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Key points
- Defining biological sex by sex organs or chromosomes is unscientific due to numerous exceptions.
- Biological sex is traditionally defined by gametes (sperm and egg), essential for direct reproduction.
- This definition is incomplete, as intersex conditions can be adaptive through kin selection.
- A complete definition of biological sex should include male, female, and intersex.
What is biological sex? It seems like a question with an obvious answer: male and female, of course. You might point to internal or external sex organs, or sex chromosomes (XX for females, XY for males), or genes (such as SRY, the maestro responsible for kicking off male development).
These answers are only part of the story—and they work well enough for most humans. But for all humans? Not quite. And when it comes to the rest of the biological world, those explanations crumble like sandcastles under the tide of nature’s diversity. Let’s unpack some of the most common misconceptions and work our way toward a more scientifically grounded understanding of what biological sex truly means.
1. Why External Sex Organs Can't Define Sex
Most animals don’t have the kind of external sex organs that humans do. Even among mammals, the human stereotype of “male and female” anatomy doesn’t always hold.
Consider the spotted hyena. They live in matriarchal societies where adult females are 10 percent larger than males. Females sport an external sex organ called a pseudo-penis, which looks almost indistinguishable from the real thing (Figure 1). Adding to the illusion, they also have a scrotal sac. The resemblance is so convincing that even experts can struggle to tell males and females apart.
What’s behind this evolutionary quirk? The answer lies in the harsh, food-scarce landscapes of Africa, where spotted hyenas roam. Faced with the demanding nutritional requirements of pregnancy, females evolved larger bodies and more aggressive temperaments to outcompete males for resources by boosting their androgen levels. As the result of elevated levels of male hormones, females also develop male-like external sex organs (1).
2. Why Internal Sex Organs Aren’t the Full Story
Much like external sex organs, internal sex organs aren’t always dedicated solely to the production of sex cells—sperm or eggs—as illustrated by the fascinating case of the European mole (Figure 2).
Female moles are biological multitaskers, sporting gonads that function like a mixtape with dual tracks. These gonads include both ovarian tissue, responsible for producing eggs, and abnormal testicular tissue, which doesn’t produce sperm. During the breeding season, the ovarian tissue takes center stage, producing eggs that are fertilized and lead to the birth of offspring.
After the babies are born, the ovarian tissue shrinks, and the testicular tissue takes over, cranking out testosterone in large quantities (2). This surge in testosterone boosts the mother’s aggression, helping her to compete for food and fiercely protect her young. This case shows that internal sex organs aren’t a definitive marker of biological sex.
3. Why Sex Chromosomes Don’t Define Biological Sex
Most animals don’t have sex chromosomes to begin with. Even in mammals, the familiar XX/XY system isn’t as universal as we like to think.
Take Australia’s platypuses and echidnas, for example. These egg-laying mammals defy expectations with five pairs of sex chromosomes. Meanwhile, some rodents and bats have done away with the Y chromosome altogether, leaving a system of XX for females and X0 (a single X chromosome) for males.
Humans, too, complicate the picture. While most of us adhere to the XX/XY framework, variations like X0 (Turner syndrome), XXY (Klinefelter syndrome), XXX (trisomy), XYY, and even more complex combinations like XXXY or XXXXY exist within our species. While these individuals often face infertility, some—such as those with XXX or XYY configurations—are fully fertile or nearly so.
What evolutionary purpose these variations serve remains a mystery. But one thing is clear: sex chromosomes don’t hold the key to defining biological sex.
4. Why Genes Can’t Definitively Determine Sex
Just like sex chromosomes, the genes responsible for sex determination and development are far from inflexible. In mammals alone, mismatches between genetic and anatomical sex have been documented in several species—including humans.
A striking example came to light during the 1996 Atlanta Olympics. Eight athletes participating in women’s events, all anatomically female, were found to carry the SRY gene—the genetic trigger for male development. The explanation? Most of these athletes were naturally intersex due to a condition called androgen insensitivity syndrome (AIS). Individuals with AIS are potentially male (XY) but their bodies are unresponsive to androgens, the hormones that drive male physical traits. As a result, their development follows a female pathway, including the formation of female-like external genitalia.
This example underscores just how intricate and variable sex determination truly is. It involves dozens of genes, each interacting in complex ways, producing a staggering amount of variation in sex development.
5. What Is Biological Sex? The Traditional Binary View
A definitive definition of biological sex eluded biologists for centuries until 1972, when three biologists led by Geoff Parker proposed a framework based on the size of gametes (3). Males, they argued, are the biological entities that produce small gametes—sperm—while females produce larger gametes—eggs. This categorization became the consensus among biologists, cementing a binary view of sex.
However, this consensus has come under increasing scrutiny in recent decades, as cases of intersex individuals have been documented across a growing number of species.
6. What Is Biological Sex? A New, Non-Binary View
It’s true that producing viable sperm or eggs is essential for direct reproduction. But evolution, as biologist William Hamilton demonstrated in 1964, isn’t about direct reproduction alone. Instead, it’s about passing on genes—your genes—whether through your own offspring or by helping your relatives, who share loads of your genes. This groundbreaking idea, known as kin selection (4), rewrote the rules of evolutionary success.
Think of evolution like competing in a triathlon or decathlon. Winning gold doesn’t mean dominating every single event; it’s about scoring enough points overall. Kin selection works the same way. You don’t have to reproduce directly to pass on your genes. If your relatives thrive because of your efforts, you’re still in the running for nature’s ultimate prize: genetic immortality.
Nowhere is this concept more vividly illustrated than in the collective lives of social insects. In a beehive, ant colony, or naked mole-rat society, most individuals are unsexed (essentially, intersex) workers or warriors. They don’t waste time reproducing; that job belongs to the queen. Her offspring carry not just her genes, but also the genes of her loyal, sterile workers, who ensure the colony thrives.
Birds and mammals play the same game along kin selection. For example, young birds often forgo breeding by themselves to help their parents raise siblings. Monkeys and apes cooperate extensively with close kin. Humans aren’t all that different. Every time we chip in to help a sibling, set up a college fund for our kids, or scribble names into a will, we’re playing the same evolutionary game.
When seen through the lens of kin selection, biological sex becomes more than a binary split between sperm-producers and egg-makers. Intersex individuals, who may not produce viable gametes, still contribute to the evolutionary game without direct reproduction. In this light, the strict male-female binary doesn’t just seem incomplete—it's outdated.
(For more details please see my talk at Harvard Radcliffe Institute)
References
Smith, J. E., & Holekamp, K. E. (2019). Spotted Hyenas. Encyclopedia of Animal Behavior, Second Edition, 2019, 190–208.
Barrionuevo, F.J., Zurita, F., Burgos, M. and Jiménez, R., 2004. Testis-like development of gonads in female moles. New insights on mammalian gonad organogenesis. Developmental biology, 268(1), pp.39-52.
Parker, G.A., Baker, R.R. and Smith, V.G.F., 1972. The origin and evolution of gamete dimorphism and the male-female phenomenon. Journal of theoretical biology, 36(3), pp.529-553.
Hamilton, W.D., 1964. The genetical evolution of social behaviour. I & II. Journal of theoretical biology, 7(1), pp.1-52.
