Sex Is a Spectrum: The Biological Limits of the Binary

by Agustín Fuentes

Why human biology is far more expansive than the simple categories of female and male

Being human entails an astonishingly complex interplay of biology and culture, and while there are important differences between women and men, there is a lot more variation and overlap than we may realize. Sex Is a Spectrum offers a bold new paradigm for understanding the biology of sex, drawing on the latest science to explain why the binary view of the sexes is fundamentally flawed—and why having XX or XY chromosomes isn’t as conclusive as some would have us believe.

In this lively and provocative book, leading biological anthropologist Agustín Fuentes begins by tracing the origin and evolution of sex, describing the many ways in the animal kingdom of being female, male, or both. Turning to humans, he presents compelling evidence from the fossil and archaeological record that attests to the diversity of our ancestors’ sexual bonds, gender roles, and family and community structures, and shows how the same holds true in the lived experiences of people today. Fuentes tackles hot button debates around sports and medicine, explaining why we can acknowledge that females and males are not the same while also embracing a biocultural reality where none of us fits neatly into only one of two categories.

Bringing clarity and reason to a contentious issue, Sex Is a Spectrum shares a scientist’s perspective on why a binary view of sex and gender is not only misguided but harmful, and why there are multitudes of ways to being human.

Genres: Science, Nonfiction, Anthropology, Biology, Audiobook, Gender, Queer

202 pages, Hardcover

First published May 6, 2025

About the author

Agustín Fuentes, trained in Zoology and Anthropology, is the Edmund P. Joyce C.S.C. Professor of Anthropology at the University of Notre Dame. His research delves into the how and why of being human. Ranging from chasing monkeys in jungles and cities, to exploring the lives of our evolutionary ancestors, to examining what people actually do across the globe, Professor Fuentes is interested in both the big questions and the small details of what makes humans and our closest relatives tick. He has published more than 150 peer reviewed articles and chapters, authored or edited 19 books and a three-volume encyclopedia, and conducted research across four continents and two-million years of human history. His current explorations include the roles of creativity and imagination in human evolution, multispecies anthropology, evolutionary theory, and the structures of race and racism. Fuentes is an active public scientist, a well-known blogger and lecturer, and a writer and explore for National Geographic. Fuentes’ recent books include “Race, Monogamy, and other lies they told you: busting myths about human nature” (U of California), “Conversations on Human Nature(s)” (Routledge) and “The Creative Spark: how imagination made humans exceptional" (Dutton).

Reviews

[Corvus · Rating 5 out of 5]

I come from a generation wherein discussions of gender often highlighted the importance of differentiating gender from sex. In these conversations we would often discuss diversity within gender, while also at least partially conceding that sex was binary, with the exception of an often ill described or tokenizing inclusion of intersex individuals. Many of us knew things were more complicated, but we needed a simplistic argument in order to fight for our rights, be they feminist, lgbq, trans rights, etc. More recent understandings of biology have shown that sex is a lot more variable than once assumed. This made sense to me, but I often didn't have the education or ability to confidently argue things one way or the other. This has become especially difficult with how algorithms have become so horrifically bad at nudging folks towards accurate information, making one's attempt at research through a highly monetized search engine often futile. Furthermore, much of the discussion around sex is dominated by archaic patriarchal structures within sciences or by bad faith actors with no scientific credentials who defend dated and debunked ideas about sex and gender. I went into Agustín Fuentes' book Sex is a Spectrum with high hopes but realistic expectations. This book surpassed both of them.

Fuentes is a biological anthropologist and primatologist who writes in extremely enlightening ways on these topics. I enjoy thinking and learning about evolution, including how it affects the human species psychologically and anthropologically. Unfortunately, I often go into readings with gritted teeth, wondering if I'm going to encounter something useful or something ridiculous. Some ideas of what constitutes evolutionary adaptation/advantage are dominated by people with highly archaic, patriarchal, racist, and other oppressive ways of thinking. Examples include men who argue that rape is an biological imperative of men, men who argue that the reason violent and abusive men exist is because women choose them for procreation, and a wide gamut of racist and xenophobic beliefs about evolutionary contributions of various peoples. Fuentes not only broke from these sorts of traditions, but brought many new ways of thinking about humans into my worldview. It was very refreshing to see someone make statements in the realms of evolutionary anthropology and psychology that both criticize more oppressive mainstream misconceptions while also bringing newer information (at least to me and mainstream thinking) to the table.

Some might expect this book, despite it's clear focus on biology, to lean on of gender/queer theory a lot, but there is very little of it therein. It is a matter of fact book about what biology, anthropology, primatology, and cultural histories tell us about sex in human beings. It is refreshingly grounded in hard science while also being accurate and honest in terms of what statements can be made with higher levels of certainly. Furthermore, Fuentes writes in a style that can actually be read by a wide audience. He also has condensed a massive amount of argument and information into a rather short book. Basically, this book is accessible and readable which feels more critical than ever given how this topic is treated in the world these days.

Fuentes hits all of the topics that I've run into or questioned myself regarding the biology and (false) binaries of sex. These include everything from primate evolution, chromosomal variation, gender expression, sex assignment at birth, secondary sex characteristics, hormonal variation, environmental and cultural influence and interaction with biology (and vice versa,) behavioral differences between sexes and genders, diversity within sexes and genders in a variety of categories, and so on.

Within these topics, Fuentes' makes many well supported arguments. There are multiple ways that sex binaries are defined, ranging from chromosomal variation (which most of us never know we have,) physical growth and development, behavior and psychology, expression (or not) of bodily functions, and so on that very rarely fit into two distinct categories. Intersex individuals are not aberrations or afterthoughts, but normal parts of human sex variation and have a wide variety of traits and expressions. Dividing humans into two binary sex categories results in reductive, inaccurate, and all around bad science and medicine due to the massive variability and overlap of attributes tied to sex. Sex and gender are biocultural (meaning culture and environment interact with and influence biology and vice versa) which causes variability across the globe. There is much variability across sex in all primate species and humans are no exception. The helplessness of human infants resulted in evolution of cooperative, (bio and non) family structure that is not in line with archaic ideas many have about gender and family. There are no "male brains" and "female brains" (an argument I have unfortunately seen some trans folks latch onto) and the variability, diversity, and overlap in brain function cannot be applied to one sex or another. There are other well argued points, but you get the picture.

It is important to state that Fuentes is not at all making the argument that there are no differences between humans of various genders and sex assignments. I found the discussion of things like aggression and violence to be particularly interesting. He does not argue that everyone is the same and that patriarchal imbalance with abuse and violence does not exist. Rather he argues that claiming that this is a purely biological trait that men are predisposed to have is dangerous and inaccurate. Cultural influence combined with a variety of factors play into this far more than human sex biology alone does.

In the last section of the book, he speaks specifically about some of the topics where folks attend to onto binary thinking the most: Sexuality, family composition, medical research and practice, cardiac disease, organ transplants, pregnancy and birth, sports (wide ranging in and of themselves,) restroom assignment, and so on. He very clearly discusses where studies have shown differences and how well we are able to say where those differences come from. You may have already heard that it's not nature versus nurture, but how these two things interact. His discussions on the biocultural nature of sex took this idea to a new level for me as his emphasis on both biological and anthropological research really shows how damaging these binary categories are in many situations.

Things in life are generally far more complex than we like to admit and sex is no exception. There are much better ways to categorize people than along and inaccurate binary. For instance, all humans have testosterone in different amount and it can be more appropriate to categorize people based on testosterone levels than on sex.

I could continue blathering on, but I will stop here because I want everyone to go read this book. An optimistic take would be that people from a variety of belief systems, if willing to open their mind just a little bit, would be able to take in this book and be changed by it. I've been changed by it.

This was also posted to my storygraph and blog.

[Sherif Gerges · Rating 1 out of 5]

Surprisingly thin and awful. Given Fuentes’s position at Princeton, I expected careful engagement with the arguments, especially the scientific ones. Instead, this brief book leans on weak strawmen. Much of the “science” reads as reverse-engineered to fit a prior commitment to a particular view of human rights rather than argued on its own terms. For context, I’m a geneticist with a PhD in psychiatric genetics, so I guess I have something to say on the matter.

The book opens with what I take to be Fuentes’s greatest strength; surveying the diversity of sex biology across the animal kingdom, but the discussion meanders. The early chapters are initially intriguing, yet they seldom ask why these patterns exist, and the extended detour will likely lose non-anthropologists.

Soon we pivot to human biology and one of Fuentes’s inspirations, Simone de Beauvoir with her famous quote “One is not born, but rather becomes, a woman. No biological, psychic, or economic destiny defines the figure that the human female takes on in society.” It’s a powerful line, culture undeniably shapes identity, but taken to mean that womanhood is wholly a social invention or performance, it collapses. What’s puzzling is that elsewhere Fuentes (rightly) says gender and sex reflect an interplay of biology and culture, yet he endorses a statement that appears to deny any biological grounding for gender. Confusing.

Other claims are either wrong or epistemically careless about what we know and don’t know. On page 3 he writes, “we now know that human brains don’t come in ‘male’ and ‘female’ versions.” - but this is wrong. if you take almost any brain on earth, you can sequence the DNA and quickly determine if it belonged to a man or woman; does this not constitute a significant difference? If this is meant to imply that chromosomal context (XX vs. XY) is irrelevant to brain biology and disease prevalence, it’s simply misleading. We may not yet have precise mappings from chromosomal/hormonal differences to specific behaviors, but that is a statement about current limits of evidence, not evidence of absence. Rather than say “we don’t know yet,” the book jumps to sweeping conclusions.

This tendency shows up again when he gestures to “brain-imaging studies,” echoing Gina Rippon–style arguments. Neuroimaging is valuable, but it is also coarse and often underpowered for fine-grained questions; it’s not the last word on sex-linked neurobiology. The book doesn’t seriously engage functional genomics or genetic evidence, nor the simple fact that the brain is the most complex organ on Earth, sometimes not finding an effect reflects our tools, not the nonexistence of the phenomenon (just as pre-germ-theory medicine couldn’t “see” bacteria).

Finally, Fuentes wades into sports, and this is the nail in the coffin for me. not because I oppose inclusion (that’s for sports bodies to decide), but because Fuentes relies on rhetorical sleight of hand. He notes that the fastest-kicked goal in the 2023 Women’s World Cup exceeded the fastest in the 2022–23 Premier League, and attributes this to "training" (!!!!). But cherry-picking a single statistic doesn’t overturn the overwhelming distributional differences between men and women. The top recorded ball speeds (roughly ~130–210 km/h by one estimate) are all by men, and any Premier League side versus a top women’s team would almost certainly be a blowout. Training matters; but it doesn’t erase gaps in sex performance.

Most grating is the repeated move where he insists there’s “rarely a perfect one-to-one correlation” between biology and gendered experience. Of course, almost nothing shows a perfect correlation. That framing dodges the substantive point - biological sex remains the strongest predictor of phenomena like menstruation and average muscle strength. Setting “perfection” as the standard is a strawman, not a rebuttal.

[Becca McCracken · Rating 5 out of 5]

As a Catholic, I was almost afraid to read this book. However, I am an open-minded person who believes God made each person unique on purpose. I am always in a quest for knowledge so that I can better understand my fellow humans, especially those who's reality is far different than mine.

This book was very well written. It helped me realize that gender has not changed. Today, we just have better terms to describe how traits manifest in a person. There is a lot of really great science in this book. It helped me conclude that today's gender debate is truly more about fear and lack of understanding than it is about right and wrong.

Worth the read for both liberals and conservatives.

[Tutankhamun18 · Rating 3 out of 5]

This book is worth reading and has a great overall message but a few things that limit my rating.

Firstly the good:

It is a short book that captures a whirlwind tour through the basic reason for sex and gametes with different investment levels (anisogamy - species with differently sized gametes being produced by different individuals vs. isogamy) and how the rest of the organism does not come in two binary “types”. I liked learning that internal fusion of egg and sperm necessitate a womb, an environment where this can happen. I also liked learning about the concept of a 3G male and a 3G female plus gender. How this allows us to remove the assumption that animals, who cannot declare their gender or sex identity to us, fit into a binary based only on their external genitals. Also the fact that human babies are so much less developed than many other mammals upon birth and that raising them really was always a group effort.

Further the explanation of sex differences in height or other features and how this is usually “average woman height is shorter than average male height” but that the range of overall lap is incredibly high to the point where this distinction between women and men is arbitary and mainly there as a result of experiment set up. How removing the binary of women and men ccould reveal much more interesting things about the data. Also how the entire body is biocultural and influenced by a number of factors some biological and some cultural and that this cultural difference can also be part of what explains “differences between men and women”.

“Most research on human sex biology uses "sex at birth" as the key category for dividing up subjects by "sex" in the study. However, "sex at birth" usually means a designation based on a classification of the genitals of the newborn. There are usually no tests for chromosomal com-plements, gonad types, or anything else along those lines. As such, the "sex at birth" category is a little sloppy, and sometimes misses the mark. What "sex at birth" is hoping to measure is really what is called the 3G category of sex. The three Gs are genes, gonads, and genitals. A "3G female" is a human who has XX twenty-third chromosomes, ovaries, and a clitoris/vagina/ labia. A "3G male" is a human who is XY and has testes and a penis/ scrotum. The assumption is that a look at the genitals gives you a solid inference of the other 2 Gs and thus that this works as the classification system and the working definition of
"sex."? The rationale for using this categorization is that the 3 Gs are highly but not absolutely correlated with one another.
Thus, while 3G categories reflect a set of typically patterned variation related to reproductive biology, they are not even close to 100 percent accurate at capturing the actual range of human biological variation, even in relation to the 3 Gs."”

Organs are biocultural: “A lifetime of being in a particular body in a particular culture can shape organs in specific ways via patterns of endocrine exposures, pregnancy (or not), alcohol and drug use, social, economic, nu-tritional, and psychological differences in lived experiences of gender/sex, age, race, socioeconomic class, caste, and so on.”

The less good:

Despite these brilliant features of the book, it was a bit dry to read. In the sense of it lacked a bit of enthusiasm or personal investment/connection from the author. This made certain sections drag despite being full of really interesting information and concepts.

Another critique is the referencing. It seemed a bit sparse to me and often the same sources would be used multiple times, which for a book about biology is just a shame. Furthermore and perhaps more concerningly, the author cited their previous books so much: by far the most referenced source (sometimes in combination with other sources to be fair). While I enjoy when authors reference their own work and position themselves on a topic, I felt the author didn’t really position themselves within the text but only in the references and this raised some questions to me.

[Julie · Rating 3 out of 5]

I don't why but I did not expect this to be as scientific as it was. It was still an informative read but I was hoping for a bit of a mis of biological vs sociological look at Sex as a spectrum. But it you are looking for a in depth look at the biological side and know nothing beyond the basics this is a good starting read for you. Most of the science in the book is basic knowledge and there is nothing ground-breaking in the text. The only brief look at the social construct side of sex holds the book back a bit. It really shows just the science side which isn't the part of the sex as a spectrum issue that many fail to understand. It fails to looks at the complexities of sex beyond the science.

Despite that, this is still a worthwhile read and it is quite a quick and short introduction of the complex subject of sex.

[Alissa · Rating 4 out of 5]

This is a pretty dry read but the facts and science are straightforward and worthwhile. While there wasn’t a ton of commentary, I appreciated that that gave me space to reflect on the topic and have my own revelations on biology and how gendered expectations have impacted us as the human species. Recommended for people who read the Other Olympians and are curious about more of the science around sex and gender.

Thank you to Libro.fm for the free educator ALC.

[alex · Rating 4 out of 5]

I didn't find this as dry as other reviewers, but I wouldn't necessarily describe it as accessible, either. What it is is informative and timely, and I thought Fuentes' narration brought a clarity, lucidity, and sensitivity to the audiobook that was much appreciated.

Thanks to Highbridge Company and Libro.fm for the ALC.

[Cretino · Rating 2 out of 5]

Excelente livro pra quem não estudou biologia a fundo, especialmente pra turma que acha que sexo e gênero são preto no branco.
Dei nota baixa porque esperava mais aprofundamento e discussão do tema, mas ainda recomendo. Hehe

[Elizabeth · Rating 3 out of 5]

Rating: 3.5

Interesting read! It was definitely more scientific than expected but the author did a wonderful job at making the information feel accessible.

[Alysa · Rating 4 out of 5]

Important book which many people should read. Would have liked to see it wrapped up tighter in the last chapter.

[Jared Hamby · Rating 3 out of 5]

This book was interesting but so much of this subject is beyond my understanding that I often got lost in the weeds. The book just reiterated the fact that there is so much about this topic that I do not understand, so who am I to judge others or say what is or isn't normal or proper.

[Daria Bondar · Rating 2 out of 5]

Маніпуляція фактами. Було цікаво почитати про тварин though

[Carl Hjelmen · Rating 5 out of 5]

A quick read overall. As a biologist, I felt the amount of detail given was nice, especially with the inclusion of references. I can see how other reviewers felt it to be a bit dense in details, but it is not something I noticed in my reading. this is definitely a book I'll recommend to my biology students who are interested in the complexities of sex

[Elizabeth · Rating 3 out of 5]

My review is published in the American Journal of Human Biology here: doi.org/10.1002/ajhb.70136

[Laura · Rating 3 out of 5]

Every aspect of the human experience of sex biology is deeply biocultural and gender is central in that experience. An ova is not a woman and a sperm is not a man.
Sex IS a spectrum. This is not propaganda, or delusion. This is biological fact. (says this biologist with a PhD, albeit not in sex or gender-related studies). We know the fact that sex is a spectrum in other animals, where sex determinants are much more complicated, mixed, and at times reversible. Intersex individuals have been reported among the modern human population for a loooong time as well. And evolution does not align neatly into the "women as only-baby makers and caretakers" either. Related species tell us that. Archeological finds of extinct hominids suggests that. Our recent (as in, a few centuries ago) social norms suggest that--every country I've visited has some version of the saying: "It takes a village to raise a child," because it usually DID take a village to raise a child. Parents, grandparents, siblings, and neighbors all contributed to child rearing, all the time. So Fuentes is basing this book on facts, and states those facts mentioned above. His strongest writing was for me in the sections discussing sex for other animals, and their reproductive habits. However I wish each chapter had a clear TL;DR at the end, or better summaries in general. The writing was very dry and may make it hard for someone to finish this book. The discussion on human gender was very shy and didn't include discussions on how brain development may be affected by hormones in utero, nor did it expand on our current understanding of gender identity and neurobiology. Sometimes I wasn't sure the conclusions were correctly stated (there was a discussion on 3G males and incidence of certain mental disorders which should have included the fact that many brain-development genes are situated on the X chromosome, and that may explain why XY individuals have a higher rate of certain neurological conditions). Overall, I think Eugenia Cheng did a better job of introducing these concepts in her book "X + Y." I wanted to rate this book higher, but I found it at times confusing and too dry; also too scant on details in certain chapters.

[Maron Gerardo · Rating 5 out of 5]

Un libro excepcional, amé cada página. Me encantó el uso de categorías como 3g males y 3g females porque es más sencillo hablar de cuerpo adecuado y no de género. Es más fácil referirte a procesos biológicos que a procesos sociales. Te alejas de autor rellenos epistemológicos como decir "hombre" e imaginar una sería de consecuencias sociales y estereotipadas. Un libro muy claro y conciso. Para mí, este y Evolutions Rainbow de Joan Roughgarden son indispensables para entender la biología humana .

[Rachel · Rating 5 out of 5]

I'm surprised by the overall rating of this book! To me, it's a clear 5. It was an efficient and fact-based takedown of the sex binary in a way without getting too wordy. Definitely on the science-y side, but that's what is needed in this case! Handled sex vs gender in a really thoughtful way. I did it as an audiobook which may have allowed some of the more technical stuff to pass by without me getting stuck in the weeds. Very much recommend.

[Beth Green · Rating 4 out of 5]

This audiobook focused on scientific information that explains common misconceptions of binary definitions/designations. It was a little dry at times and required some focus to grasp all the biology, but it was generally accessible to readers and really interesting. It briefly discussed the social/cultural side of gender and sex, so additional reading should be paired with this science-focused book for a fuller understanding of the topic.

[Sarah · Rating 4 out of 5]

This was a well done scientific overview of the spectrum of sex and the common misconceptions of the binary definitions. While I wouldn't recommend this to someone brand new to the topic/concept of looking outside of the binary, I would recommend this to someone looking for something in the topic that specifically covers the 'science side' rather than culturally.

[Caroline Elizabeth · Rating 5 out of 5]

Per usual, Agustín’s writing is engaging and informative, and this book is a timely one. Backed by data (and lack thereof) and Agustín’s scientifically-trained interpretation, this work addresses common historical and contemporary arguments regarding sex and gender in the social, cultural, biological, and political spheres. I recommend this book to EVERYONE.

[Quedesmadre Saul · Rating 4 out of 5]

This read helps to see through the power of inculturation and interpretations that are passed through one’s environment. Along the way, Fuentes makes
It easy for the novice to understand the biology and science behind all the similarities between 3G XX and XY. Try to read this one and suspend your belief system and stick to the science explained therein.

[miki larson · Rating 4 out of 5]

The misconceptions we hold about sex and gender affect everyone in our society, not only those individuals that do not fit within the binary. Incredibly informative and thorough, this book helped me form a better understanding of the science and culture around sex.

[Chet Taranowski · Rating 4 out of 5]

A good summary of the research refuting the "binary" theory. Essentially males and females are more alike that it might seem at first glance. However the brief book gets a bit technical and demands either really close reading or previous knowledge of human biology.

[balonciega · Rating 4 out of 5]

solo leí los primeros 3 capítulos y el último porque había mucha información de humanos, y ciertamente me aburren un poco los humanos T_T
creo que le doy 3 estrellas pero un bonus porque me salteé capítulos

[Heather · Rating 3 out of 5]

Very science focused examination of sex. Only briefly explores the social component of sex/gender.

[Debs · Rating 4 out of 5]

3.5 stars

[francomedia · Rating 3 out of 5]

Note to myself: I need to read some parts again en español.

[Sara Goldenberg · Rating 2 out of 5]

It's well-written but I didn't actually learn anything. I'm fairly well-versed in the subject so that might be on me.