Jim Gaffigan has this joke about a scientist who mistakes the male seahorse for the female and then stubbornly refuses to admit his mistake after learning that the one he’d called the male is pregnant. It’s a funny bit, but it hinges on a common misunderstanding that sex designations are arbitrary.

The biological definition of males and females is entirely based on the size of the sex cells, called gametes, that they produce. Males produce the smaller gametes and females produce the larger ones. When scientists discover a new sexually reproducing species, gamete size is how they determine the sexes. In humans, males release between 200 and 500 million tiny sperm in each ejaculation while females have a lifetime supply of around 400, much larger, eggs.

This initial asymmetry in gamete size may not seem like a big deal, but it leads to a whole cascade of evolutionary effects that result in diverging paths for each sex, which we call sexual selection. If male and female bothers you, call them little and big, but regardless of what you call them, this foundational cell-sized difference in gamete size has profound effects on both evolution and behavior.

Sexual reproduction that involves the union of gametes of different sizes is called anisogamy, and it sets the stage for phenotypic differences between males and females. Robert Trivers laid down the basic argument, which he later described in biblical terms (“the scales fell from my eyes”), in one of the most cited papers in biology, when he wrote “What governs the operation of sexual selection is the relative parental investment of the sexes in their offspring.” This fundamental insight provides the framework for understanding the emergence of sex differences across all sexually reproducing species.

Because male animals can produce millions of sperm cells quickly and cheaply, the main factor limiting male reproductive success is their ability to attract females, whereas the primary limiting factor for females, who, in humans, spend an additional nine months carrying the baby, is access to resources. The most reproductively successful men (e.g., Genghis Khan is likely to have had more than 16 million direct male descendants) can invest little and let the chips fall where they may, while the most successful women are restricted by the length of pregnancy.

By replacing “female” with “the sex that invests more in its offspring,” however, we can extract Trivers's general argument, and one of the most falsifiable predictions in all of biology—the sex that invests more in its offspring will be more selective when choosing a mate, and the sex that invests less will compete over access to mates. Find a single species where the sex that invests less in offspring is choosier, and the entire theory is disproved. The brilliance of this insight is that it explains both the general rule and all of the exceptions. Because of the initial disparity in investment, females will usually be more selective in choosing mates. Trivers explained something that had puzzled evolutionary biologists ever since Darwin and also explained why the male seahorse, which really does get pregnant, is choosier, and the female seahorse is bigger.

The assertion that male and female is an arbitrary classification is false on every level. Not only does it confuse primary sexual characteristics (i.e., the reproductive organs) which are unambiguously male or female at birth 99.8 percent of the time with secondary sexual characteristics (e.g., more hair on the faces of men or larger breasts in women), it ignores the very definition of biological sex. Although much has been made of the fact that sex differences in body size, ornamental features, hormonal profiles, behavior, and lots of other traits vary widely across species, that these differences are minimal or non-existent in some species, or that a small percentage of individuals, due to disorders of development, possess an anomalous mix of female and male traits, none of it undermines or challenges this basic distinction. Sex is binary. There is no third sex.

In the 50 years since Trivers’s epiphany, we seem to have done everything we can to forget it. John Money in 1955 was the first to introduce a distinction between biological sex and gender roles. Prior to Money, gender was almost exclusively used to refer to grammatical categories (e.g., masculine and feminine in Spanish). But the major change came in the 1960’s when feminists first adopted it to distinguish social and cultural differences (gender) from biological differences (sex). By 1988 gender outnumbered sex in all social science journals and in the following decade the sex vs gender usage ratio in scientific journals had gone from 10 to 1 to less than 2 to 1. The last twenty years have rapidly accelerated this trend, and today this distinction is rarely observed.

This major change is part of a larger movement to deny the effects of biology in humans altogether and the dominant view in the social sciences has now become that human sex differences are almost entirely socially constructed. In this interpretation, all differences in outcomes between men and women are the result of bigotry, and all we need to do to eliminate them is change children's beliefs by encouraging gender neutral play.

Despite these assertions, human sex differences are among the most robust and replicable findings in the social sciences. The strongest effects are in physical abilities, such as throwing distance or speed, spatial relations tasks, and some social behaviors, like assertiveness, where men have the advantage. Women, meanwhile, tend to have an edge in being more extraverted, trusting, and nurturing. The largest sex differences, however, involve mate choice and are in behaviors that emerge out of Trivers’ theory of parental investment, with women giving more weight to traits that signal a potential partners ability to acquire resources (e.g., socioeconomic status and ambition), while men give more weight to traits that signal fertility (e.g., youth and attractiveness).

The evidence that sex differences in behavior have a biological origin is overwhelming. There are three main methods that scientists use to determine whether or not a trait is rooted in biology. The first hallmark of a trait that is likely to have evolved by natural selection, is that the same pattern is seen across cultures. This is because it is unlikely that a characteristic, like husbands being older than their wives, is culturally determined if the same pattern is seen in every country on earth—like the odds of getting heads 200 times in a row. The second indication that a trait has biological origins is if it is seen in young children who have been less exposed to culture (e.g., baby boys are more aggressive than baby girls). Third, a similar pattern (e.g., males are more aggressive) seen in closely related species, such as other apes or mammals, also suggests an evolutionary history. Many human sex differences, such as more aggression in males or choosier females, hit the trifecta.

If these differences are so common, why has the opposite message—that these differences are either non-existent or the result of social construction—been so vehemently proclaimed? The reasons are almost entirely political. The idea that any consequential differences between men and women have no foundation in biology is appealing because it creates the illusion of control. If sex differences are hardwired into human nature, they are more difficult to change. In other words, if biology underpins sex differences, we might have to learn to accept them. But if gender role “theories” are correct, all we need to do to eliminate them is give kids gender neutral toys.

Acknowledging the role of biology also opens the door to the unwelcome possibility that unequal outcomes for men and women should not just be expected, they might even be desirable. Consider the so-called gender equality paradox whereby sex differences in occupations are higher in countries with greater opportunities for women. Countries with the highest gender equality, like Finland, have the lowest proportion of women who graduate college with degrees in stereotypically masculine STEM fields, while the least gender equal countries like Saudi Arabia, have the highest. Similarly, the female to male sex ratio in stereotypically female occupations, like nursing, are 40 to 1 in Scandinavia but only 2 to 1 in countries like Afghanistan. This is an inconvenient finding for gender role theorists because it suggests that women and men have different preferences which they act out given greater choice, and it is only a “paradox” if you assume that sex is a social construct.

It is understandable, however, that some might fear that any concession to nature or evolved differences between men and women will be used to perpetuate discrimination. But is the fear of abuse so great that lying about biological sex differences is the only alternative? The rhetorical contortions required to assert that gender and sex are nothing more than chosen identities requires increasingly incoherent arguments and inscrutable jargon. This not only subverts confidence in science; it also leads to extreme exaggerations designed to silence those who don’t agree. The lengths to which previously trusted institutions like the American Medical Association now go to deny the impact that hormones have on development are extraordinary. These efforts are also likely to backfire when gender neutral terms handed down by elites, like “Latinx,” are opposed by 98 percent of those they are supposed to protect.

The heart of the problem lies in conflating equal opportunity with being equal, and in our utter failure to respect and value our differences. For two billion years, sexual selection, governed by an initial disparity in the size of the sex cells, has driven a cascade of biological differences between males and females, while at the same time ruthlessly enforcing another type of equality between the sexes. The fact that it takes one male and one female to reproduce guarantees equal average reproduction of the sexes, while the fact that mothers and fathers will each contribute a nearly equal amount of DNA to both their sons and daughters ensures equal genetic representation in the next generation.

Although this may not be the kind of equality some want, we need to move beyond simplistic ideas of hierarchy, naively confusing different with better and worse, or confusing dominance with power. In the logic of evolution, there are many paths to power and neither sex is superior. Better simply means more copies, and dominance only matters if it leads to more offspring.

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The assertion that children are born unisex and are molded into gender roles by their parents delegitimizes other scientific claims. If you can’t be honest about something every parent knows, what else might you be lying about? It leads to inane propositions, like the assertion that transgender men can give birth by a pro-choice doctor testifying to Congress, and endangers the most vulnerable parts of our population.

When people are shamed into silence about obvious male advantages in sports, or children are taught that sex is grounded in identity rather than biology, masculine girls and feminine boys may become confused about their sex or sexual orientation, and harmful stereotypes take over where none existed. Boys are told that if they like dolls, they are really girls, while girls who display interests in sports or math are told they are boys, born in the wrong body. This restrictive thinking shames people for feeling misaligned with their birth sex and pushes them to change their bodies to reflect this new identity. How is this progress? The rapid rise in the number of young girls with gender dysphoria is a warning sign of how dangerously disoriented our culture has become. Feminine boys, who might have ended up being homosexual, are encouraged to start down the road towards irreversible medical interventions, hormone blockers and infertility.

The push for a sexless society is a stupendously arrogant and utopian vision. It cuts humans off from our biological history and promotes the delusion that we are not animals. Sex is neither simply a matter of socialization, nor is it just a matter of choice. Making these assertions—failing to understand the profound role that an initial asymmetry in gamete size plays in sexual selection—is like trying to referee a game in which you’ve never bothered to read the rules, and it is an ignorance that we can’t afford.