374 - The evolutionary biology of testosterone: male development & sex-based behavioral differences

This episode features Carole Hooven, an expert in evolutionary biology with a focus on testosterone and sex-based behavioral differences. The conversation delves into the biological and evolutionary underpinnings of sex differences, particularly the role of testosterone in male development, the hormonal influences on behavior, and how social dynamics interact with biology. Topics include embryological sex differentiation, hormonal fluctuations across the lifespan, behaviors shaped by testosterone such as aggression and play, the impact of modern society on these evolutionary drives, and the cultural challenges surrounding discussions of sex differences.

Early Interest and Evolutionary Perspective

Carole Hooven's interest in testosterone began growing up in a family with three older brothers, driving her curiosity about male behavior and what motivates it compared to females. This personal background, combined with extensive travels—especially experiences in culturally distinct environments like Egypt, and time spent studying primates in Uganda—triggered her fascination with the evolutionary origins of human behavior. Encountering profound cultural variations and observing chimpanzee sex differences, which often parallel human patterns, helped her appreciate that sex hormones like testosterone play fundamental roles not only physiologically but also behaviorally, rooted deeply in reproductive strategies.

Sex Determination and Embryological Development

The podcast explains that sex determination in mammals, including humans, begins with chromosomal differences where females are typically XX and males XY. However, chromosomes define sex but do not wholly determine phenotypic sex, which is influenced heavily by hormones. Early in embryonic development, gonads are undifferentiated and can develop into either testes or ovaries. The SRY gene on the Y chromosome in XY embryos triggers testes development around five to six weeks of gestation. These testes then produce hormones, particularly testosterone and anti-Müllerian hormone, which guide the masculine development of internal ducts and external genitalia.

A major emphasis is on the role of testosterone conversion to dihydrotestosterone (DHT) via the enzyme five-alpha reductase. DHT has a much stronger affinity for androgen receptors and is crucial in masculinizing external genitalia like the penis and scrotum. Cases of five-alpha reductase deficiency illustrate how important DHT is; affected individuals present with typically female external genitalia at birth but masculinize at puberty due to testosterone's other effects.

Hormonal Variations in Non-Mammalian Species and Anomalies

The discussion briefly touches on non-mammalian species such as sharks and hyenas. Despite females appearing as or more aggressive than males in some species (e.g., female spotted hyenas), males typically still have higher testosterone levels. Female aggression in such species may be influenced more by maternal androgens during fetal development or by estrogens, rather than adult circulating testosterone. This highlights that while testosterone is the key androgen in males, the manifestations of aggression and sex differences in behavior can have complex, species-specific mechanisms.

Testosterone's Critical Role in Brain and Behavior Development

One of the core takeaways is that sex differences in behavior—such as activity levels, aggression, and play styles—stem primarily from exposure to testosterone during critical developmental windows in utero and shortly after birth (mini-puberty). Between around 8 to 20 weeks of gestation, male fetuses experience a testosterone surge that shapes brain development and sexual differentiation. This hormonal environment causes male brains to develop differently from female brains, which develop under lower testosterone exposure.

Postnatally, a transient "mini-puberty" occurs in boys at around 1-3 months, marked by increased testosterone levels that possibly influence activity levels and early temperament. In contrast, testosterone levels in boys and girls during childhood are similar and generally low, explaining why behavioral differences at that age reflect the prenatal hormonal impacts rather than current testosterone levels.

Male and Female Play and Aggression Patterns

The podcast sheds light on the differences in play behavior between boys and girls, contextualizing them in evolutionary biology. Boys engage more in rough-and-tumble play which serves an adaptive function to learn social hierarchies, boundaries, and competitive skills essential historically in male-male competition for mates and resources. Such play helps regulate aggression and hierarchy in groups.

Female aggression often manifests differently, being more indirect or social, such as reputational denigration or relational aggression, with evolutionary roots in competition for status or mates but without the same overt physical confrontations typical in males. This distinction extends across many species and plays into socialization and cultural norms.

Aggression, Testosterone, and Social Context

Testosterone is a major contributor to male aggression and risk-taking behavior, supported by evidence from humans and other mammals where males are disproportionately involved in violent crime and intra-sexual competition. However, social, cultural, and environmental factors modulate this raw biological tendency. Rates of violence vary by society and legal structures, showing testosterone is not solely determinative. Instead, it predisposes, while socialization shapes expression.

The podcast also addresses the misconception that biological explanations mean immutability, emphasizing gene-environment interactions and the importance of culture in shaping behavior. It cautions against glorifying or demonizing either masculine or feminine behaviors, noting that both have adaptive aspects and social costs.

Evolutionary Perspectives on Reproductive Strategies and Gametes

Carole discusses how differences in gamete production underlie many sex differences. Females create all their eggs upfront—an energetically costly, limited resource—while males produce sperm continuously and abundantly. This asymmetry shapes evolutionary reproductive strategies: females invest heavily in offspring quality, nurturing, and survival, while males compete more for mating opportunities.

This difference further explains patterns of parental investment, sex-specific behavior, and social roles. She explores hypotheses for why eggs are made en masse early in life, including the idea that attrition of eggs reflects a selection process for quality over time.

Testosterone and Fatherhood

A significant and fascinating point is the link between fatherhood and testosterone levels. In both humans and other animals, testosterone tends to decrease when males engage in active parenting, facilitating nurturing behaviors and reducing aggression or mating drive. This hormonal modulation supports offspring survival and pair bonding.

Conversely, elevated testosterone is associated with mating effort and status-seeking. The episode underlines that this is not a simplistic "high testosterone equals bad dad" scenario, but rather a natural hormonal adjustment to social role and environmental context.

Hormonal Mechanisms: Testosterone, DHT, and Aromatization to Estrogen

The conversation explains the complex hormonal pathways involved in sexual development and adult physiology, including testosterone's conversion to dihydrotestosterone for masculinizing external genitalia, and its aromatization to estrogen, which has its own important effects.

A noteworthy study they discuss involved chemically castrating men and restoring testosterone with or without aromatase inhibitors, showing that optimal health, body composition, mood, and libido require both testosterone and estrogen. This challenges outdated clinical perspectives that sought to minimize estrogen levels in men undergoing testosterone therapy.

Role of Estrogen in Male Physiology and Behavior

While estrogen is often only associated with females, it has crucial functions in males—both physiologically and behaviorally. In rodent models, estrogen from aromatized testosterone masculinizes the brain. However, humans differ in this mechanism, and masculine development in people primarily depends on androgen receptor activation rather than estrogen.

Cases of complete androgen insensitivity syndrome—where XY individuals with testes but nonfunctional androgen receptors develop phenotypically female—underscore the importance of functional androgen signaling in male biology. Despite low or absent testosterone signaling, these individuals have estrogen and tend to have normal female-typical sexual behavior.

Adult estrogen in men affects mood, libido, and body composition, with low estrogen linked to negative outcomes. The interplay between testosterone, estrogen, and receptor sensitivity is complex and critical for understanding male health.

Testosterone Replacement Therapy (TRT) and Clinical Considerations

The podcast also tackles the contemporary issues around testosterone replacement therapy in men. While testosterone naturally declines with age, particularly in Western populations, the decline is less pronounced in hunter-gatherers who live with different energetic and environmental conditions.

TRT has clear benefits for symptoms such as reduced muscle mass, frailty, and low libido, but dosing decisions should match individual symptoms rather than aiming for a fixed youthful level in every case. Importantly, behavioral effects such as increased aggression are uncommon within physiological dosing ranges.

A critical gap in clinical practice is the absence of routine tests for androgen receptor density or function, causing variability in individuals' responses to the same serum testosterone levels. This partly explains why some men feel profoundly better with TRT and others do not.

Hormonal Regulation and Social Signaling

Hooven emphasizes that sex hormones also provide social and physiological feedback, signaling health, reproductive potential, and social role readiness. They decrease during illness or stress adaptively, a process disrupted by exogenous hormone use.

Therefore, hormone therapy represents a trade-off between symptomatic benefits and potential alterations to natural feedback systems, which must be individualized and carefully managed.

Young Men and Testosterone Use

The conversation cautions about the increasing use of testosterone supplements among younger men, highlighting the risks of suppressing endogenous hormone production and impairing fertility. Unlike older men, younger men may suffer long-term adverse effects and dependency, emphasizing the need for regulated use and patient education.

Masculinity, Culture, and Future Work

Toward the end, Hooven discusses her upcoming book focused on masculinity, examining the cultural narrative, biological realities, and social challenges facing men today. She aims to explore why masculinity is increasingly under scrutiny and how denial of sex differences fuels misunderstanding in education, economics, and social policy.

She highlights the importance of open, evidence-based dialogues free from ideological bias, advocating for compassionate acknowledgment of biological sex differences alongside social flexibility and acceptance of individual variation.

Personal Reflections and Challenges

Carole shares her difficult professional experience following public discussions on sex differences, which led to accusations of transphobia and the loss of a long-held academic position. This painful episode has reinforced her commitment to science communication grounded in evidence and openness, despite cultural resistance.

She also reflects personally on hormone replacement therapy after surgical menopause, noting profound effects on mood, energy, and physical wellbeing, and stresses the value of individualized, informed hormone management for both men and women.