Nathan H. Lents on Gender Diversity, Evolution, and the Science of Sex

Author(s): Scott Douglas Jacobsen

Publication (Outlet/Website): The Good Men Project

Publication Date (yyyy/mm/dd): 2025/02/22

Nathan H. Lents, a biologist at John Jay College, discusses gender diversity in biology, arguing that sex is a spectrum rather than a rigid binary. He critiques the ideological resistance to this perspective, particularly among older scientists, and emphasizes that natural selection depends on variation. Lents draws parallels between opposition to gender diversity and creationist arguments, highlighting intellectual rigidity in both. He advocates for a scientific approach that acknowledges diverse sexed bodies and behaviors. The discussion explores how human sexuality evolved over millions of years, challenging cultural assumptions about monogamy and gender roles while promoting adaptability as a key evolutionary advantage.

Scott Douglas Jacobsen: I want to make sure I get my facts correct here. Today, we are here with Nathan H. Lents.

He is an American scientist, author, and professor of biology at John Jay College of Criminal Justice, City University of New York. He specializes in forensic science, genetics, cell biology, and evolutionary biology. He has written several books, including Not So Different: Finding Human Nature in Animals and Human Errors: A Panorama of Our Glitches, from Pointless Bones to Broken Genes.

His latest book, The Sex Factor: How Evolution Influences Human Behavior, explores the evolutionary history of sex, gender, and relationships. He is a fellow of the Committee for Skeptical Inquiry, a well-regarded organization. Lents advocates for science communication and critical thinking and regularly contributes to media outlets and public discussions on human evolution. Thank you very much for joining Small-Town Canadians. I appreciate it.

Prof. Nathan Lents: My pleasure. Thanks for the invitation.

Jacobsen: You’re welcome. So, first question: Your recent article, “Get Gender Ideology Out of Biology!,” critiques ideological influences in biological science. This topic has come up in some secular humanist and related community discussions recently.

It made a brief appearance in the news and then faded away. What prompted you to write this piece? And what are the key misconceptions the public holds regarding discussions about sex and gender?

Lents: To be honest, the title of that article was intended to provoke a reaction from certain groups. There is a common perception that progressive views on gender are based on feelings and ideology, while science and biology stand in opposition.

I am pushing back against that idea because there is strong scientific support for the notion that gender diversity is a natural phenomenon. Gender expression exists in other animals, and sex is not strictly reducible to gametes and binaries. 

Sexual characteristics appear in a wide range of forms, and defining maleness and femaleness is more complex than people often assume. While these complexities may not affect daily life for most people, they are crucial to understand from a biological perspective.

In reality, categorizing sex and gender with strict definitions and rigid boundaries is problematic because that is not how nature works. Life does not conform to binary classifications — it moves fluidly in and out of categories. Diversity, creativity, flexibility, and adaptability are the true hallmarks of life.

Strict definitions with hard boundaries are human constructs. That does not mean they are wrong or useless, but we must recognize that we created them.

And when we encounter biology that challenges definitions, the answer is not to call the biology wrong or aberrant but rather to rethink our definitions. People get freaked out by this because we have these cherished institutions and ideas. But we revise our understanding of many things as we learn more.

That’s what you do in intellectual pursuits. The more you learn, the more you have to revise your understanding. I can’t think of any scientific discipline where things did not turn out to be far more complicated than we initially thought.

That is the history of Western thought over the last 400 years. Every single subject we sought to understand became significantly more complex the deeper we investigated it. And it is fitting that gender, sex, and sexuality are finally receiving that same level of scrutiny.

We are now recognizing that these concepts are complicated, diverse, and messy — that they shift between categories without regard for our preconceived notions. At present, conservative reactionaries are trying to convince people that they have science on their side while portraying gender activists as driven purely by feelings and ideology. That is the narrative they push.

But I push back on that. There are strong scientific reasons to view gender as encompassing more diverse categories and expressions, both in humans and animals. When we observe animal behavior, we see how poorly it fits into two strict categories — male and female — so it would be unusual if humans were any different.

It was ideological bias that prevented us from understanding monogamy for centuries until DNA-based paternity testing forced us to reconsider what monogamy means in animals. Ideology also led us to ignore the diverse ways animals engage in reproductive behaviors. That was ideology at work.

So yes, I do want to remove ideology from biology — but that takes me in a different direction than some biologists. Some believe everything is reducible to two categories: male and female. I disagree with that, both in terms of the morphology of sexed bodies and in behaviors.

Jacobsen: Now, could the title imply getting gender ideologies out of biology in the sense that multiple ideological camps are approaching these issues differently, influencing how we interpret even those stances? From a broad perspective, what are the ideologies positioning themselves as the arbiters of truth? And where has science shifted the goalposts of those definitions, at least slightly — if not significantly — toward a more realistic perspective?

Lents: That’s a good point, and you are right that multiple ideological camps are involved. The biggest problem has been our historical tendency to view things strictly through a binary lens. That ideological perspective still dominates, and moving beyond it would benefit us significantly.

When people argue that sex should be seen as more of a spectrum, they are not claiming that everything we understand about males and masculinity is wrong or that femininity and femaleness do not exist. That is not the point. What it means is that sex exists on a continuous spectrum with two major peaks.

If you visualize a bell curve with two humps, that is a bimodal distribution. A good way to illustrate this is by considering a sex difference such as height.

Height — most men are taller than most women, right? So you have an average height difference between men and women. Those two peaks are distinct, but there is a significant amount of overlap.

There are some tall women. There are some short men. But overall, you still see two different peaks with some individuals falling in between.

Now, apply that same concept to basal metabolic rate. That is another significant difference in our bodies. On average, men have a much higher basal metabolic rate than women — typically by about 400 to 500 calories per day. That is not an insignificant difference.

But again, it exists on a spectrum. There are individuals who do not fit within their sex-typical category. Take red blood cell count or white blood cell count, for example. Men tend to have more red blood cells, while women tend to have more white blood cells. But there are people who do not fit neatly into those categories.

You can observe this pattern throughout the body — in different organs, tissues, hormone levels, and other biological traits. You can see it in testosterone levels, in estrogen levels, in the size of the corpus callosum, in the ratio of white matter to gray matter in the brain, and in the preoptic area of the hypothalamus.

When you examine all the sex differences in the body, they consistently form a spectrum — with average differences but significant overlap. This tells us that sex differences do not fit into two neatly defined categories. There are always individuals who fall somewhere in between.

Some individuals exhibit sex-atypical traits. And if you examine enough biological markers, most people will have at least one trait that places them outside their expected category.

Some men, for example, develop breasts. They may not be large, but they are larger than the breasts of some women. Some men have wide hips. Some women have strong upper-body musculature. There are always individuals who do not fit perfectly into either category based on at least one measure.

In reality, we are all mosaics of sex differences. No one fits perfectly into their designated category across every biological trait.

Why does this matter?

If you define sex purely by which gamete an individual produces, you are centering sperm and egg production over all other aspects of biological sex. Yet, all the other aspects of an individual’s biology are far more relevant — to themselves, to those around them, to their daily lives, and to their health.

For example, women develop lupus nine times more often than men. That has nothing to do with sperm or eggs. Men develop Parkinson’s disease two to three times more often than women. That also has nothing to do with sperm or eggs.

Whether it is heart attack risk, stroke susceptibility, or certain cancers, sex-based differences exist throughout the body. Even men can develop breast cancer. And if we reduced their classification to “sperm-producer,” we might dismiss breast cancer as irrelevant to men.

In fact, because male breast cancer is often overlooked, it tends to be more lethal when it does occur.

Recognizing that sex exists on a spectrum reinforces the idea that biology and sex-influenced traits permeate every aspect of our bodies. Every cell, tissue, and organ matters when considering sex differences — not just sperm and egg production.

Reducing sex to gametes ignores the rest of human biology. Another important factor is that there are individuals who are intersex — those who do not fit neatly into a sex-typical category for most biological measures or who fall somewhere in between. Discussions about sex differences often focus primarily on genital anatomy, since that is what most people fixate on.

To deny that intersex individuals straddle the categories of maleness and femaleness is to exclude them from our sex-based classification system entirely. That strips them of dignity, value, and respect. The vast majority of intersex people are perfectly healthy and fertile, and they are important members of society. Even if we adjust our understanding of sex categories solely for the benefit of this small minority, it is still worth doing.

That is why I argue that viewing sexed bodies as a spectrum aligns more closely with the biological realities that matter to us. And this is not just the case in humans — it applies to other animals as well. Many species include individuals who do not conform to the expected appearance, behavior, or physiology associated with the gametes they produce. To ignore this diversity is to overlook crucial aspects of these animals’ roles in their social ecologies.

In many species, minority sex variants play essential roles in maintaining social balance. That is one of the main themes of my book — it explores the various ways animals can be male and the various ways they can be female. It is not just about sperm and eggs; it is about everything they do with their bodies.

Jacobsen: Sexuality has evolved over several hundred million years. That is a long time — though I am told that about 1/5 of Canadians believe the maximum is only 10,000 years.

Lents: Yeah, that number is surprisingly high.

Jacobsen: I know. Iceland is the only country where, at least several years ago, surveys showed that no one under 30 held that belief. Either the world was created outright, or — my personal favorite, which I think is both more accurate and more hilarious — it was all designed by a committee. All these trade-offs, negotiations, and inefficiencies make more sense when you think of nature as the result of committee decisions.

Lents: It does, yes.

Jacobsen: That idea is only one step away from saying that nature itself is just a series of committee meetings. But back to the main point — sexuality has evolved over hundreds of millions of years. As far as I understand, and I am not a specialist, Homo sapiens speciated around a quarter of a million years ago.

Lents: Fully anatomically modern humans appeared around 200,000 years ago.

Jacobsen: Evolution takes a long time to speciate, depending on reproductive cycles. So what about human sexuality? How does this larger evolutionary story — spanning several hundred million years, say 500 million — connect to the story of the last 200,000 years of human existence?

Lents: That is a great question because a central thesis of my book is that we are a product of our evolutionary past. Understanding our natural history helps us make sense of the present. One of the fascinating patterns we see across human cultures worldwide is the presence of enduring themes in social and sexual behavior.

I am particularly interested in pre-agrarian societies — those that existed before farming. The transition to farming and settled life set us on a different trajectory, one that distanced us from our natural biology. Instead of following purely biological instincts, we began constructing a social biology, and from that point, cultural evolution took off at an accelerated pace.

However, if we look at foraging societies today, we see an incredible diversity in how they live. They inhabit different environments and construct their family structures in unique ways, but they still share some commonalities. One notable pattern is the tendency to form dyads — mating pairs of two individuals. While not strictly universal, this pattern is widespread, which suggests that humans have a natural inclination toward pairing up.

But what we do not commonly see is that these dyadic pairings necessarily lead to strict sexual monogamy. Many cultures practice pair bonding without exclusive sexual fidelity. In some cases, pair bonds are temporary, while in others, they exist within polygynous or polyandrous systems, meaning they are not always one-to-one relationships. The notion of permanent, lifelong, sexually exclusive marriage is actually a cultural peculiarity. Only a small minority of societies have historically adhered to it as a strict norm.

What does this tell us? It tells us that humans are highly adaptable in how we form families and pursue sexual relationships. Flexibility in social and sexual structures is a defining feature of our species. Unlike some animals that follow rigid reproductive strategies, humans do not have strict, predetermined ways of organizing their social lives.

In many aspects of biology, certain species exhibit much less flexibility, but humans are the ultimate generalists. This adaptability is evident not just in relationships but in all aspects of our existence. We can survive in an astonishing variety of environments. We can thrive on vastly different diets. We can use our bodies in countless ways to suit our surroundings. We are not designed for any single way of living — we are designed to be generalists. That adaptability has been one of our greatest strengths.

Jacobsen: So David Deutsch was right.

Lents: What’s that?

Jacobsen: So David Deutsch was right — we are the universal constructors.

Lents: Yeah, exactly. We construct our own environments. That is what our big brain is for — to manage complex social relationships and to navigate new situations. A significant part of our intelligence is dedicated to maintaining social bonds, but another critical function is our ability to enter an unfamiliar environment and figure it out.

Unlike other species, we do not have to wait for biological adaptations through mutations and natural selection. Instead, we actively engage with our surroundings, determine what needs to be done, and then pass that knowledge along to others. That is culture — the collective intelligence shared within a community.

Given how adaptable and flexible we are in other areas of life, why would those same tendencies not apply to our sexuality and gender? I am not saying that everyone is inherently pansexual or polyamorous or that modern perspectives on sexuality are just anomalies of contemporary culture. However, if you had been born in a different time and place, you would likely think differently about your sexuality and your relationships.

A great deal of what we perceive as the “natural” or “normal” human condition is simply cultural inheritance. That does not necessarily mean it is bad — some aspects of cultural tradition are beneficial — but it also does not mean it is some sacred, predetermined destiny. The idea that our current sexual norms are inevitable is simply not true.

If you look at foraging societies worldwide, they approach sexuality differently. Many exhibit a general openness toward sexual behavior, far more permissive than what modern societies consider acceptable. Sexuality in hunter-gatherer cultures is often more fluid, with greater acceptance of premarital and adolescent sexual exploration. In many of these societies, it is believed that gaining sexual experience is essential for becoming a well-adjusted adult. Imagine the radical notion that you improve at something through practice.

I wish modern culture would take more time to critically examine institutions handed down to us as though they were sacred and unchangeable. Many of them are arbitrary — they could have developed in a completely different way.

Jacobsen: I interviewed Michael Behe years ago and asked almost the similar questions to Kenneth Miller because that was fun and interesting. You have engaged with intelligent design creationists — who often resist being labeled as such, despite adhering to the same philosophical and epistemological framework.

When engaging with ideological perspectives, whether in politics, social issues, or scientific discussions, do you notice any patterns in how these interactions unfold? Specifically, do you see similarities between engaging with intelligent design proponents and addressing ideological debates about gender and biology?

Intelligent design creationists are more obvious in their approach, especially given that many of them were associated with the Wedge Strategy when it was first introduced. At that time, they were largely middle-aged and older American men of European descent, typically Protestant and often evangelical — except Behe is Catholic and Berlinski is agnostic–highly educated — all of them. So do you notice any commonalities in those interactions?

Lents: I do. When people become deeply attached to a particular way of thinking or an ideological framework, it makes them intellectually inflexible. I see that pattern among intelligent design proponents, but I also see the same rigidity in those who resist the idea of gender diversity.

And the fact that they are mostly older is not a coincidence. It is well-documented that as we age, we become less open to new ideas. That is a well-established psychological principle.

Here is a true story. Last week, I gave a talk at Washington University in St. Louis — a prestigious institution — to their evolutionary biology group. In my presentation, I invited them to consider gender diversity in the study of animal behavior. I presented multiple examples, using their own data, to demonstrate that they were missing a simpler explanation: gender diversity. Instead of acknowledging this straightforward framework, they had relied on more convoluted explanations, such as female mimicry or imperfect mate choice.

Many of the prevailing explanations for observed behaviors in animals are unnecessarily complex when gender diversity offers a much more intuitive answer. So, I laid out these examples for the audience.

In the room, there were about seven or eight faculty members — all older, with white hair and wrinkles — who furrowed their brows and sat in silence. You could tell they did not agree, but they also did not challenge me. They did not ask questions, they did not critique my argument, they just sat there, unmoved.

Meanwhile, the rest of the room — about 30 to 40 younger attendees, including postdocs and graduate students — were nodding their heads and snapping their fingers in agreement. They understood the concept immediately because they were open to it. They could see how the framework of gender diversity provides a clearer and more consistent explanation for animal behavior.

It really is a simpler way to interpret these observations — accepting that there is more than one way to be male and more than one way to be female. That explanation aligns with the data and follows Occam’s razor, which tells us that the simplest explanation is usually the correct one.

I do see a generational divide. Older biologists tend to be committed to rigid categorical labels and definitions that constrain diversity into simplistic frameworks. In contrast, younger scientists are much more open to the idea that diversity is an inherent feature of sexuality, not an anomaly.

Sexual diversity, like all other aspects of biology, is shaped by adaptability, flexibility, mutation, diversity, and creativity. These characteristics define biological systems. Once this diversity is introduced into nature, natural selection begins to operate on it.

Natural selection requires variation. Many people focus solely on selection, but they forget that selection cannot function if every organism is identical. Selection only works when variation exists. In fact, variation is the fundamental starting point of evolution by natural selection. You must first have variation, and that variation must be heritable. Only then can selection act upon it to determine the direction evolution will take.

We need to acknowledge that diversity in sexed bodies and gendered behaviors is a natural part of life. It is expected. It provides ecological and social resilience to a species. We already understand that diversity is the best insurance policy against an unpredictable future. We recognize this principle in all other areas of biology — physiology, survival strategies, and behavior.

Yet, for some reason, many resist applying the same logic to sexual behaviors and sexual morphologies. I am inviting biologists to approach the study of diverse sexed bodies and gendered behaviors with the same openness they apply to other aspects of biological research.

Jacobsen: Professor Lents, thank you very much for your time today. I appreciate it.

Lents: It was nice to meet you. Let me know when this goes live.

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