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In 2007, I was studying abroad in Perugia, Italy. I had been there for five weeks, my eyes wide with the excitement of navigating a foreign culture, my heart aflutter over a nerdy boy I’d met at a classical music recital. It all seemed like a glorious dream, until it became a nightmare. On November 1, a local burglar named Rudy Guede broke into the apartment I shared with three other young women, two Italian law interns and a British exchange student named Meredith Kercher. Meredith was the only one home that night. Rudy Guede raped her, stabbed her to death, and then fled the country to Germany.

Before the forensic evidence came back, showing unequivocally that Rudy Guede was responsible for this crime, the police and prosecution focused their attention on me. It was a logical place to start. Of all the roommates, I knew Meredith the best. I was the one who discovered that our house was a crime scene and notified the police. They told me I was their most important witness, that any small detail I might remember could be the clue they needed to find out who had done this to poor Meredith.

Over the five days after Meredith was murdered, the police questioned me for a total of 53 hours, without a lawyer and almost entirely without a translator.

A week later, I was in jail, charged with Meredith’s murder. Two years later, I was convicted and sentenced to 26 years in prison. I went on to win my appeal and in 2011 I was acquitted, after four years incarcerated. Even after this vindication, however, so convinced were the Italian authorities that I was guilty, they overturned my acquittal, put me on trial in absentia for the same crime, convicted me again, and sentenced me to 28.5 years in prison. It wasn’t until 2015 that my legal nightmare ended when I was definitively acquitted by Italy’s highest court per non aver commesso il fatto — for not having committed the act. How and why did this happen? A big part of the answer has to do with cognitive bias and motivated reasoning.

Over the five days after Meredith was murdered, the police questioned me for a total of 53 hours, without a lawyer and almost entirely without a translator, and all in a language I spoke about as well as a ten-year-old. I was young (20), scared, and naïve to the ways of the criminal justice system. My final round of questioning went long into the night as they deprived me of sleep, of food, and of bathroom access. When I told them over and over again that I didn’t know what happened to Meredith and that I was at my boyfriend’s house that night, they refused to accept my answers. They slapped me, and they told me that I had amnesia, that I was so traumatized by what I’d witnessed that I had blocked it out.

Instead of listening to what I was telling them, they pushed me to “remember” something I didn’t remember, namely meeting my boss, Patrick Lumumba, at my house that night. Why? They’d found a text message on my phone. I worked at a local cafe, and Patrick had given me the evening off on the night Meredith was killed. I had thanked him, and I texted him back in my broken Italian, “Ci vediamo più tardi,” my best attempt at “See you later.” But to the Perugian authorities, this English idiom didn’t translate as a casual, “I’ll see you when I see you.” To them, it meant I had made an appointment to meet Patrick later that night. You met Patrick, they told me. We know you brought him to the house. I told them that was wrong countless times, but they wouldn’t believe me.

Motivated reasoning was already in full effect among the investigators. Some early lost-in-translation moments outside the house once the police arrived had given them suspicions about my candor. There was confusion over whether Meredith regularly locked her door or merely closed her door (in Italian, the word for “to lock” is “to close with a key”). And as there was nothing obviously stolen from the apartment, they leapt to the conclusion that the break-in — the rock, the smashed window — was staged. The prosecutor, Giuliano Mignini, even assumed, as he said much later in a documentary about the case, that only a woman would cover the body of a murder victim with a blanket. Oh really? And how does he know this?

I was simply naïve to the fact that police lie to suspects to get a confession, even a false one.

My behavior was also grossly misinterpreted. With a flurry of panicked Italian whipping past me, I often didn’t understand what was happening. When my other roommate looked into Meredith’s room once they kicked the door down, she started screaming about what she saw. But I never saw into Meredith’s room, and I didn’t quite understand what she was so upset about. The idea that Meredith had been killed was just so far out of my world of possibilities that I couldn’t even imagine it. So when I stood outside the crime scene, looking dazed but not obviously hysterical, this was interpreted as me looking cold and unemotional in the face of my roommate’s murder, a fact I had yet to fully comprehend.

It didn’t help that I did lie to them early in my questioning. My Italian roommates, who were both big pot smokers, begged me not to tell the police about the marijuana, to deny that anyone in the house smoked it, because they’d lose their law internships if anyone found out. Coming from pot-friendly Seattle, and thinking this was small potatoes and quite irrelevant to what had happened to Meredith, I did what they asked me. But the police had found evidence of marijuana in the house, and they knew I wasn’t being honest. I came clean immediately, but it was too late. That small lie, coupled with the other misunderstandings, were anchoring biases that shaped how the investigators interpreted everything afterward and led them to believe that I was withholding something, that I wasn’t telling them the whole truth about that night. Hence their erroneous certainty that the benign text message to Patrick was evidence of something nefarious.

Photo of Amanda Knox by Patrik Andersson

My own biases led me to trust them. I was nearly 6,000 miles from home, my friend had just been murdered, the killer was on the loose, and I was scared. I thought if anyone would keep me safe, if anyone had my best interests and wellbeing in mind, it was the authorities. This was my fundamental prior belief, shaped by my own privileged upbringing: the cops are the good guys. I’d never had a bad interaction with the police. I had no reason to think they would lie to me. So when they did lie, when they told me that Raffaele, my boyfriend of a week, had turned on me and denied my alibi (he hadn’t), when they lied that they had evidence that I was home that night (they didn’t), I tried to make sense of it. If they weren’t lying, then what other explanation was there? After hours and hours of this intense pressure, I started to believe them that I did have amnesia, and I honestly tried to remember what might have happened. I tried to imagine meeting Patrick like they said I did. They typed up a statement from these blurry incoherent ramblings, and, utterly exhausted, well past the edge of my sanity, I signed it. I was simply naïve to the fact that police lie to suspects to get a confession, even a false one.

It didn’t matter that I recanted that statement almost immediately once I was out of the pressure cooker of the interrogation room and had recovered my senses and reasoning. That false admission sealed my fate. And it became the biggest anchoring bias that would shape the case for the next eight years and my own reputation to this day.

Once you see how the false confession I signed became a fundamental prior for the investigators and prosecution, everything else starts to make sense.

We know how unreliable such interrogation methods are from DNA exonerations. According to the Innocence Project, nearly one in four proven wrongful convictions involves a false confession. And yet, it’s so hard for anyone who hasn’t been through a coercive interrogation to understand how a person could sign false statements implicating themselves or others. Even the police on the other side of the table don’t understand it. They truly think they’re just cracking a suspect and getting them to admit the truth. But they are suffering from a cognitive bias that we are all susceptible to, the idea that our own experience of the world is a reasonable reference. They’ve never signed false statements, so why would a suspect?

Once you see how the false confession I signed became a fundamental prior for the investigators and prosecution, everything else starts to make sense. When the forensic evidence came back weeks later implicating a sole perpetrator, Rudy Guede, they had to find a way to fit this new evidence with their prior belief. This is just what humans do.

A recent study argues that nearly all the cognitive biases we are susceptible to — confirmation bias, the anchoring bias, the framing effect, and so on — can be reduced to “the combination of a fundamental prior belief and humans’ tendency toward belief-consistent information processing.” The prosecution, holding my coerced statements as a fundamental prior belief, tried to force the new forensic information implicating Rudy Guede to be consistent with the idea that I was present that night. And thus, with no evidence, and contrary to my own character and history, they invented a motive and wove a story out of whole cloth about a sex game gone awry and a three-way murder plot involving Rudy, a man whose name I didn’t even know, and my boyfriend of a week.

Source: Statista

It was never a satisfying answer to me that the people responsible for my wrongful conviction were evil, or uniquely incompetent. And once I learned about how common wrongful convictions are even in the U.S., this was even more obvious to me. I wanted to know why this had happened to me, and how mostly well-intentioned people who wanted to repair the rend in the fabric of their community, to bring a perpetrator to justice, and to bring closure to Meredith’s grieving family, could have gotten it so, so wrong. Nothing has been more illuminating for me on this question than diving deeply into the research on cognitive bias.

Most of the specific biases I’m about to discuss are reducible to a general pattern of a fundamental belief and belief-consistent information processing, but I find the added specificity useful to help me see these types of errors in my own thinking.

The anchoring bias is the tendency to rely on the first piece of information, regardless of its validity, when interpreting later information. Thus, early suspicion against me shaped how all later evidence was interpreted. This has also impacted my reputation, and explains why I still receive so much vitriol. Despite my definitive acquittal, the first thing most people heard about me was that I was a suspected killer, and that colors everything else they ever hear about me.

And if they persist in believing conspiracy theories about my guilt, they are helped along by the base rate fallacy, the tendency to ignore general information and focus only on the specifics of one case. Those who think I’m guilty rarely look at general information regarding murders and wrongful convictions. If they did, they’d see how vanishingly rare it is for women to commit knife killings against other women, and how common the errors in my case were. It features all the hallmarks of wrongful convictions, many of which result from cognitive biases themselves.

The most general form of this is often called confirmation bias, the tendency to seek out information that confirms a hypothesis and ignore information that disconfirms it. With wrongful convictions, this is known as tunnel vision. The anchoring bias of an initial hunch shapes the investigators’ search for more information. They magnify the significance of any tiny thing that confirms the anchor and write off large things that don’t. Thus, much weight was put on a kiss between Raffaele and me, while the fact that my DNA was not present in the room where the murder happened and that it was impossible to have participated in such a violent struggle without leaving any traces of oneself, was ignored. This is sometimes called the conservatism bias, the tendency to insufficiently revise one’s prior beliefs in light of new evidence.

The conjunction of many small cognitive biases by the authorities and media is enough to explain the massive debacle the case became.

Then there’s the salience bias: the tendency to ignore unremarkable items and focus on striking ones. The prosecution did this to me, and many people continue to succumb to this bias still. Malcolm Gladwell makes this mistake in his analysis of my case. Like the prosecution and tabloid media, he overlooked the copious moments of unremarkable behavior and highlighted the few moments of so-called “odd” behavior, putting great explanatory weight on them and framing me as someone who acts guilty despite my innocence.

That bias is tied in with the fundamental attribution error: the tendency to overemphasize personality-based explanations for others’ behavior and to de-emphasize the role of context. Thus, in judging my behavior in those early days, people ignored the fact that I was alone, far from home, my roommate had just been murdered, and the killer was on the loose. It was a scary and traumatizing experience, and people react in many ways to trauma. Instead, people often strip my behavior from this context and conclude that I must be weird, “off,” or suspicious. This same bias often comes into play when people reflect upon the false statements I signed. Instead of explaining those false statements by the brutal and coercive context I was in, they leap to a conclusion about my character, that I must be an untrustworthy liar.

Selection bias magnifies all these initial biases by shaping what gets reported. There are no news articles from 2007 about all the moments that I looked sad, or scared, or exhausted. There are no deep-dive articles about my perfectly benign upbringing, complete lack of a history of violence or mental illness, about my strong community and loving family. But one small moment caught on camera of me seeking comfort from the young man I’d met five days previous, sharing a chaste kiss while confused and scared, gets endlessly republished, repeated, and played on loop.

At trial, a host of other cognitive biases came into play. Stereotyping was used to paint me as an American “girl gone wild,” though I was in fact a nerdy poetry and language student. The rhyme as reason effect, in which something that rhymes is seen as more truthful, was used against me. Thus, the moniker “Foxy Knoxy” shaped opinion of me as sly and devious. In Italian, they translated this as Volpe Cattiva, the wicked fox.

The framing effect was used repeatedly at trial to present benign behaviors as suspicious. She ate pizza after her friend had been murdered? Why wasn’t she wasting away, sobbing? Literally, the fact that I ate pizza was used against me as evidence that I was not sufficiently morose, as if a grieving and scared person can’t also be hungry.

All of that framing was repeated for eight years of trial, and it affects me to this day through the continued influence effect, the tendency to believe previously learned misinformation even after it has been corrected. My reputation has not been fully restored. Many people still think that, even if I’m not guilty of murder, I must have had something to do with the crime, or I must have somehow brought suspicion upon myself.

The proportionality bias is our tendency to assume that big events have big causes, when often they are caused by many small things. The massive decade-long series of trials with global media coverage doesn’t need an underlying conspiracy as a cause. It doesn’t require that I was grandly suspicious, nor does it require that the authorities were grandly corrupt. The conjunction of many small cognitive biases by the authorities and media is enough to explain the massive debacle the case became, but the proportionality bias leads us to think there must be a bigger reason.

As far as my continued reputational damage, I can thank the illusory truth effect, the tendency to believe a statement is true if it’s easier to process or has been repeated many times. “Amanda Knox is Bad” is a lot simpler than explaining the miscarriage of justice. This is related to the availability cascade, in which a collective belief is seen as more plausible through repetition in public discourse. The hundreds (thousands?) of media articles painting me as a killer have shaped this perception that many people still have of me.

People wrongly assume that there can only be one true victim, and that if we are to honor the victim of the original crime, we must deny that anything wrong happened to the person wrongfully convicted. In truth, wrongful convictions multiply victimhood.

I try to counter that perception by acting honorably and putting thoughtful work into the world. However, the structures of social media and psychological factors create further selection bias. If I tweet about criminal justice reform, I get maybe a dozen retweets. If I make a joke about my wrongful imprisonment, the tweet spreads far and wide, and I pop onto others’ radar in that context. They don’t see the vast amount of serious work I do, and only see the highly retweeted joke, and conclude that I’m purely flippant.

And then they judge me for making light of a tragedy, but fail to distinguish between the tragedy that befell Meredith and the one that befell me. This is the zero sum bias, assuming incorrectly that if one person gains, another must lose. In this case, they assume that respecting my victimhood by the Italian justice system is tantamount to disrespecting Meredith’s victimhood for being murdered by Rudy Guede. I’ve coined my own term for this specific situation: the single victim fallacy.

You see it often in wrongful conviction cases. People wrongly assume that there can only be one true victim, and that if we are to honor the victim of the original crime, we must deny that anything wrong happened to the person wrongfully convicted. In truth, wrongful convictions multiply victimhood. Meredith is a victim of murder. I am a victim of a miscarriage of justice. Both our families are also victims of this miscarriage of justice, which has denied them closure and put our families through hell. Because of this single victim fallacy, I am told I should never joke about the injustice I suffered, because it is conflated with the injustice done to Meredith by someone else. Because of this fallacy, I am told to shut up and disappear.

These cognitive biases have caused a lot of pain in my life, and in the lives of others touched by this case. And they have also gotten in the way of potential healing. I still hope one day to be able to come together with Meredith’s surviving family in recognition of our shared and overlapping victimhood from the actions of Rudy Guede and the Italian authorities. But as far as I know, they remain in thrall to the single victim fallacy.

I don’t know if that day will ever come, but in the meantime, I take solace in the fact that I have such a great opportunity to see these cognitive errors up close. I was able to see how poorly many people judge this complicated case that took over my life, particularly the facts and the individuals involved in it. To see how wrongly they judge me. This makes me a better thinker. It helps me to better avoid all the cognitive biases that caused my wrongful conviction, that led to slanderous media coverage, and that are still responsible for the hate I regularly receive.

And I would be remiss if I didn’t point out the bias blind spot, the tendency to see yourself as less biased than others. Knowing these biases exist doesn’t make me immune to them. I know I can fall prey to them just as much as the people who imprisoned me. So if you have to have a fundamental prior belief that shapes your reasoning, let it be a belief in your own susceptibility to cognitive bias.

“Fashion” and “Style” typically refer to products and practices that are valued in specific cultures and time periods. In the contemporary U.S., vintage clothing, athleisure, and oversize water bottles are in style, whereas cigarettes, neckties, fur coats, wood paneling, shag carpets, and white wall tires are out.

Ideas also vary in their popularity and prestige across cultures and times. These days, mindfulness, sustainability, plant-based diets, and pet parenting are fashionable, while eugenics, homophobia, hierarchical work environments, and colonialism are not.

Perhaps the most fashionable current idea is that the binary distinction of females and males—and girls and boys, and women and men—is scientifically incorrect and harmful. Instead, leading social scientists, activists, and even professional journals and organizations, have adopted the view that sex should be considered a nonbinary variable, either a continuous spectrum or something with more than two categories.

Yet, the traditional, binary view of sex, despite being unpopular, is basically correct. Crucially, I am confident that holding this view is in no way at odds with being fully respectful to individuals who are transgender or intersex. Here are eight reasons for affirming that biological sex is binary.

Let’s review each of them in more detail.

1. Evolutionary biologists define sex based on gametes, a binary framework. Although scientifically fruitful, this doesn’t work for many individuals.

Reproduction is fundamental to life. Although there are many ways of reproducing, one basic distinction is that of asexual reproduction versus sexual reproduction. In asexual reproduction, sometimes called cloning, an organism reproduces without the participation of another organism. Although asexual reproduction occurs in some vertebrate animals, including reptiles as large as Komodo dragons, it is rare. Instead, most vertebrates, including humans, only reproduce sexually, meaning that two organisms combine their genetic material to produce offspring.

In all vertebrate species, including humans, there are two kinds of sexual reproducers: females and males. Females contribute an egg, which is a gamete (i.e., sex cell) package with a large nutrient bundle and no ability to move. Males contribute sperm, which is a gamete package with no nutrients, but excellent mobility, usually because of a flagellum, a miniature tail. In humans, a single egg is roughly 100,000 times larger than a single sperm.

In all vertebrate species, including humans, there are two kinds of sexual reproducers: females and males.

Most evolutionary biologists, including Richard Dawkins and Jerry Coyne, define sex according to gametes, whether the individual produces eggs (female) or sperm (male).1, 2, 3, 4, 5 This gametes framework undergirds our understanding of reproduction across all sexually reproducing animal species. For instance, in terms of parental responsibility, producing eggs is a high investment strategy whereas producing sperm is a low investment strategy. Building on this initial pattern, in all placental mammals, females, but not males, gestate offspring and provide milk once they are born.

Although the gametes framework has proven highly fruitful for evolutionary biology, it has two key drawbacks when applied to humans. One is that gametes in mammals can’t be observed easily—we don’t lay eggs as do all birds and most reptiles. A second drawback is that, although this framework works well for species or populations, it doesn’t work for many individuals. Postmenopausal women generally don’t produce eggs and neither do women who have had their ovaries removed. Boys don’t produce sperm, and neither do men who have had their testes removed.

2. People in all societies define sex based on reproductive traits, another binary framework. This framework builds on the gametes definition and is far more practical.

Because of the limitations of the gametes framework, I propose the following two “new definitions” of sex based on reproductive traits that are related to gametes.

In humans, a female should be defined as an individual who possesses, or is on a trajectory to possess, or previously possessed, the traits necessary for reproduction as an egg producer; these traits include ovaries, fallopian tubes, uterus, cervix, and vagina. These traits are often referred to as the primary sex characteristics.In humans, a male should be defined as an individual who possesses, or is on a trajectory to possess, or previously possessed, the traits necessary for reproduction as a sperm producer; these traits include the testes, penis, and scrotum.

I put “new definitions” in quotes to make the point that, although I am arguing we should use this pair of definitions, it is more accurate to say that we should acknowledge our traditional use of them, rather than thinking of them as new. This is because people in all human societies distinguish between males and females, and, historically, they must have done this by observing a newborn’s external genitals. What other option could they have had?

Even in modern industrial societies, where blood and tissue tests are often available, genital inspection is still the main method of sex categorization, although that often now occurs by looking at ultrasound images months prior to birth.6

Of course, outside of the birthing room, most of us rarely examine anyone’s genitals to learn their sex. Instead, we use various traits that typically, but not always, co-occur with the primary reproductive traits. These include secondary sex characteristics, which are physical traits that are not necessary for reproduction but that usually accompany the respective set of reproductive traits, particularly after puberty. In women, these include enlarged breasts and widened hips; in men, they include facial hair and Adam’s apples. There are other relevant traits, including men’s generally lower vocal pitch and their greater height and upper body musculature. Even facial shape is extremely helpful: adults can, with high accuracy, correctly determine another adult’s sex from pictures alone.7, 8

In addition, many components of fashion and style serve, in part, to signal the wearer’s sex. For example, women typically have fuller lips than men, and the current popularity of lip filler procedures can be understood as means of amplifying femininity. Many other products and practices work similarly, including jewelry, piercings, tattoos, hairstyles, makeup, and clothing. Thanks to all this biological and cultural information, most of us interact with many people each day and rarely have doubts about whether a person is female or male.

Even in modern industrial societies, genital inspection is still the main method of sex categorization.

Nonetheless, it bears stressing that the initial assignment of a newborn’s sex, apparently in all societies, is based on external genitalia. In fact, this assignment can be considered part of our universal folk knowledge, “common sense” that sometimes turns out to be correct. Moreover, this universal folk knowledge about sex isn’t limited to knowing that sex is assigned based on external genitalia; it also includes recognizing that there are two, and only two, kinds of human reproducers, that only females bear children, and that females can only reproduce if they have had sexual intercourse with males.

Is this folk knowledge about sex universal, meaning that it has occurred in all human societies that ever existed? Logically, of course, nobody can prove that any belief or behavior is universal; there could always be some exceptional society that has never been studied.9 Nonetheless, we can be highly confident that these patterns are universal or nearly so. The reason is that, for the past few hundred years, cultural anthropologists, ethnographers, missionaries, and explorers have traveled the world documenting exotic beliefs and practices, such as taboos against eating nutritious foods or body modification that involves painful procedures. This attention to describe cultural beliefs extends to sexuality. We know, for example, that in some traditional societies, people believe (incorrectly) that a woman can only get pregnant if she is inseminated repeatedly;10 in other societies, people believe (incorrectly) in partible paternity, meaning that a child can have two biological fathers;11 in others, people believe (incorrectly) that a woman becomes impregnated by a spirit but that a penis must first open the vagina to allow the spirit’s entrance.12 If there were a society where people believed in a third manner or mode of reproduction or were unaware that penile-vaginal intercourse is, with very few exceptions, necessary for conception, we can be confident anthropologists would have studied it and described it by now.

3. Sex is consequential for many reasons, including that it is women who have babies, not men.

In addition to the ones just noted, humans hold other universal beliefs and practices. A fundamental practice in all societies is that there are distinct words for girls and boys, women and men, mother and father, and daughter and son.13 These words exist everywhere because they indicate reproductive roles, and reproduction is always potentially consequential. Even in contemporary countries with sub-replacement level fertility, a substantial portion of women bear children, an event with major consequences for education, work, recreation, family life, and, indeed, survival.

Healthcare is a particularly notable sphere where sex, based on reproductive traits, is consequential. Practitioners aiming to provide the best care frequently must consider if their patient is female or male. Diseases that differ substantially in prevalence, manifestation, or treatment, include Alzheimer’s, COVID-19, depression, diabetes, influenza, pneumonia, and several kinds of cancer.14

Some have asked whether our society would be better off if we simply ignored sex. Although this seems desirable in some situations, it is not practical in many others.15

4. Third genders are nonbinary, but they do not challenge the biological sex binary, reproductive traits framework.

Another frequent question is whether societies with so-called third genders pose a problem for the binary, reproductive traits framework. For example, in Samoa, there is a third gender kind of person called Fa’afafine, which roughly translates to “in the manner of a woman.” Fa’afafine are biological males who often dress in female-typical clothing, adopt feminine names, and do female-typical, people-oriented jobs such as teaching and nursing; they are exclusively attracted to male-typical, masculine men as sexual partners.16

Third gender individuals either do not reproduce or they reproduce in the typical male or female manner.

Fa’afafine are accepted in Samoa as a third kind of person, neither a typical male nor a typical female. However, everyone there recognizes that Fa’afafine do not reproduce in a third way; they rarely or never reproduce, and, if they do, they do so as men. In other societies, there are other kinds of third gender individuals, including biological females who adopt male-typical roles and, in other societies, individuals who do not conform to either a male-typical or female-typical role.17 However, the story is essentially the same everywhere: everyone knows that third gender individuals either do not reproduce or they reproduce in the typical male or female manner.

Many people, even educators and policymakers, express confusion about third gender individuals and claim they challenge the idea that sex is binary. However, this is attributable to many scholars and activists using the terms “sex” and “gender” interchangeably. They will say things such as, “The Fa’afafine embody a third gender and sex.”

Gender has a rich and broad web of meanings, but we don’t need to unpack them all to address the confusion. We only need to remember that what we can call gender roles associated with biological sex—that is, doing male-typical or female-typical things—can be nonbinary. That is, there can be three or more genders or gender roles, and there can be intermediate genders. Further, there are many other traits that are associated with biological sex that are nonbinary; these include skeletal traits, hormones, and personality. None of this, however, contradicts the binary reproductive traits framework discussed above. Again, there are exactly two kinds of sexual reproducers, male and female, and people in all societies—even societies with third genders—recognize this binary distinction, and they recognize it as consequential.

5. Intersex individuals challenge the binary, reproductive traits framework, but they don’t invalidate it because they do not reproduce in a third way.

A more substantial challenge to the binary, reproductive traits framework comes from intersex individuals, who are sometimes described as having DSDs (Disorders or Differences of Sexual Development). These individuals are born with genetic, hormonal, or physical characteristics that are unusual for males or females. For example, a person might have male-typical chromosomes (e.g., XY) yet their phenotype or appearance may be female. A critical point is that, unlike most third gender individuals, intersex individuals typically do not possess a full set of traits necessary for reproduction, and often they are unable to reproduce. Frequently discussed intersex conditions include complete androgen insensitivity syndrome (CAIS), congenital adrenal hyperplasia (CAH), 5-alpha reductase deficiency (5-ARD), and ovotesticular disorder (also called true hermaphroditism). In some communities, some intersex conditions occur fairly commonly (e.g., 5-ARD), and people with this condition may be described as embodying a third gender.18

Source

There is debate about the frequency of intersex conditions with some writers claiming that 1 in 60 live births is intersex and others suggesting the true frequency is roughly 1 in 5,000. The debate largely centers on what counts as a true intersex condition. Using a very broad definition,19, 20 anyone who doesn’t fit their “exacting criteria” for being a typical male or female should be considered intersex, making the prevalence relatively high. In this perspective, a man with a short but functional penis could be called intersex, as could a boy with hypospadias (i.e., their urethra opens on the underside of their penis instead of at the tip) or a woman who bore three children but learned later in life that her androgenic hormones were unusually high.

The binary nature of human reproduction is about as complete a binary distinction as one can find in the natural world.

However, if we reserve the label intersex for individuals who do not possess all traits necessary for successful reproduction, whose chromosomal sex does not match their phenotypic sex (e.g., XY female), or for whom there was genuine uncertainty about their birth sex—all of which might be called classic intersex conditions—the more accurate estimate is roughly 1 in 5,000.21 This estimate is not particularly controversial; it was cited without challenge in 2020 in a progressive editorial in the New England Journal of Medicine advocating for the removal of sex designations on birth certificates.22

Regardless of whether they are fairly common (1 in 60) or truly rare (1 in 5,000), some intersex individuals do not fit neatly into the binary, reproductive traits framework. Nevertheless, they don’t invalidate the framework for two reasons.

First, many definitions of real-world phenomena— including of species, vegetables, or games—become fuzzy or must admit exceptions if scrutinized.23 This is true even of binaries that we usually take for granted:

• Nearly all of the earth’s water can be classified as freshwater or saltwater, yet there are transitional, intermediate areas, which are sometimes called brackish.
• When coins are flipped, they land on one side or the other, heads or tails, however, coins occasionally land on their edge.
• In all societies, people are considered dead or alive, yet, with modern medicine, some individuals may exist in intermediate states, including comas and brain death.

A second, and more crucial, reason that intersex individuals don’t invalidate this framework is that this framework is based on female and male modes of reproduction, and intersex individuals do not reproduce in a third way. All intersex individuals who reproduce, reproduce in either the male or the female manner. To summarize: the binary framework accommodates all modes of sexual reproduction in humans, but it does not (quite) accommodate all humans.

It’s worth stressing that, as Dawkins has eloquently explained,24 the binary nature of human reproduction is about as complete a binary distinction as one can find in the natural world.

6. Hyde and colleagues’ (2019) popular nonbinary definition of sex is indefensible.

What about the nonbinary definitions of sex? Because these have become influential, one might assume that they are better than the binary definitions. I want to note that there is apparently no nonbinary definition of sex that has been recognized as being the best. Nevertheless, a good place to start is with a review article by Hyde and colleagues entitled, “The Future of Sex and Gender in Psychology: Five Challenges to the Gender Binary.”25 This article was published in 2019 in the American Psychologist, a leading journal of the American Psychological Association, the largest psychological society in the world. Hyde is one of the most influential sex and gender scholars, and this article has already been cited more than 1,100 times—a very high number for any academic article, particularly one published so recently. Hyde et al. offer this definition of sex: “The term sex is used here to refer to biological systems involving the X and Y chromosomes, pre- and postnatal sexual differentiation, and hormones that influence sexual differentiation of the external genitals, which, in turn, serve as the basis for sex assignment at birth.”

This is a terrible definition, on several counts. The first issue is that it fails to acknowledge the unifying or organizing feature of the properties included in the definition—that unifying feature being, of course, reproduction. Imagine someone defined a door as: “A human-created object, situated in a house, dwelling, or vehicle, that can be comprised of various materials, that can be decorative, and that can have other objects affixed to it.” These stated properties of doors are all true, yet this is an inadequate definition because it doesn’t state that the chief purpose of a door is to serve as a movable barrier. Similarly, it would be a poor definition of an eye if someone listed many of its components (e.g., cornea, lens, retina) but failed to mention that the eye’s function is to see (that is, to transduce some property of some portion of the electromagnetic spectrum into neural impulses).

A second issue is that Hyde et al. have not explained why a new definition of sex is needed. A basic principle of communication is that if there is an established definition, one should not alter it, or introduce a new one, without justification. Of relevance here is that the gametes definitions of sex developed by biologists is well established—it has been used for more than 100 years and has proven extremely fruitful. (Yes, it’s true that earlier in this article I proposed defining sex based on reproductive traits, however, I explained why this is a sensible extension of the gametes definitions and that it is best viewed as an acknowledgment of our intuitive, universal folk knowledge.)

A third problem is that Hyde et al. have not provided a constructive, practical definition of sex. To make this clear, recall that according to the reproductive traits framework, an individual is female if they possess (or did possess or will possess) the traits necessary for reproduction as an egg producer, and they are male if they possess (or did possess or will possess) the traits necessary for reproduction as a sperm producer, or they are outside the binary if they don’t possess traits for either of these modes of reproduction. There will always be a few edge cases requiring further careful consideration. That said, the vast majority of people can be categorized easily by the reproductive traits framework.

Hyde et al. don’t offer anything similarly useful. They do not state what, according to their definition, the various kinds or categories of sex are—whether it be three, four, five, or more categories. Or, if they consider sex a gradient or spectrum, rather than a categorical variable, they do not specify what concepts or variables that gradient represents. This is worth emphasizing: Based on those very definitions, one cannot categorize a single person as female, male, a third sex, or some intermediate sex. Practically speaking, a 99.9 percent easy classification (i.e., the reproductive traits framework) is preferable to a 0 percent classification!

What issues do Hyde et al. address in their article? One emphasis is defining additional terms, such as “gender,” “transgender,” “cisgender,” “nonbinary,” “agender,” and “genderfluid.” This creates the impression that sex is mixed up with these other terms. The other approach Hyde et al. take—and this comprises most of the article—is to argue that most traits that typically differ between women and men are affected by many factors that can be placed on a gradient or spectrum. These traits include the amounts of various hormones, the size of various brain structures, and the amount or frequency of cognitive abilities, social behaviors, romantic attraction, and feeling like a man or a woman. Although it’s true that these traits are nonbinary, that is not at odds with the reproductive traits framework. The take-home message of that framework is not that all sex-differentiated traits are binary; it’s that the two modes of reproduction—and the reproductive traits that support them—are binary.

So, how does the review by Hyde et al. deal with the fact that reproduction is binary? It doesn’t—it simply ignores the issue. In particular, although it is more than 20 journal pages and over 14,000 words, the words “gamete,” “baby,” “child,” “pregnancy,” “mother,” or “parent,” do not appear, except in parentheses or in the references section. There are a few mentions of “reproductive phases” and “reproductive structures,” but there is no acknowledgment that people bear and raise children, that these events are important, or that reproduction should be considered when defining sex.

Of course, Hyde et al. might argue that ignoring reproduction is a feature, not a bug—indeed, one that justifies their new definition of sex. Perhaps so, but this then raises the question of what trait should serve as the foundation for defining sex. Since they do not provide any answer, I submit the reason is that no nonbinary definition is defensible.

Without a theoretically defensible definition of sex to provide the endpoints of a gradient or spectrum, no individual could be placed on any female-male spectrum for any trait.

Imagine that someone proposed that the amount of testosterone a person is exposed to prior to birth should be the foundation for defining sex, an idea that seems reasonable given that such exposure is known to correlate with variation in several typically sex-differentiated important areas, including play preferences, work preferences, cognitive abilities, and sexual orientation. Although apparently reasonable, one might argue that it is in fact an adult’s current level of circulating testosterone, not their prenatal exposure, that is the more salient measure; another might suggest that prenatal estrogen exposure as the key measure; a fourth might posit that yet another hormone is critical.

The resulting situation is actually more nebulous because other scholars might just as reasonably claim that hormones are no more important than brain shape, skeletal anatomy, personality, or other areas, and each area has many possible measures. There is simply no single area or measure that is more defensible than the others.

In addition, a more fundamental problem is that any scoring system requires a reference point, and scholars who do not accept a binary definition of sex have committed themselves to abandoning the reference point of binary sex. This issue is relevant to every kind of trait—behavioral, anatomical, physiological—that might be considered a candidate for characterizing sex along a spectrum. Without a theoretically defensible definition of sex to provide the endpoints of a gradient or spectrum, no individual could be placed on any female-male spectrum for any trait.

7. No better nonbinary definition has been offered.

If the definitions and ideas proposed by Hyde et al. in 2019 are unworkable, then perhaps there is some other framework that works better? Though I follow the scholarship and discussions in this area closely, I’ve yet to encounter one.

In debates and essays, scholars arguing against the “sex is binary” position, including Alice Dreger26 and Steven Novella,27 invariably echo the points made by Hyde et al., including that some intersex individuals do not fit the binary and that most traits related to sex are not binary. However, these scholars never cite nor develop a viable nonbinary alternative framework, and they fail to acknowledge the fact that all humans who reproduce do so in either the male or female mode, never in a third or intermediate way.

I’ve long taught the course, “Psychology of Sex Differences,” and so am familiar with the leading textbooks in this field, and have participated in a content analysis of the leading textbooks.28 These books also deny that sex is binary yet fail to offer any constructive nonbinary alternative. Furthermore, although they invariably address reproduction and childrearing in later chapters, when they first introduce and define the terms sex and gender in their opening chapters, they do not acknowledge either that reproduction exists or that it should be incorporated into a definition of sex.

8. Nonbinary definitions of sex are popular because they are viewed as being progressive. However, one can hold progressive political views while still retaining the traditional binary view of sex.

If the binary framework of sex based on reproduction is solid and the nonbinary alternatives are so weak, why have the nonbinary alternatives gained so much traction? Why is the nonbinary view presented not only in texts but especially in the media?

The short answer is that scholars, especially evolutionary biologists, haven’t done an adequate job in making the binary basics accessible. While concentrating on the logical and heuristic power of the gametes framework, including its applicability to species where individuals can alter their sexual strategies or reproduce asexually, evolutionary biologists have neglected to provide intuitive and practical definitions for nonspecialists. I hope this article has made the case for how and why a reproductive traits framework—which builds on the gametes framework and makes explicit humans’ universal folk knowledge—remedies this problem.

Those with intersex conditions don’t fully fit the traditional binary pattern, and neither do gay people, or third gender individuals.

A second reason nonbinary definitions of sex have become popular is that they seem to accommodate better the diverse and varied biologies, psychologies, and experiences of individuals who do not conform to the traditional binary pattern, i.e., that most people have male or female reproductive traits, and this difference is accompanied by a corresponding package of female-typical or male-typical nonreproductive traits.29 Among many others, these include secondary sex traits (e.g., enlarged breasts, facial hair), normative social roles and interests (e.g., homemaker, breadwinner), heterosexual orientation, and identifying with one’s biological sex (cisgender). Those with intersex conditions don’t fully fit the traditional binary pattern, and neither do gay people, or third gender individuals. Further, in the U.S. and other Western societies, traditionally there has not been any third gender category or categories, but this is changing as increasing numbers, especially of younger people, identify as transgender.

Although there have always been people who don’t fit the traditional binary pattern, in recent years the visibility of these individuals—and the respect and legal rights afforded to them—has increased tremendously. A major contributor to this change has involved rejecting the presumed practical and moral superiority of the traditional binary pattern. Some scholars call this the cisgender heteronormative pattern. Whatever the nomenclature, the critical point is that more and more individuals have recognized that it is fine and safe to be, for example, a biological woman who is sexually attracted to women, a biological man who identifies as a woman, a biological woman who prefers fixing cars rather than crocheting baby blankets, or an intersex person whose reproductive traits do not allow categorization as being strictly male or female.

For many, myself included, such rejection of the superiority and dominance of the traditional binary pattern represents true ethical and political progress. However, doing this does not require abandoning the scientifically accurate view that sex is binary.30, 31