The Biological Basis of Gender Roles

Patriarchy is best understood as an emergent outcome of long-term evolutionary pressures.

Written by Marc J. Defant.

In recent decades, the idea that sex and gender are merely social constructs has spread far beyond university departments. From policy debates to classrooms to online discourse, the claim that biology plays little or no role in shaping male and female differences is often treated as a settled truth. Proponents argue that categories like “male” and “female” are fluid, that behaviors associated with masculinity and femininity are learned, and that any biological differences are insignificant or irrelevant. But how well do these assertions stand up to scrutiny?

Research in biology, neuroscience, psychology, and cross-cultural studies paints a rather different picture. While culture undoubtedly shapes how people express themselves, biology exerts a profound influence on sex, behavior and gender identity. Denying this reality means replacing science with ideology and undermining policies that promote fairness in sports, education and healthcare. What, then, does science actually say about sex and gender?

The origins of social constructionism

The idea that gender is socially constructed rose to prominence during the feminist movements of the 1960s and 1970s. Before that period, the word “gender” was mostly used in grammar to describe masculine and feminine articles or forms of words. Feminists argued that what societies call “masculine” or “feminine” traits are shaped by culture, not biology. Philosopher Judith Butler took this further, describing gender as “performative”. It is not something you are, but something you “do” through repeated behaviors and social rituals.

Other thinkers questioned whether even biological sex could be treated as fixed. Biologist Anne Fausto-Sterling proposed that sex exists on a continuum, citing intersex conditions as evidence. She even claimed that there are five sexes in addition to male and female–herms (true hermaphrodites), merms (male pseudohermaphrodites) and ferms (female pseudohermaphrodites). From this perspective, the familiar male-female binary is simply a social convention imposed by doctors, scientists and governments.

At its most extreme, social constructionism suggests that any claim to objective truth—including biological science—merely reflects the biases and power structures of those in authority.

Evolutionary psychology

Let’s start with first principles: What defines biological sex?

In species that reproduce sexually—including humans—sex is determined by the type of gametes produced. Males produce small, motile sperm. Females produce large, immobile eggs. This basic division is universal among mammals. Chromosomes direct the process: most males have XY chromosomes, and most females have XX. The genetic instructions guide the development of gonads, hormone production, and secondary sexual characteristics. Less we forget, sex exists for the purpose of reproduction. In fact, many intersex individuals are infertile (e.g., androgen insensitivity syndrome, Klinefelter syndrome, Turner syndrome, and ovotesticular DSD).

Some feminists claim that intersex conditions disprove the binary. However, intersex conditions are rare and typically result from developmental complications. They do not constitute a “third sex.” Estimates of intersex prevalence vary widely depending on definitions, but rigorous analyses suggest it affects less than 0.02% of births—far less than commonly claimed. This doesn’t diminish the humanity of individuals with intersex traits. But it does illustrate that exceptions don’t overturn the rule.

Evolutionary psychology provides a biological framework for understanding sex-based differences in human behavior, drawing on insights from biology (e.g., hormones and fetal development), neurology and economics. Central to this perspective is the fundamental asymmetry in reproductive investment: males produce vast numbers of sperm and can father multiple offspring in a short period, whereas females produce a limited number of eggs, endure pregnancy and frequently raise children with limited paternal support.

As a result, women tend to adopt more cautious mating strategies, while men tend to adopt more opportunistic ones. Although cultural and economic conditions shape how these predispositions manifest in modern society, evolutionary psychology emphasizes that biological constraints on reproduction continue to influence sex differences today. These differences, shaped over hundreds of thousands of years in hunter-gatherer societies, remain embedded in human nature. Modern culture has not existed long enough to fundamentally reshape these evolved traits.

The 300 percent increase in brain size among our hominin ancestors—from australopithecines roughly 4 to 7 million years ago to Homo sapiens—occurred in a remarkably brief span of geological time. While there many competing explanations for this expansion, what is clear is that the increase in brain size had profound consequences for infants and mothers. Because the birth canal can accommodate fetuses only up to approximately 60 percent of adult brain size, human infants are born neurologically immature. Roughly 25 percent of brain development occurs in utero, with the remaining 75 percent taking place after birth, demanding prolonged maternal care. This extended period of dependency contributed to the evolution of pair bonding and significant paternal investment—an uncommon pattern in mammals.

Sexual selection, a concept first proposed by Charles Darwin, offers additional insight into the evolution of sex differences. Darwin observed that certain male traits—such as the peacock’s extravagant feathers—appeared to provide no clear survival benefit and might even impose costs, such as increased visibility to predators. He identified two mechanisms of sexual selection that enhance reproductive fitness.

Intrasexual competition: Among male gorillas, for instance, size is a decisive factor in winning access to females. The victorious male passes on his genes, resulting in sexual dimorphism: male gorillas are 1.5 to 2 times larger than females. The result is a true patriarchy where large alpha-males control female fecundity.

Intersexual selection (mate choice): Female weaverbirds choose mates based on nest quality, prompting males to build elaborate structures. By exercising this selectivity, females increase the likelihood that their offspring will inherit beneficial traits. Female mate choice often favors indicators of paternal investment. As David Buss notes: “A woman in our evolutionary past who chose to mate with a man who was flighty, impulsive, philandering, or unable to sustain a relationship found herself raising her children alone and without benefit of the resources, aid, and protection that a more dependable mate might have offered.”

Men, however, also display strong preferences in mate selection. Male choice frequently prioritizes physical attractiveness, which typically signals health fertility, and genetic quality (a characteristic often denigrated by feminists as objectification). Traits such as facial symmetry, clear skin, and a favorable waist-to-hip ratio (usually associated with a low body mass index) are cross-cultural indicators of reproductive potential. While cultural ideals of beauty vary, these biological cues remain consistent across populatios.

Patriarchy, the pay gap and the glass ceiling

From the perspective of social construction, patriarchy is viewed as a sociocultural system in which men collectively hold dominant power and authority, while women are systematically marginalized. This framework sees gender hierarchies not as natural or biologically inevitable, but as socially constructed through institutions, norms, and cultural narratives that perpetuate male dominance.

The gender pay gap is frequently touted as the result of an oppressive patriarchy where men discriminate against women. However, research indicates that it largely reflects evolved behavioral strategies. Traits such as male competitiveness, risk-taking and resource acquisition, as well as female caregiving, social cohesion and mate selectivity, represent adaptive solutions to ancestral reproductive challenges. Today, these traits manifest in differences in occupational choices, hours worked, risk tolerance and preferences for work-life balance.

One of the largest contributors to the gender pay gap is occupational choice. Research consistently shows that men and women gravitate toward different fields, leading to significant wage differences. Men are more likely to enter higher-paying industries such as engineering, technology and finance, whereas women more often pursue careers in education, healthcare and social work—fields that typically offer lower pay but provide greater flexibility and job security.

When wage data are adjusted for industry and occupation, the gap narrows substantially. Harvard economist Claudia Goldin has demonstrated that men and women in the same jobs with comparable hours experience far smaller pay disparities, suggesting that career choices, rather than direct discrimination, account for much of the difference. Men are also overrepresented in physically demanding or hazardous occupations such as construction, oil and gas and logging—fields that offer wage premiums to compensate for risk. Women, by contrast, tend to avoid high-risk jobs, a pattern consistent with well-documented sex differences in risk tolerance.

Another major factor influencing the pay gap is the difference in hours worked and career continuity. On average, men work longer hours than women, especially in fields that reward extended availability (workers who can commit to long, unpredictable hours) with higher earnings. Men are more likely to accept overtime and pursue promotions requiring greater time commitments, which leads to compounding income advantages over time.

A Stanford study of over one million Uber drivers provides an illustrative example. Uber’s pay system is fixed, transparent, and algorithm-driven, minimizing the potential for managerial or customer bias. Yet men still earn about 7% more per hour than women. This gap is explained by three factors: men drive faster, take more late-night or high-crime-area shifts, and accumulate greater experience by working more hours. These findings highlight how even in neutral, bias-free platforms, behavioral and preference differences contribute to wage disparities. Merit simply trumps diversity.

Many women, particularly mothers, prioritize work-life balance and often reduce hours or exit the workforce temporarily to manage family responsibilities. These career interruptions significantly impact lifetime earnings. Negotiation also plays a role. Men initiate salary negotiations more frequently and assertively than women, resulting in higher starting salaries and faster wage growth. A Pew Research Center study found that women are more likely to modify their careers for family needs, often seeking roles with flexible hours or remote options—a pattern that aligns with ancestral caregiving roles

Indeed, women tend to prioritize other interests over careers (e.g., children and family). Difficulties getting tenure in universities for women compared with men are legendary. For example, on average, women have lower h-index scores (a metric used to measure both the productivity and citation impact of a researcher’s published work).

Historical social experiments, such as the early 20th-century Israeli kibbutzim, attempted to eliminate gender roles entirely. Men and women initially performed identical work, and children were raised communally. Yet over time, women gravitated toward child-rearing and less strenuous tasks, while men assumed physically demanding or leadership roles. Mothers rejected communal childcare, marriages formalized, and private family units re-emerged. Anthropologists observed that these changes arose not from male domination but from women’s own preferences.

The “glass ceiling” is frequently cited as a barrier preventing women from advancing into leadership roles. However, differences in risk-taking, negotiation and career prioritization arguably matter more. While social factors play a role, biological influences—such as the impact of testosterone on competitiveness—help explain why more men pursue and attain high-stakes leadership positions.

Research also challenges the view that discrimination is the primary driver of gender disparities. For example, a recent study found that nations with the greatest gender equality—such as Finland, Norway and Sweden—paradoxically have the lowest proportion of female STEM graduates, while less gender-equal nations such as Algeria and Turkey produce more. This “educational-gender-equality paradox” suggests that women in egalitarian societies feel freer to pursue personal interests, often choosing fields outside of STEM despite equal opportunity.

patriarchy is best understood as an emergent outcome of long-term evolutionary pressures. Over hundreds of thousands of years, women tended to prefer men who demonstrated traits that enhanced survival and reproductive success: reliable paternal investment, physical strength for protection and hunting high-protein game, and a willingness to take risks when necessary. These preferences, repeated across countless generations, gradually shaped male behavior and social structures.

Hormones and development

While chromosomes draw the blueprint, hormones execute much of the work that differentiates male and female bodies and brains. Testosterone is the primary driver of male puberty. It is a key hormone underlying the physical and behavioral divergence between human males and females, with its effects most evident during critical developmental periods. Before puberty, testosterone levels in boys and girls are generally below 2 nmol/L. At puberty, however, male levels surge to 7.7–29.4 nmol/L, while female levels remain low at 0–2.5 nmol/L. This sharp hormonal increase drives the development of secondary sexual characteristics in males, forming the basis of sexual dimorphism. Elevated testosterone levels were indirectly selected for by women in ancestral hunter-gatherer societies.

Testosterone also plays a crucial role during earlier developmental windows. The postnatal “mini-puberty” in male infants (1–6 months) and the prenatal testosterone surge at 8–24 weeks of gestation influence the organization of neural circuits that contribute to male-typical behaviors.

Beyond physical traits, testosterone shapes psychological and behavioral sex differences. Elevated testosterone in males is associated with aggression, dominance and risk-taking—traits linked to evolutionary pressures such as male–male competition. Evidence from congenital adrenal hyperplasia further supports testosterone’s role in early behavioral differentiation: females with prenatal testosterone exposure are more likely to display male-typical play behaviors, such as rough-and-tumble activity. Testosterone also enhances spatial reasoning, a domain in which males often excel.

The sharp post-pubertal divergence in testosterone levels between human males and females underpins the emergence of distinct physical and psychological traits aligned with ancestral evolutionary roles. These biologically grounded differences, reinforced by prenatal and early postnatal testosterone surges, challenge claims that sex exists along a continuous spectrum by instead highlighting a bimodal hormonal distribution.

In the context of athletic performance, testosterone’s role in promoting greater muscle mass and strength in males yields substantial physiological advantages, justifying the need for sex-specific competition categories. Collectively, these findings affirm that physical and behavioral sex differences are primarily rooted in biology and shaped by testosterone’s evolutionary function in optimizing male reproductive success.

By contrast, female secondary sexual characteristics are driven predominantly by estrogen and progesterone, which surge at puberty in females. These hormones promote the development of breasts, wider hips and increased subcutaneous fat—traits associated with increased brain size of the human fetus. Additional features, including higher-pitched voices and smoother skin texture, reflect evolutionary adaptations related to mate attraction and parental investment.

Brains are not Blank Slates

Beyond physical differences, male and female brains are organized in measurably distinct ways. Neuroimaging research shows that men, on average, have higher overall brain volume, particularly in regions involved in spatial processing, such as the parietal cortex and hippocampus. These capacities would have been critical for navigating large territories, tracking prey, and coordinating hunting strategies—tasks primarily performed by males in hunter-gatherer societies.

In contrast, women exhibit proportionally greater cortical volume in areas associated with language, social cognition, and emotion regulation, including the prefrontal cortex and parts of the limbic system. These regions support abilities essential for nurturing children, maintaining social cohesion, and interpreting nuanced emotional cues—skills central to the cooperative caregiving and foraging roles typically undertaken by females.

Functional brain studies further reveal that men tend to rely more on lateralized brain activity, favoring one hemisphere for specific tasks, which may support focused problem-solving during solitary tasks like stalking game. Women, meanwhile, show greater bilateral brain activation, allowing for integrated processing of emotional, linguistic and relational information—a likely advantage in social environments involving child-rearing and group coordination.

These neurological patterns align with observed sex differences in cognitive performance: males generally score higher on tests of mental rotation, spatial navigation and targeting accuracy, while females tend to excel in verbal fluency, emotional recognition and social communication. Importantly, these averages do not imply that all men or all women conform to strict stereotypes. However, their cross-cultural consistency strongly suggests they are not merely products of socialization, but rather the outcome of long-term evolutionary pressures.

The idea that sexual orientation is purely a social construct is also undermined by neuroscience and endocrinology. Brain imaging studies have identified structural differences in regions linked to sexual attraction. For example, part of the hypothalamus differs in size between heterosexual and homosexual men.

While scientific evidence for biological sex differences has accumulated, some professional organizations have moved in the opposite direction. The American Psychological Association (APA) has published guidelines describing traditional masculinity as potentially harmful. Critics point out that most of the relevant traits are actually adaptative. Risk-taking and competitiveness, for example, helped males secure resources and protect their communities. Emotional restraint can be an asset in dangerous or high-stakes situations. While it is true that men are statistically more prone to aggression, they are also deeply capable of protection, sacrifice and nurturing—qualities that make them not only effective guardians but also devoted and caring fathers.

Why it matters

The question of whether sex and gender are biological is not merely academic. It affects real lives and policies. In sports, it influences eligibility and fairness. In medicine, it guides treatment decisions. In education, it shapes how we approach learning differences and developmental needs. Acknowledging biological differences does not imply inequality of worth or capability. It simply recognizes that men and women are, on average, different in ways that matter. Denying these differences doesn’t eliminate them—it only makes it harder to design effective systems, while potentially discriminating against males.

There is no question that society and culture shape how we express our identities. But biology lays the foundation. Sex is binary. Gender differences, while variable, have consistent biological roots. Sexual orientation shows clear markers of genetics. A balanced approach respects individual dignity without discarding scientific reality. When institutions elevate ideology over evidence, they risk undermining trust in science itself.

Marc J. Defant is a professor of geology/geochemistry at the University of South Florida. He has written for Skeptic and Popular Science, and was a guest on the Joe Rogan podcast.

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