, and how to measure them, that involve the contextual judgments of researchers. Specifically, these researchers assert that since reviews show that for many traits, males are just as or more variable than females, no special controls are required for the study of female rodents due to the estrous cycle, nor, for that matter for other potential sources of sex-specific variability, whether male or female (Shansky 2019), when conducting male female-comparisons. For these scientists, studies showing similar within-group variability across males and females support the ease and practicality, without the need for more organisms, time, and resources, of adhering to mandates to include both males and females in biomedical research and report and analyze sex disaggregated results.

The constitution of sex subgroups (male and female) as representing “sex as a biological variable” and as yielding knowledge about sex, even without measuring sex-related biological variation such as gonadal hormone state, is a stark and striking departure from early arguments for sex inclusion mandates. Evidence that estrous cycle-related hormonal states produce female-specific biological variability, advocates argued at the time, showed the need for thoroughgoing study of females as well as males in all research studies (e.g., Beery and Zucker 2011). A touchstone article arguing for the NIH SABV mandate by neuroscientist and policy architect Larry Cahill, “Why Sex Matters for Neuroscience,” for instance, prominently featured evidence that estrous cycle state produces behavioral effects of large magnitude as an example of “copious sex influences on brain anatomy, chemistry, and function” (Cahill 2006, 477). Similarly, Jill Becker, another SABV policy architect and current editor of the journal Biology of Sex Differences, has long argued for the importance of measuring ovarian steroids in study designs that include females (e.g., Becker et al. 2005). Today, however, her position has evolved. Explicitly recognizing that how one conceptualizes the variable of sex may depend on pragmatics and the research question being asked, she now advises that, “for those investigators initiating research on female rats, power calculations based on data from males would likely be sufficient to determine the number of female subjects needed in order to see a sex difference… [I]nclusion of intact females, without regard to estrous cycle, and intact males is a valid approach to learn about females in neuroscience research” (Becker, Prendergast, and Liang 2016, 7).

The operationalization of “sex as a biological variable” in the diverse literature produced in the estrous cycle variability debate yields a concrete example of how sex, in preclinical laboratory practice, is a pragmatic, plural, and contextual construct. A sex contextualist analysis helps answer the question of why researchers deem some forms of sex-specific variation to be non-sex-related. Specifically, a contextualist framework makes clear how pragmatic constraints in the laboratory and in research policy administration inform a particular operationalization of male and female sex in the laboratory. Researchers as well as policy makers supporting sex inclusion research mandates are interested in what is required to allow researchers to easily incorporate the requirement into practice and what is sufficient to merely detect a difference between male and female subgroups. The presence of a policy mandate makes these pragmatic, contextual choices uniquely salient in this case. Fundamentally, advocates of sex inclusion policies in preclinical laboratory science are committed to the importance of the sex subgroup as a category of analysis—or, as Becker et al. put it in the passage quoted above, to research designs adequate “to see a sex difference” (2016, 7). The outcome of the process is the production of simple comparative research designs that represent sex as a discrete, binary biological variable sufficiently measured by comparing means between male and female subgroups.

If we understand operationalizations of sex in preclinical research as always involving contextual judgments (Richardson 2022), both choices—to exclude or include estrous cycle measures in sex-difference research design—are examples of sex contextualism in practice. Conceptualizing the estrous cycle as a sex-related biological variable that must be measured as a part of any sex comparison is a pragmatic contextual judgment that centers gonadal variation as a particularly salient hypothesized sex-related mechanism. In contrast, comparing male and female subgroups without measuring such estrous cycle-related variability is a contextual judgment that favors utilizing sex category as a proxy for sex. Sex contextualism helps us to see that these two constructs of sex are not the same and that plural approaches to operationalizing sex as a biological variable co-exist, even within a single model system and scientific discipline. In this case study, we see this in the radically divergent constructs of sex arrived at by Levy et al., Prendergast et al., and others, as compared to Zajitschek et al., Rocks et al., and others. Recognizing this pluralism is vital to evaluating inferences, generalizations, and mechanistic hypotheses resulting from any claims of sex difference that emerge from preclinical research.

A sex contextualist approach does not tell us whether one way of constructing sex as a biological variable is more objectively true, sound, or empirically adequate than another, but it does make visible the contextual dimensions of the operationalization of sex in laboratory models, opening them to scrutiny. For example, the Levy et al. study found that 85% of the time, individual mouse identity among females predicted their behavioral repertoire better than any variable including sex; furthermore, Levy et al. recorded high intra- and inter-individual spontaneous exploratory behavior for both males and females (both were slightly higher for males). As coauthor Robert Datta explained to a reporter: “If you give me any random video from our pile, I can tell you which mouse it is. That’s how individualized the pattern of behavior is” (Pecheva 2023, np). The news piece continues: “we find that each female mouse exhibits a characteristic pattern of exploration that uniquely identifies it as an individual,” and the same is true for males; these results are characterized as “surprising” (Pecheva 2023, np). Alternative approaches to the operationalization of sex-related variation that use other approaches than sex category, and offer a different vision of what sex-inclusive science that advances gender justice might look like (Smiley et al. 2024; Massa, Aghi, and Hill 2023), might allow a different interpretation of these data. In contrast to Levy et al.’s strong conclusions about the need for more sex comparisons, perhaps the highly individualized behavioral repertoires identified in this study present a challenge to binary sex as an important and meaningful category to retain for analysis in future work on mouse exploratory behavior.

The debate over measuring variability in preclinical sex difference science shows that operationalizing “sex as a biological variable” in rodent research requires scientists to make contextual judgements at multiple decision points, each of which becomes a new source of potential incoherence. What results from laboratory-based experimental designs that include male and female model organisms is a riddle—not a simple finding of a difference, but a difference that rests on establishing or assuming other axes of similarity. That is, on the one hand, it is argued that scientists must study female and male biology in preclinical laboratory experimental research in order to capture differences that could be relevant to advancing the health of human women. But operationalized as a binary sex comparison within a single experimental design, studying sex as a biological variable requires, at the same time, the assertion that females are similar enough to males to be included in the same study design.

The paradoxical construction of the estrous cycle as both intrinsic to femaleness and extrinsic to sex itself is present throughout the sex difference literature. This convention of distinguishing sex itself from estrous state is iterated in the title of the Y. Zhao et al. 2018 study previously discussed: “No effect of sex and estrous cycle on the fear potentiated startle response in rats.” Levy et al. (2023) assert that estrous cycle is “a defining feature of the female internal state” (1358), but then proceed to argue that accounting for estrous state is unnecessary for the consideration of sex differences. Similarly, Mogil and Chanda’s (2005) review of nociceptive sensitivity on mouse tail withdrawal tests found that “sex itself accounted for a rather modest percentage of the overall variance” (2), where “sex itself” refers solely to whether the individual was a female or male and explicitly does not include estrous cycle hormonal profile. As seen above, even Rocks et al. slip into this riddle, describing estrous state as “sex-specific factor that is more precise than sex” (11).

This riddle of variability—of which kinds of biological variability constitute sex and should be the focus of experimental laboratory model organism research that includes and compares male and female subgroups, in the face of multiple dimensions of difference, similarity, and variability—reveals that sex does not simply “fall out” of comparisons of males and females as a coherent biological variable that causes differences between individuals or subgroups. It is operationalized through a series of assumptions about what “intrinsic sex” is, materially and statistically realized in a study design and informed by research contexts, including policy, in a sea of uncertainty produced by a dynamic background of many sources of variation, some of which mediate or interact with the thing that is called sex. The issue here for the researcher interested in sex differences is how to operationalize the variable of sex distinct from entanglements and contingencies across varied contexts, in order to get at the “true” effects of sex. Writing in the journal Biology of Sex Differences, Smarr and Kriegsfeld (2022) use the metaphors of “white noise in a signal” (2) and “Brownian motion offering precise location measurements for particles” (2) to describe this challenge of locating sex amidst the dynamic complexity and uncertainty of many sources of variability.

The riddle of variability is likely to be persistent, requiring resolution each time a sex-split study design (whether binary or nonbinary) is contemplated. Notably, the estrous cycle debate is just one example of the riddle of variability in sex difference science in laboratory model organisms. Laboratory housing is a large mediator of variability for rodents, representing an interaction between an entirely artifactual laboratory variable and the social-behavioral repertoire of these species. For example, Prendergast et al.’s review of articles that reported housing conditions found that single-housed mice exhibited 37% lower variability when compared to group-housed (Prendergast, Onishi, and Zucker 2014, 1). Similar to estrous cycle, there may be sex-specific dimensions to the structure of variability generated by different housing conditions. It is well known, for instance, that male rodents housed together in a group exhibit much higher levels of fighting behavior than females housed in groups, though such potential sources of variability in males receive far less attention than variables such as the estrous cycle in females.

Like estrous and group housing, average body size often systematically differs between male and female rodents by a ratio as high as 2:1. Body size/weight influences behavioral repertoire and health measures; thus, without adjustments for body size when conducting sex comparisons, a researcher could erroneously attribute to sex a difference that is better explained simply by differences in size. This is well recognized in the literature. For example, the Y. Zhao et al. (2018) study of sex differences in startle response discussed above did not adjust for body weight, but noting that “it cannot be fully excluded that the sex differences in overall startle response was related to differences in body weight between males and females” (31). A 2022 review in the journal Nature Communications on “the unresolved issue of whether sex differences in phenotypic traits in mice may be explained by sex differences in body weight” (Wilson et al. 2022, 1) concluded that body size explains a large amount of variation in rodent behavior, either singly or in interaction with other variables. Dayton et al. (2016), looking at sex-specific variability in rat phenotypes, found that “the phenotype of body weight exhibited a significantly different variance between males and females,” which “corresponded to an increase in magnitude of the measured trait, that is, male rats were larger than female” (Dayton et al. 2016, 1141).¹ In short, if male and female rodents are, for example, different sizes, caged differently, and experience different regimes of hormonal cycling, multiple forms of contextual and contingent variation may need to be controlled for to make valid comparisons between males and females as groups and to reason about which part of any difference between these groups can be attributed to “sex.”

Here, sex contextualism offers some modest normative guidance. In the face of multiply-entangled forms of variability, similarity, and difference, researchers can make visible the constraints and decision points that go into operationalizing the construct of “sex as a biological variable.” Specifically, sex contextualism invites researchers to generate thick characterizations of how sex, in a particular study, is “pragmatically constituted in an observational frame,” rather than presenting sex-related findings as unmediated read-outs of “sex itself” (Richardson 2022, 10). Researchers can make context visible by “contextually defin[ing] sex-related biological variables” (10), “justify[ing] their choices in how they operationalize sex” (10), and appropriately characterizing sex-related variables as producing “variation between and within sex-defined classes in a context-sensitive manner” (Richardson 2022, Table 1, 12). In this way, contextualist frameworks that rigorously specify the partiality, contingency, and artifactuality of particular operationalizations of sex in laboratory model organism research not only better describe research practices, but also offer an ameliorative approach in the face of the risks of sex essentialism, clarifying the strengths, weaknesses, and generalizability of inferences made about sex differences from these studies.