Recent years have seen a concerted effort to overthrow the longstanding scientific consensus that “male” and “female” represent two real, discrete biological categories in humans. The Oxford philosopher Amia Srinivasan, for instance, rejects the notion that biological sex is “natural,” “pre-political,” or “objective,” claiming instead that it is “a cultural thing posing as a natural one.” UC–Riverside’s Gender and Sexualities Chair, Brandon Andrew Robinson, openly claims that we “should stop teaching that sex is biological” because we “assign meaning to certain things . . . because of dominant gender ideologies.” In this view, categorizing people as male or female is not only biologically incorrect but also harmful and oppressive.

Early attempts at debunking the two-sex model sought to expand the number of sexes beyond two. Consider Brown University professor Emerita Anne Fausto-Sterling, who claimed in the 1990s that the “two-party sexual system” in humans was “in defiance of nature,” and that there were instead “at least five sex categories, and perhaps even more.” However, the additional “sexes” she proposed simply corresponded to various intersex conditions, not new sexes akin to the functional reproductive roles of producing either sperm or ova that define males and females universally across all taxa.

More modern attempts to debunk the binary nature of sex have moved away from trying to discover new sexes. Instead, they call for the elimination of sex categories entirely in favor of viewing sex as a continuous, though perhaps bimodal, spectrum consisting of many traits. Such ideas have found refuge in the pages of Nature and popular-science magazines like Scientific American.

Because the sex binary has been so thoroughly stigmatized as inherently “oppressive” and invalidating of transgender and “non-binary” identities and experiences—cardinal sins of our age—this effort has started an arms race among activist scientists to create the least binary model of sex imaginable. Since the “bimodal spectrum” concept still entails two of something, it must be abandoned. After all, the bimodal distribution of sex-related traits may problematically stem from an underlying property that remains fundamentally binary.

In pursuit of this goal, a “Multimodal Sex literature survey team” composed of researchers from UC–Berkeley and Loyola University Chicago has been assembled to “re-imagine a more inclusive framework for biological sex.” On January 27, 2023, the team produced their first pre-print paper, titled “Multimodal models of animal sex: breaking binaries leads to a better understanding of ecology and evolution.” The paper argues that sex is best viewed as “a constructed category operating at multiple biological levels” rather than bimodal or binary.

In saner times, such a paper would perhaps bring a small chuckle from a journal editor before issuing a swift rejection. But current times are far from sane, and the quick ascendance of fashionable pseudoscience in academia on the biology of sex is ample reason to worry that this paper will not receive the withering review it deserves.

Time and again, the paper argues with ideological opponents who simply don’t exist. Take the authors’ baseless but repeated claim that the binary model of sex necessarily requires that any and all measurable genetic, hormonal, morphological, or behavioral difference observed between the sexes must also be a strict binary (i.e., these traits will have no overlap):

‘Sex’ is often semantically flattened into a binary model, for which individuals are classified as either ‘female’ or ‘male.’ A more expansive definition of sex is bimodal—with most individuals falling within one of two peaks of a trait distribution. However, even a bimodal model is an oversimplification, since ‘sex’ comprises multiple traits, with variable distributions. Individuals may possess different combinations of chromosome type, gamete size, hormone level, morphology, and social roles, which do not always align in female- and male-specific ways or persist across an organism’s lifespan. Reliance on strict binary categories of sex fails to accurately capture the diverse and nuanced nature of sex.

The authors then quote renowned transgender biologist Joan Roughgarden, who similarly fails to grasp the meaning of the sex binary: “the biggest error in biology today is uncritically assuming that the gamete size binary implies a corresponding binary in body type, behavior, and life history.” I am not aware of any biologist having ever made this claim, but this (perhaps willful) misunderstanding leads the authors to believe that if they’re able to locate a sex-related difference that does not conform to an absolute binary, then they will have successfully refuted the claim that only two sexes exist.

When biologists speak of sex being “binary,” we mean something very straightforward. There exist only two sexes, which are fundamentally rooted in the binary classification between sperm and ova. Males have the function of producing small gametes (sperm), and females large gametes (ova). Other measurable sex differences beyond gametes (which include genetic differences, hormone levels, and average morphological and behavioral differences) are either a cause or consequence of this fundamentally binary and definitional distinction between males and females, and they need not be binary. Put simply, not all sex differences are differences of sex.

The authors then present three “case studies” that they claim demonstrate how the multiple “independent levels of sex” surveyed in the previous sections can be integrated “in a multimodal framework.” Every example intended to undermine the binarity of sex actually reinforces it. For instance, the first case study looks at several “sex role reversed” species that “defy ‘traditional’ expectations of social sex roles,” such as “male competition and female parental care.” But the simple fact that the authors are capable of identifying a system where females and males behave in ways that are not “traditional” demonstrates that being male or female is something entirely separate from possessing behavioral traits like competition or parental care.

The second case study claims to investigate “the evolutionary consequences of more than two sexes.” Yet the examples provided of species purported to have more than two sexes, such as white-throated sparrows and two species of Pogonomyrmex ants, show no such thing. The sparrow species simply has a chromosomal mutation (inversion) that produces two male and female color morphs (having a white stripe or a tan stripe). Because each color morph prefers to mate with the opposite color morph, chromosome 2 “behaves like” another sex chromosome. But having more than two sex chromosomes is not the same as having more than two sexes. While this species may offer insight into how sex chromosomes may have evolved, it doesn’t have “four sexes,” which would require four distinct gamete types. As for the ants, the author of the paper they cite describes the system as two ant species that “each comprise two distinct types of female and two distinct types of male.” But “two distinct types” of females and males does not equal four sexes; it equals two sexes—males and females.

The third and final case study looks at “intrasexual polymorphisms,” which describe differences observed within a sex. “Not all members of the same sex look and behave the same way,” the authors claim, as though this were a novel insight. However, they go on to claim, “Collapsing intrasexual polymorphisms into a female-male binary erases extensive multivariate phenotypic variation.” The very act of referring to these polymorphisms as “intrasexual” means that the sex binary remains.

The arguments presented throughout the paper derive from a fundamental misunderstanding of the universal defining property of all males and all females across all taxa: having the function of producing sperm or ova, respectively. That any individual scientist, lab, or “survey team” could claim to be expanding the boundary of our knowledge on a topic that they do not understand at its core is embarrassing.

The justification for their “multimodal animal sex” model is that a binary understanding of sex “fails to accurately capture the diverse and nuanced nature of sex.” But the fundamental binarity of sex serves as a central organizing principle that affords us a much deeper understanding into patterns of evolved sex differences in nature than would be possible by looking at individual traits in isolation. If scientists ignore the fundamental causes producing general patterns, we will sacrifice true understanding and turn science into mere stamp collecting.

Perhaps a more revealing motivation for the multimodal model comes when the authors discuss the alleged role that “binary language” is playing in fueling “legislation targeting [transgender and gender nonconforming] people.” “[B]iologists,” they claim, must “push back against misunderstandings of the biology of sexual phenotypes that enact harm on marginalized communities.” Further, “uncritically applying a simple binary . . . completely erases the biological realities of [transgender and gender nonconforming] and intersex people.” Binary language in biology must therefore be abandoned.

But a biologist’s job is to describe and explain the natural world as accurately as possible, not to protect or affirm the identities of “marginalized communities.” Scientists are being increasingly required to incorporate political initiatives into their research programs to stay competitive for grants and promotions. Biologists can accomplish this by purporting to debunk the oppressive sex binary. It should come as no surprise that forcing scientists to inject politics into their research comes at the cost of scientific rigor.

Photo by John Keeble/Getty Images


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