The Sex Binary vs. Sexual Dimorphism

These related concepts must never be conflated.

Reality’s Last Stand is a reader-supported publication. Please consider becoming a paying subscriber or making a one-time or recurring donation to show your support.

A scientist’s job is very straightforward: describe the natural world as clearly and accurately as possible. This is done through making careful observations, identifying important questions the answers to which would further our understanding, constructing testable hypotheses to answer those questions, and conducting properly controlled experiments.

Because scientists are human, which is to say fallible, it is impossible to completely eliminate bias from the scientific process. Until recently, most scientists agreed that bias was bad and should be eliminated or reduced as much as possible. This is why many safeguards have been implemented to eliminate or reduce bias, such as double-blind peer review, conflict of interest disclosures, and study pre-registration. But there has been a recent push within the sciences insisting that new bias needs to be introduced to counter biases they assert are currently skewing scientific research to the detriment of underrepresented minority groups.

Nowhere is this newly introduced bias more rampant than on the topic of biological sex. This is because it is widely believed among activists that binaries are not only unnatural, but inherently bad. Nature, they claim, is inherently “queer,” meaning that rigid categories are more likely to be relics of human bias rather than naturally occurring phenomena. Binaries are viewed as de facto evidence that privileged and powerful groups have simply constructed them to either maintain power or obtain more of it. Since binaries are viewed as tools of oppression, activists have put the sex binary directly in their crosshairs.

More and more papers are coming out purporting to have debunked the antiquated notion of the sex binary in favor of new “spectrum” or “multimodal” models of sex. Because “scholarship” on this topic is rooted primarily in passionate political activism instead of the dispassionate pursuit of truth, academic rigor has been thrown out the window and peer review has become little more than a cabal of ideological gatekeepers.

The most common error made by activist scientists to debunk the binary nature of sex should be immediately obvious to any half-competent biologist: they conflate two very distinct concepts—the sex binary and sexual dimorphism.

The sex binary refers to the two gamete types—sperm and ova—that serve as the universal defining features of the two sexes. Males have the function of producing sperm, and females, ova. Sexual dimorphism, in contrast, refers to the differences in size, shape, color, behavior, and other physical and behavioral characteristics between males and females of the same species.

While these two things are related, they must never be conflated. However, some prominent figures appear to be confused about their distinction.

Alice Dreger, a historian of science and author who is most known for her work on the history of medicine’s relationship with intersex conditions, flatly stated on Twitter that human beings “are not sexually dimorphic.”

In another tweet on the same day, Dreger said we need to “Stop insisting that sex is simply dimorphic in humans because these realities make you uncomfortable.”

I have never heard the term “simply dimorphic” before, but she appears to believe the word “dimorphic” implies or requires being totally non-overlapping with respect to some trait(s).

This confusion is becoming widespread.

Several weeks ago I dismantled a recent pre-print that argued sex in animals was “multimodal” instead of binary or bimodal. The evidence they repeatedly trotted out in support was the existence of overlapping distributions of traits between the sexes—i.e. sexual dimorphism. This multimodal sex paper served as the basis for a Twitter thread by Princeton anthropologist Agustin Fuentes, which he wrote in response to my recent Wall Street Journal essay on the sex binary. I refuted his “fabulous takedown” of my article here.

Fuentes appears to have learned little-to-nothing from that exchange, because yesterday he published an article in Scientific American titled “Here’s Why Human Sex Is Not Binary.” His argument relied heavily on conflating the sex binary and sexual dimorphism. He (again) cited the multimodal sex paper as evidence.

Here’s an example of his inability to separate the sex binary from sexual dimorphism.

Among our fellow mammals, which are less freewheeling because of the twin constraints of lactation and live birth, there are varied connections between gametes and body fat, body size, muscles, metabolism, brain function and much more.

While sperm and ova matter, they are not the entirety of biology and don’t tell us all we need to know about sex, especially human sex.

Nobody denies that gametes are connected in “varied” ways to “body fat, body size, muscles, metabolism” or “brain function,” but these are downstream consequences of sex, not sex itself. Additionally, nobody to my knowledge has ever said that gametes are “the entirety of biology.” They are, however, the fundamental defining property of what it means to be male and female.

I will now cover the distinction between the sex binary and sexual dimorphism is a bit more detail.

The Sex Binary

Sexual reproduction involves a complex life cycle in which a gamete with half a set of chromosomes combines with another to produce a zygote that develops into an organism composed of cells with a full set of chromosomes. Some species are isogamous, which means they reproduce by combining gametes of equal size. Other species are anisogamous, which means combine gametes of unequal size. Only anisogamous species have “sexes” (i.e. males and females), because males and females are denoted by the size of the gamete they have the function to produce. Males have the function of producing small gametes (sperm), and females large gametes (ova). Because there is not a third sized gamete, there are only two sexes.

This two sex system rooted in sperm and ova is the sex binary, and it has evolved independently many times across a wide range of organisms due to its stable efficiency. Large eggs contain more nutrients that result in faster embryonic development and more viable offspring, and many small sperm make fertilization numerically more likely to occur.

Sexual Dimorphism

The sex binary has variable downstream consequences on bodies. Because eggs are more costly to produce than sperm, and females typically bear the time and energy costs of carrying and bearing offspring, females are generally the limiting factor of male reproductive success. This is a phenomenon known as Bateman’s Principle, and it (along with other ecological factors) has resulted in sexual dimorphism: the divergent—but often overlapping—morphological, physiological, and behavioral adaptations that males and females have evolved to optimize their reproductive output.

While the distinction between sperm and ova is binary and discontinuous, sexual dimorphism is a sliding scale. With regard to body size, some species, like deep sea anglerfish, exhibit extreme levels sexual dimorphism where females can be hundreds of times larger than males.

Female and male deep sea anglerfish

Other species, like many birds, display very little sex-related size differences, though they often exhibit considerable dimorphism in coloration and courtship rituals. Male and female humans are moderately dimorphic in body size, somewhere between gibbons (negligibly dimorphic) and gorillas (highly dimorphic).

Some aspects of sexual dimorphism arise from genetics, while others emerge as a consequence of hormonal differences. In species where males are larger than females, this is typically due to male-male competition for access to females. Color dimorphism, in contrast, is often attributed to sexual selection, where females choose males based on the quality and intensity of their coloration, which is usually a reliable and direct indicator of fitness. In other cases, male ornaments may be selected if females just happen to prefer them, even if they are not a direct indicator of health. This is called the “sexy son hypothesis,” and it occurs when there is a correlation between female preference and male ornamentation, ensuring that any sons produced will have a mating advantage. If female preference for ornaments becomes widespread and intense, this can result in extremely exaggerated male ornaments and displays, a process known as Fisherian runaway selection.

Sexual dimorphism does not require or imply that sex differences are absolute. In humans, for instance, males and females differ in average height and weight, yet considerable overlap between the sexes exists. Everyone has seen women who are taller than some or most males, for instance. The existence of overlap with respect to any trait does not negate the sex binary. Rather, these average sex differences are a downstream consequence of the binary nature of sex rooted in gamete type.

Thus, despite the nuanced complexities of sexual selection, the basic tenets of anisogamy often give rise to a generalizable and highly predictive set of traits that influence the reproductive success of individuals over time.

Conclusion

It is crucial to keep the sex binary and sexual dimorphism as distinct concepts. The sex binary refers to the fundamental defining property of males and females, which is rooted in the type of gamete they produce—sperm versus ova, respectively. Sexual dimorphism, on the other hand, refers to the divergent morphological, physiological, and behavioral adaptations that males and females have evolved to optimize their reproductive output.

Whether a result of ignorance or insidiousness, there now appears to be legions of biologists who appear unable or unwilling to understand or accept the clear distinction between these two distinct phenomena. We must therefore call out this confusion wherever it occurs so we can avoid the negative consequences that often arises when they are used interchangeably.

Reality’s Last Stand is 100% reader-supported. If you enjoyed this article, please consider upgrading to a paid subscription or making a recurring or one-time donation below. Your support is greatly appreciated.