Stewart-Williams reveals why gender equal societies show wider sex differences

Human self-understanding often stalls at the boundary lines of polite conversation. Few topics trigger this intellectual freeze quite like the scientific investigation of sex differences. When we look at the history of science, the reluctance is understandable. In the nineteenth century, scholars like Gustave Le Bon used biological arguments to dismiss female intellect, famously writing that highly accomplished women were as rare as a two-headed gorilla. That legacy of prejudice left a lasting scar. It created a defensive posture that treats any talk of evolved sex differences as a backdoor attempt to justify inequality.

But avoiding facts does not foster justice. Evolutionary psychologist Steve Stewart-Williams, in a deep dialogue on Modern Wisdom hosted by Chris Williamson, argues that modern evolutionary science acts as an antidote to historical sexism, not a continuation of it. By mapping the real, often modest psychological variations between men and women, researchers replace crude caricatures with precise, empirical data. Understanding the evolutionary architecture of our minds is not about keeping anyone in their place. Rather, it is about developing an accurate map of human nature so we can build a society that respects people as they are.

The Allure of Cross-Disciplinary Insight

For those who fall in love with evolutionary psychology, the experience mirrors a childhood realization of how the phases of the moon work. Suddenly, disparate, confusing fragments of human behavior click into a coherent framework. This intellectual awakening often traces back to foundational texts like The Moral Animal by Robert Wright. This book, which transformed the careers of both Williamson and Stewart-Williams, demonstrates how evolutionary theory serves as a unifying lens, linking biology, anthropology, and psychology.

Yet, the public conversation surrounding these insights remains deeply tarnished. A critic-preparedness mindset immediately labels any fascination with sex differences as a sign of latent prejudice. This accusatory tone prevents us from looking at our species with honest curiosity. It blocks the path to self-knowledge, leaving us with a shallow, purely ideological understanding of why we act, love, and compete the way we do.

Decoupling the Definitional Binary from Variable Traits

To understand the evolution of sex, we must first clear away the semantic confusion that dominates current debates. Biology defines sex through a single, elegant parameter: gamete size. This is a definitional truth across the vast majority of sexually reproducing species, a phenomenon known as anisogamy.

The Mechanics of the Gamete Barbell

In anisogamous species, reproduction relies on two distinct reproductive cells:

• Large Gametes (Eggs): Richly provisioned with nutrients to ensure offspring survival, but expensive to produce and limited in number.
• Small Gametes (Sperm): Highly mobile, stripped of excess baggage, designed purely for speed and localization, cheap to produce in massive quantities.

This division represents an evolutionary barbell. Medium-sized gametes are evolutionary dead ends. They lack the mobility of sperm and the nutrient payload of eggs. Consequently, selection pressures push reproducing populations away from the unstable middle ground (isogamy) and toward the two extreme endpoints of egg and sperm production. This biological binary underpins the entire natural history of sex.

Variation Within the Binary

While the biological definition of sex is binary, the physical and behavioral traits (phenotypes) that branch out from this foundation are highly variable. We see massive within-sex variation in height, vocal pitch, risk-taking, and nurturing instincts. To point to a tall woman or a shorter man as a refutation of average sex differences is a fundamental category error. Variation does not erase the reality of biological generalizations; it is the very raw material upon which natural selection acts.

The Six Pillars of Evolved Differences

How do we know that psychological sex differences are not purely the product of cultural socialization? Sociocultural theorists argue that parents, media, and peer pressure shape young minds into gendered roles from infancy. While evolutionary psychologists do not deny the power of culture, they point to six distinct lines of evidence that suggest a robust, unlearned biological contribution to human behavioral sex differences.

+-----------------------------------------------------------------+
|              THE SIX PILLARS OF BIOLOGICAL EVIDENCE            |
+-----------------------------------------------------------------+
|  1. Early Lifespan Emergence                                    |
|     - Traits appear before significant socialization occurs.     |
|                                                                 |
|  2. Resistance to Cultural Suppression                          |
|     - Differences persist despite active efforts to damp them.  |
|                                                                 |
|  3. Long-Term Historical Stability                              |
|     - Core psychological profiles remain steady over centuries. |
|                                                                 |
|  4. Prenatal Hormonal Correlates                                |
|     - In-utero testosterone levels predict future behavior.     |
|                                                                 |
|  5. Cross-Cultural Universality                                 |
|     - Patterns repeat across wildly diverse global societies.   |
|                                                                 |
|  6. Phylogenetic Continuity                                     |
|     - Shared behavioral patterns appear in related primates.    |
|                                                                 |
+-----------------------------------------------------------------+

Early Lifespan Emergence and Cultural Resistance

Many sex differences show up long before cultural socialization can take root. Differences in risk-taking and physical aggression emerge as soon as toddlers are mobile. From early childhood, boys end up in emergency rooms for risk-related injuries at far higher rates than girls.

Furthermore, these differences often persist despite active cultural pressure to suppress them. Almost every human society actively discourages physical aggression in young males, yet post-puberty, male physical violence spikes universally. Teachers and parents often reprimand boys for aggression more than girls, not out of bias, but because boys exhibit the behavior far more frequently.

The Hormonal Underpinnings of Behavior

Our endocrine systems provide a clear bridge between biology and behavior. Levels of prenatal testosterone exposure in the womb correlate with specific behavioral trajectories later in life, regardless of biological sex.

This connection is clearly illustrated in females born with Congenital Adrenal Hyperplasia (CAH), a condition that exposes developing fetuses to unusually high levels of prenatal androgens. As children and adults, girls with CAH consistently exhibit more male-typical traits. They show a preference for mechanics and things-related toys over dolls, report lower average interest in traditional parenting, and are significantly more likely to identify as bisexual or lesbian.

Cross-Cultural Patterns and Animal Comparisons

If culture were the sole author of sex differences, we would expect to find at least one society where women commit the vast majority of physical violence, or where men do the bulk of infant care. We find neither. Across all nations where data is collected, males commit over ninety percent of homicides.

This pattern is not unique to humans. When we look at our closest living relatives, chimpanzees, the numbers are virtually identical: male chimpanzees commit ninety-two percent of lethal violence, and represent seventy-three percent of the victims. This deep phylogenetic continuity suggests that our behavioral profiles are anchored in a shared evolutionary heritage.

The Evolutionary Engines of Divergence

To explain why these differences evolved, we must look at the concepts of reproductive variance and parental investment. Most psychological variations between the sexes stem from a fundamental asymmetry: the difference in maximum potential offspring.

Reproductive Variance and the Battle for Status

Because sperm are cheap and eggs are costly, the ceiling for male reproductive success is astronomically higher than for females. A single male can theoretically father hundreds of children, while a female is biologically constrained by gestation, birth, and lactation. This difference creates a higher reproductive variance, or skew, in males.

In our ancestral past, a small percentage of highly successful males fathered a large portion of the next generation, while a significant number of males fathered none at all. For females, reproductive success was much more evenly distributed. This high-stakes lottery selected for male traits like extreme competitiveness, physical risk-taking, and status-seeking. To lose the competition for status was, in evolutionary terms, to face genetic extinction.

Parental Investment and Mate Choice

Robert Trivers refined this understanding by showing that the sex making the larger mandatory investment in offspring (usually the female) becomes the limited resource for which the other sex competes. Because women face the heavy physical demands of pregnancy and lactation, they evolved to be highly selective when choosing mates.

In contrast, the minimal biological investment required from a male to produce an offspring—a single sex act—favors a stronger baseline interest in casual sex and sexual variety. This behavioral difference is reflected in modern data. The trait of sociosexuality (no-strings-attached sex) consistently shows a large sex difference, with men scoring roughly one standard deviation higher than women on average.

+-----------------------------------------------------------------+
|                  PARENTAL INVESTMENT ASYMMETRY                  |
+-----------------------------------------------------------------+
|             FEMALES             |             MALES             |
+---------------------------------+-------------------------------+
|  - High obligatory investment   |  - Low obligatory investment  |
|    (gestation, lactation)       |    (single sex act)           |
|  - Lower reproductive ceiling   |  - High reproductive ceiling  |
|  - Low reproductive variance    |  - High reproductive variance |
|  - Selected for selectivity and |  - Selected for competition,  |
|    resource-seeking             |    risk, and status-seeking   |
+---------------------------------+-------------------------------+ 

The Paradox of the Egalitarian State

Perhaps the most surprising finding in modern evolutionary sociology is the gender equality paradox. Traditional social roles theories predict that as a society becomes more egalitarian, less patriarchal, and more supportive of gender neutrality, psychological sex differences should shrink and eventually disappear.

The empirical data reveals the exact opposite. In highly progressive, affluent, and gender-equal nations like those in Scandinavia, sex differences in personality traits, spatial rotation abilities, and career preferences are significantly larger than in less egalitarian nations.

The Freedom to Diverge

This paradox makes perfect sense under an evolutionary lens. In poorer, more traditional societies, survival demands take precedence over personal preferences. A woman in a developing nation may pursue a engineering degree because it is her only path to financial security, regardless of her intrinsic interests.

However, when a society removes financial barriers, provides a robust social safety net, and actively encourages free choice, people are finally clear to do what they actually want to do. Under these conditions, the subtle, biological tilts in our interests are not overridden by economic necessity. Men and women diverge naturally because they have the freedom to do so. The push for absolute fifty-fifty representation in every career field ignores the reality that equal opportunity does not lead to equal outcomes when baseline preferences differ.

The Dual Edge of Risk and the Danger of Denial

Ignoring or denying these evolved differences carries real societal costs. When we attribute every occupational gender gap entirely to systemic bias, we create a culture of unnecessary resentment. We launch costly, coercive interventions that fail to achieve their goals because they target the wrong causes, occasionally resulting in reverse discrimination.

The Lifespan Penalty of Male Biology

Consider the realm of physical health. Men consistently die younger than women in almost every culture. Much of this longevity gap is driven by testosterone, which builds upper-body strength and status-seeking drives but also weakens the immune system and increases cardiovascular risk.

Historically, eunuchs in the Korean Chosen Dynasty lived significantly longer than intact males of the same social status, showing the direct biological cost of male hormones. This risk-taking profile cuts both ways:

• The Downside: Men make up eighty-eight percent of Darwin Award winners, dying in spectacular acts of self-inflicted stupidity.
• The Upside: Men also receive ninety percent of Carnegie Hero Awards for risking their lives to save strangers.

If we could lower male mortality rates to match female rates, we would save more total years of life than if we cured cancer entirely. Yet this massive health disparity receives remarkably little public attention because we are conditioned to view gender issues solely through a lens of male privilege.

The Pitfalls of Unisex Expectations

Denying sex differences also harms clinical outcomes. Autism presents differently in girls than in boys, often lacking the repetitive behaviors that doctors use for diagnosis. Because autism is viewed as a predominantly male disorder, thousands of girls go undiagnosed and lose access to early intervention.

Similarly, heart attacks present differently in women, often causing shortness of breath rather than the classic shooting arm pain. Minimizing these physical realities puts lives at risk. An intellectually honest society must learn to hold two thoughts in its mind at once: we can treat men and women with equal respect and dignity, while fully acknowledging the beautiful, complex biological differences that make us human.