#374 - The evolutionary biology of testosterone: how it shapes male development and sex-based behavioral differences, | Carole Hooven, Ph.D.
Carole Hooven, a human evolutionary biologist and former Harvard lecturer, discusses how prenatal testosterone shapes male development and lifelong behavior, distinct male/female aggression styles, and the implications of denying biological sex differences. The conversation also covers testosterone's role across the lifespan and hormone therapy.
Deep Dive Analysis
17 Topic Outline
Carole Hooven's Journey to Studying Sex Differences
Cross-Species Sex Differences in Aggression and Behavior
Embryonic Sexual Differentiation: Chromosomes, SRY Gene, Hormones
DHT and 5-Alpha Reductase Deficiency in Genital Development
Prenatal Testosterone's Impact on Brain and Childhood Behavior
Gamete Differences and Divergent Reproductive Strategies
Evolutionary Basis of Sex Differences in Play and Conflict
Male Aggression, Violent Crime: Biology vs. Culture
Female Evolutionary Strategies: Nurturing, Risk Aversion, Social Aggression
Modern Society's Redirection of Male Competitive Drives
Testosterone's Role in Fatherhood and Mating Strategies
Evolutionary Reasons for Post-Reproductive Human Pair-Bonding
Estrogen's Crucial Roles in Male Physiology and Behavior
Testosterone Replacement Therapy: Natural Levels and Considerations
Personal HRT Experience and TRT Risks for Young Men
Rebuilding After Academic Controversy and Commitment to Science
Forthcoming Book: Masculinity and Denying Biological Sex Differences
8 Key Concepts
SRY protein
A protein produced by the SRY gene on the Y chromosome, which triggers the differentiation of the undifferentiated gonad into testes in XY individuals. Without it, the gonad differentiates into ovaries by default.
Wolfian ducts
A primitive duct system in embryos that develops into the male internal reproductive plumbing, such as the epididymis and vas deferens, under the influence of testosterone produced by the developing testes.
Mullerian ducts
A primitive duct system in embryos that develops into the female internal reproductive plumbing, including the fallopian tubes and uterus, in the absence of anti-Mullerian hormone.
DHT (Dihydrotestosterone)
A more potent androgen converted from testosterone by the enzyme 5-alpha reductase, primarily responsible for the masculinization of external genitalia (penis, scrotum) and prostate development in the fetus. It binds more tightly to androgen receptors than testosterone.
5-alpha reductase deficiency
A rare genetic condition where XY individuals have testes but cannot produce DHT due to a defective 5-alpha reductase enzyme. This leads to female-appearing external genitalia at birth, despite normal internal male reproductive structures and the development of male musculature at puberty.
Mini-puberty
A postnatal surge in testosterone in male infants, starting within a month after birth and peaking around three months. This period appears to have important effects on brain development, activity levels, and the lengthening of the penis.
Parental Investment (Sex Differences)
The differing energetic and time burdens males and females put into reproduction. Females invest heavily in each offspring (expensive eggs, gestation, nursing), while males invest less per gamete (cheap sperm), leading to distinct reproductive strategies and behaviors.
Androgen Receptor Density
The concentration of androgen receptors in different parts of the brain and body, which influences how effectively testosterone (or DHT) can act on cells. This genetic variation helps explain why individuals with similar testosterone levels can have different physiological and behavioral responses.
9 Questions Answered
All embryos start with undifferentiated gonads. The presence of the SRY gene on the Y chromosome triggers testes development, which then produce testosterone and anti-Mullerian hormone, leading to male internal and external genitalia. Without SRY, ovaries develop by default, and female structures form.
The primary cause is the difference in prenatal testosterone exposure; male brains develop in the presence of high testosterone during a critical window, while female brains develop in its absence. This early hormonal environment, not current childhood testosterone levels, largely drives observed behavioral differences.
Females produce energetically expensive, finite eggs and bear the high costs of gestation and nursing, leading to strategies focused on offspring survival and quality. Males produce energetically cheap, continuous sperm, leading to strategies often focused on competing for mates and maximizing quantity of offspring.
Males tend to engage in direct, physical confrontation, often to establish dominance hierarchies that reduce overall aggression. Females tend to use indirect, relational aggression (e.g., denigrating reputation), which is less physically risky and serves mating competition.
If testosterone levels are restored within the typical physiological range, even at the high end, significant changes in sexual or aggressive behavior are generally not observed. However, very young men taking exogenous testosterone should be cautious about potential impacts on fertility and the difficulty of withdrawal.
While estrogen does not masculinize the human brain in early development, it is crucial for adult male health, including bone density, mood, and libido. Studies suggest that higher estrogen levels (within physiological norms) in men on TRT can lead to better outcomes for body composition and mood.
Western men are often overnourished and have fewer energetic stressors, allowing them to maintain higher baseline testosterone levels than hunter-gatherers. This higher baseline leads to a more noticeable decline with age, whereas hunter-gatherers start lower and maintain levels more consistently due to different environmental pressures.
Mini-puberty is a natural surge in testosterone that occurs in male infants, starting within a month after birth, peaking around three months, and lasting until about six months. This period is associated with important effects on brain development, activity levels, and penis lengthening.
This behavior is linked to the 'grandmother hypothesis,' where post-reproductive individuals (especially grandmothers) contribute significantly to the survival and success of their daughters' offspring, thus indirectly ensuring the survival of their own genes. The commitment and trust of a long-term bond continue to benefit the extended family.
11 Actionable Insights
1. Acknowledge Biological Sex Differences
Appreciate and grapple with the reality of biological sex differences, as denying them can lead to misguided attempts at creating equal outcomes based solely on societal influence. Understanding these real differences is crucial for developing nuanced social approaches.
2. Engage Arguments, Not Character
When discussing controversial topics, encourage engagement with arguments and the sharing of facts, rather than resorting to character assassination. This approach fosters learning and understanding, which is vital for addressing complex societal issues.
3. Optimize Sleep, Nutrition, Exercise
Before considering testosterone replacement for vague symptoms of low testosterone, prioritize optimizing fundamental health habits such as sleep, nutrition, and exercise. These foundational elements often need to be addressed first, regardless of current testosterone levels.
4. Fathers: Engage with Young Offspring
Fathers should be physically involved with small, dependent offspring, as this can lead to a beneficial suppression of testosterone. This hormonal change can facilitate greater contentment with family life and increase attention to the mate and offspring, supporting reproductive success.
5. Caution with Young Male TRT
Young men should be extremely cautious about using exogenous testosterone, as it can be addictive and potentially cause permanent infertility. For young men with low testosterone, prioritize addressing underlying issues like sleep, nutrition, and exercise, and consider HCG to preserve gonadal function before resorting to testosterone replacement.
6. Maintain Estrogen on TRT
For men on testosterone replacement therapy (TRT) within physiological norms, allow estrogen levels to rise naturally, as higher estrogen (in conjunction with higher T) has been linked to better outcomes for body composition and mood. Avoid unnecessarily blocking aromatization unless symptoms like gynecomastia appear.
7. Assess Androgen Receptor Density
When evaluating testosterone levels and potential replacement, consider that individual responses vary significantly due to factors like androgen receptor density and genetic differences (e.g., CAG repeat length). A specific testosterone level may feel different for different individuals, suggesting the need to look beyond just the number.
8. Encourage Rough-and-Tumble Play
Allow boys to engage in rough-and-tumble play, especially if they are having fun, smiling, and laughing. This type of play is crucial for learning to work out social dynamics, developing social relationships, and responding physically, which can prevent future social difficulties.
9. Menopausal Women: Lift Weights
Women, especially those in or approaching menopause, should consider incorporating weightlifting into their routine. Carol Hooven found it made a huge difference in her well-being and physical capabilities after starting at age 58.
10. Consider HRT Post-Ovariectomy
Even if seemingly in menopause, consider hormone replacement therapy (progesterone, testosterone, estrogen) after an ovariectomy. Carol Hooven experienced significant negative changes like hair loss and plummeting sex drive after her ovaries were removed at 57, highlighting the impact of even low hormone levels.
11. Understand Male Intimacy Dynamics
Recognize that male intimacy can often involve insults, with harsher insults sometimes indicating deeper intimacy, provided they are not rejected. This understanding can help interpret male social interactions.
6 Key Quotes
I wouldn't say they're a different sex. I would say they're a different species.
Peter Attia
The only thing that differentiates the sexes cleanly and essentially is the gamete production.
Carole Hooven
We are designed to be motivated to seek out these foods and we have to expend energy to get high calorie foods, say like we would have maybe gotten honey and that would have been super rewarding and we only would have had a little bit and then we would have ran around and spent those calories and now we have chocolate bar. That's a mismatch situation that's maladaptive.
Carole Hooven
There's something that feels fair about that and to sort of backstab and not give somebody the opportunity and not to be able to work it out and to gossip behind people's backs. Yes, there's murder and rape and men are overrepresented in those horrible crimes, but we shouldn't glorify feminine ways of interacting necessarily and try to get men to be more feminine because there's a lot of issues also with typical feminine behavior.
Carole Hooven
Reality is there whether we like it or not. It's always to our benefit to understand it and to try to figure out then to use democratic processes to figure out what to do with reality or how to improve human health or whatever the issue is.
Carole Hooven
There's just only two sexes and we should learn to deal with that kind of reality.
Carole Hooven