How Hormones Shape Sexual Orientation & Behavior | Dr. Marc Breedlove
In a Nutshell
Prenatal testosterone exposure shapes sexual orientation, evidenced by masculine finger length ratios (2D:4D) and otoacoustic emissions in lesbians, equivalent ratios in gay men, and conditions like CAH increasing lesbian likelihood in women. The fraternal birth order effect raises gay probability in males with more older brothers (2% baseline to 50/50 after ~12) via maternal antibodies to Y-chromosome proteins, not social factors. Animal models (gay rams) and brain differences (smaller INAH-3 in gay men) confirm biological mechanisms, with group trends but no individual predictions; adult brain plasticity to androgens exists but orientation forms early.
These notes were generated by AI and may contain inaccuracies.
The larger the number of older brothers that a male has, the higher the probability that he is gay. This has been seen over and over and is one of the rock solid findings in human sexuality. For a baby boy born today with no older brothers, odds of being gay when he grows up are about 2%. With one older brother, odds go up by a third to 2.6%. With two older brothers, up a third again to 3.5%. It takes about a dozen older brothers to have a 50/50 chance.
Welcome to the Huberman Lab podcast, where we discuss science and science-based tools for everyday life. Andrew Huberman is a professor of neurobiology and ophthalmology at Stanford School of Medicine. Guest is Dr. Marc Breedlove, professor of neuroscience at Michigan State University, expert in how hormones shape the developing brain, particularly influencing sexual orientation. Prenatal testosterone exposure impacts finger length ratios and plays a role in sexual orientation, with correlations confirmed many times in humans and animals.
Every time a woman is pregnant with a male, a biological trace biases the likelihood that her next male offspring will be heterosexual or homosexual. These are solid biological findings with understood mechanisms in animals and humans. Prenatal hormones shape attraction to one sex and aversion to the other, forming specific circuits. Hormones also impact rough-and-tumble play and social play in kids, and the interplay of nature and nurture in male-female differences and sexual orientation.
Dr. Breedlove is a pioneer in how hormones shape brain development and psychology. Discussion approaches through biology and statistics, not politics. Covers gay rams, with implications for hormones, brain development, nature-nurture, and partner choice.
Podcast separate from Stanford teaching/research. Aims to bring zero-cost science info to public. Episode includes sponsors.
Dr. Marc Breedlove, welcome. Been 25 years since standing in same physical space. You look great.
Wanted Dr. Breedlove on since podcast launch due to work on how/why people become who they are via hormones and genes. Jump to finger length ratios and sexual orientation study published together. Other labs replicated similar experiments. Context on hormones influencing orientation independent of behavior.
In year 2000, many regarded same-sex orientation as a choice or lifestyle choice to disapprove of it. At Berkeley, no such notions; sexual orientation not a choice. Class exercise: first crush before puberty (e.g., age 6, puberty at 14). Personal anecdote: age 6-7, Marilyn Monroe on TV caused agitation, hard time sleeping, no knowledge of sex. Did not choose reaction. First crush determines lifelong attraction.
Despite animal research showing early hormones affect nervous system, doubted relevance to humans due to social influences. Brain grows at fetal rate until 9-10 years. Heterosexual world (Disney movies with Prince Charming). Social learning explains 95% straight people, not choice, like learning English without choosing or remembering.
In 1998-1999, Dennis McFaden at University of Texas published on otoacoustic emissions (ears making popping sounds, sex difference present at birth: girls more). Measured in straight men, gay men, straight women, lesbians. Lesbians had fewer than straight women, suggesting more prenatal testosterone than straight women, making attraction to women more likely. Explains 95% men attracted to women due to prenatal testosterone.
In 1999 at Berkeley, read paper on sex difference in finger ratios in 9-year-old children: 2D:4D ratio (length of second digit/index finger divided by fourth digit/ring finger). John Manning reported smaller ratio in men than women, present in children, due to prenatal testosterone. More pronounced on right hand.
Typical heterosexual male: index finger shorter than ring finger. Women: more similar lengths. Averages, not predictive for individuals.
Never worked with humans before, but Bay Area has many gay people. Went to street fairs (Castro District for gay men, Solano Street Fair in Berkeley/Oakland for lesbians). Anonymous questionnaires on sex partners/desires, handedness; xerox hands. Incentive: $1 lottery scratcher tickets (worth ~27 cents). Measured all digits twice personally. Least expensive experiment (vs. rats at $20 each). Bought 750 tickets at 7-Eleven.
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Castro for gay men, Solano/Oakland for lesbians. Measured hands (e.g., ran to third floor Tolman Hall to measure Andrew Huberman's and adviser Michael Zucker's hands; noticed differences between hands). No difference in digit ratios between gay and straight men, implying equivalent prenatal testosterone. Gay men not under-androgenized; big sex differences in sexual attitudes (multiple/younger partners, casual sex, visual porn) are masculine in gay men.
80s-90s Hollywood stereotype: gay men effeminate. Recent show on gay hockey players challenges that. Younger people know diversity; older generations less exposed.
Lesbians had more masculine digit ratios than straight women on average. Replicated by many labs (Dave Puts at Penn State, Ashlin Swift-Gant meta-analysis). Suggests lesbians exposed to more prenatal testosterone, leading to attraction to women. Time lag: prenatal exposure affects first crush 6-10 years later.
Published 2000, caused uproar. Emails accusing of lying to justify gay lifestyle (despite straight team with mixed orientations). Joke: to guess orientation from right hand 95% accurately, always guess straight (in random sample). Averages indicate group trends, not individual prediction; other factors influence ratios.
If you want to gather a big sample of people and guess their sexual orientation from their right hand, look at whether index finger is shorter than ring finger and no matter what you see, guess straight and you will be right 95% of the time.
Someone asked if cutting off index finger raises testosterone (no reverse causality).
The link to prenatal testosterone raises questions about whether behaviors associated with gay or straight men and women change hormone levels independently. For years, researchers tested if gay men had higher or lower testosterone levels, hypothesizing lower based on the effeminate stereotype. Results showed the opposite: if anything, gay men had higher levels. Gym-goers in the 1980s and '90s observed this, though steroids confounded some data.
The 2D:4D ratio sex difference exists in mice. Wendy Brown and Marc Breedlove first noted it; evolution shaped digits where the first is shortest, third longest, and second varies. Genetic manipulations making the androgen receptor dysfunctional eliminate the sex difference. Mice have more androgen receptors in growing bones of the fourth digit than the second, explaining why the fourth grows more.
Androgens like testosterone and DHT exist in men, women, and animals, organizing the body plan, including the D2:D4 ratio.
Prenatal effects on the brain are unknown directly, but Simon LeVay studied the preoptic area (POA) in the hypothalamus. In rats, the sexually dimorphic nucleus (SDN-POA) shows a huge sex difference, larger in males. LeVay found a similar nucleus in humans, larger in men than women. In gay men, it was smaller than in straight men, similar to women—indicating less androgen exposure or response, not hyper-male like finger ratios.
LeVay's 1991 Science paper caused uproar. Critiques included AIDS-related neurodegeneration in some gay samples (addressed by including straight men with AIDS, no difference) and bias accusations due to LeVay being openly gay, tied to a supposed "gay agenda." William Byne replicated it despite skepticism; AIDS deaths dropped post-epidemic, delaying his sample collection.
Widely seen as proof sexual orientation is not chosen, though LeVay noted causation direction is unknown: smaller INAH-3 (intercalated nucleus of the anterior hypothalamus-3, SDN-POA equivalent, grain-of-sand size) causing gay orientation, or vice versa? Adult brains change; in animals, removing testosterone from males shrinks sex differences in nuclei like medial amygdala within weeks.
Chicken-and-egg issue persists for INAH-3. Dennis McFadden's otoacoustic emissions and digit ratios provide prenatal evidence predating orientation. No social influence on digits; kids later used hand ratios to guess orientation, but individual prediction fails.
Group differences exist but individual prediction does not—hard for scientists to communicate.
Sex difference in digit ratios is ~0.5 standard deviations (half the height sex difference). Height averages are ~2 standard deviations apart: guessing sex from height (e.g., >5'5.5" male, < female) in 1,000 people yields ~80% accuracy (20% error). Digit ratio overlap is larger; other factors influence it—no predictive value for individuals.
Insufficient bisexual samples then; now more youth identify as bisexual. Multiple developmental pathways to lesbianism or gay maleness; prenatal testosterone plays a role, not the whole story. Stressful development or competition can impact androgens prenatally/postnatally.
Brain development birth to age 12 matches prenatal speed. Human vs. chimp brains: similar fetal growth/body size ratio until birth; chimps slow postnatally, humans continue feverish growth to ~6-10 years. Children are "fetuses outside" learning socially—human distinctiveness.
Behavior (e.g., sibling competition, sports) affects hormones, which change brain, potentially impacting preference. Bidirectional: hormones alter behavior, behavior alters hormones (e.g., competition winners/losers, election victors have testosterone spikes).
Brain plastic lifelong, including hypothalamus (e.g., POMC neurons in arcuate nucleus differentiate late, explaining obesity-driven hunger). Neocortex more plastic than hypothalamus, but relativity; synapses form/prune anywhere. Neuroscience meetings show increasing plasticity evidence yearly.
Competition raises testosterone in winners. Sex behavior: in rats, testosterone removal stops mounting (reinstated by replacement). Female odors spike male testosterone, preparing for mating.
Textbook circuits: female lordosis (brain-body), male mounting/erection/ejaculation (brain-spinal cord-body). Human sex varies; not strictly mimicking. Motor circuits easier to study than motivation/libido (scarce animal models).
60 Minutes demo: neonatally castrated male rat fails to mount receptive female (unlike intact "Romeo"), showing permanent early testosterone effects on behavior.
He mounts several times and the male rat acts like nothing's going on. Romeo eventually gives up. Drop into the cage a male rat castrated on the day of birth 90 days before, given the same hormones as the female to make her receptive. Romeo hops on and gets a beautiful lordosis response that the control male never showed. This neonatally castrated male shows very female-like patterns. Miss Stall kept asking, "Would you say this is a gay rat?" Rats don't have an orientation. Romeo happily mounts any rat thrown in the cage. Response: "This is a rat whose sexual behavior has been permanently changed because of something that happened at the very beginning of development." If given the female those hormones, she shows lordosis to whoever mounts her. Male rat mounts any rat just in case he gets lordosis out of them.
Romeo tries to mount males as well as females, which doesn't align with male human behavior. Humans are distinctive: not particular about motor behaviors but overwhelmingly interested in who the partner is. Few animals have this. Anthropologist from Mars: half the planet finds George Clooney an ideal sexual partner, the other half finds him unacceptable, even if behaviors are the same (positions, acts with genitalia). Overwhelming concern is the sex of the partner. Hard to have an animal model of that.
Chuck Roselli at Oregon studied rams that never mount females, keep mounting males. Shepherds knew this; those rams sent to slaughter. Tests: females in stocks ready for mating. Most rams mount females. Male-preferring rams mount males (intercourse to ejaculation), never females—even after 12 hours with a dozen females. Small percentage of population. Can't explain except aversive component: rams care about sex of partner.
Company identified gay rams, harvests their wool for clothing instead of slaughter. Wool clothing from gay rams saves them.
Chuck dissected preoptic areas: difference between gay and straight rams in how they process testosterone, in the preoptic area where Simon LeVay saw difference between gay and straight men. Hypothalamus/preoptic area may relate to orientation in organisms complex enough to have one, like sheep.
Neuroscience theme: antagonistic relationships (bicep/tricep, hunger/fullness). Appetitive aspect to sex: heterosexual males want females, etc. Aversive aspect: same-sex aversive to some. Sociologically relevant: mating with same sex aversive to many (not all). Biological basis possible, especially asymmetry in men. Half population wants sex with George Clooney, other half never. Reverse with Margot Robbie: men want her, many women would consider. Women more plastic: straight then fall in love with woman, feel gay. Among males, same-sex sex often aversive (context like prisons overrides). Possibly biological (brain difference) or socially inculcated. Gay rams: aversive to females, never mount despite opportunity.
Acknowledgement of aversive pathway for sexual partner choice as important as biological correlates of homosexuality. Long-form discussions prevent snippet misuse. Hypothesis: in male humans, pathway/molecule suppresses desire for same-sex, activates disgust. In women, no such pathway or less robust—explains sex differences, women more open to same-sex.
Early 90s TV gay characters dismantled effeminate stereotypes. Heterosexual women accepted gay men before heterosexual men. Now men and women comfortable.
Nikki Oipka studied mole in Tilden Park that transits testes to ovaries. Went to UCSF for babies called pseudohermaphrodites (outdated).
Congenital adrenal hyperplasia (CAH): adrenal glands slightly larger because fetus can't make adrenal steroids. Brain drives pituitary to enlarge adrenals, but they produce testosterone/androgens instead. Happens in XX or XY. In XX: prenatal extra testosterone enlarges clitoris (sometimes penis-like), labia like scrotum (no testes). Identified at birth, treated with oral adrenal steroids lifelong, shuts off excess testosterone. Intersex phenotype: phallus between clitoris/penis, skin between labia/scrotum.
Old practice: emergency cosmetic surgery to make XX look female—often unsuccessful, caused anger, anorgasmia. Now wait-and-see: no elective surgery on infants without consent (thanks to activists like Cheryl Chase).
CAH females (extra prenatal testosterone): more likely lesbians than population, but most straight. Percentage reporting lesbian orientation increases with age—possibly always there, but follow societal path then reject it.
Androgen insensitivity syndrome (AIS): androgen receptor gene on X chromosome dysfunctional. Mother passes to XY son: testes develop (Y chromosome), secrete antimüllerian hormone (suppresses müllerian ducts—no uterus/oviducts) and testosterone (no response due to receptor). Periphery female-like. Undetected at birth, noticed at puberty (no period, short vagina, no cervix, high testosterone, abdominal testes, XY karyotype). Vast majority straight, attracted to men, want families (can't carry children).
Women with Androgen Insensitivity Syndrome (AIS) are genetically XY but cannot respond to testosterone, resulting in very feminine bodies and faces. They are attracted to men, appearing as straight women. However, they are not useful for studying prenatal testosterone's role in sexual orientation because their brains either never responded to prenatal testosterone or were socialized as girls to be attracted to men. There's at least one woman with AIS who is a successful model and another who wrote a good memoir. These women are frankly quite attractive as women.
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Model of accelerators and brakes on different axes: chromosomes drive male vs. female notions (with caveats like XY bodies appearing female); prenatal testosterone plays a key role in same vs. other orientation choice; possible cultural plasticity biased toward female side; aversive signals in male sheep and observations match. Partner preference departs from linear models like testosterone makes boys attracted to girls. Better languaging is needed to separate biological sex, orientation, and other aspects to reduce confusion and conflict.
Rock-solid finding: larger number of older brothers increases probability a male is gay, first noticed by Ray Blanchard, replicated worldwide.
- No older brothers: 2% chance gay.
- One older brother: 2.6% (33% increase).
- Two older brothers: 3.5%.
- Needs ~12 older brothers for 50/50 chance, all from same mother.
In large populations, more gay men with increasing older brothers. General population: 105 boys per 100 girls born. Straight men: 105 older brothers per 100 older sisters. Gay men: 140 older brothers per 100 sisters. Linear progression; seen in Kinsey surveys. No predictive power for individuals—most men with two older brothers are straight. ~1 in 7 gay men are gay due to this effect; same genotype but prior brothers make them gay statistically.
Males engage in more rough-and-tumble play due to testosterone. Brain develops rapidly post-birth up to age 6. Hypothesis: older brothers increase such play, influencing testosterone and orientation. But older stepbrothers have no effect. Older brothers from same mother but raised apart have same effect. Not socially mediated (e.g., no bullying effect).
Maternal Immunization Hypothesis
Ray Blanchard and Tony Bogaert: Mother carries son, exposing her immune system to Y-chromosome male-specific antigens at birth via blood mixing. She makes antibodies. Subsequent sons get more antibodies crossing placenta, perturbing brain development to increase gay probability. Evidence: Mothers with this pattern have higher antibodies to male-specific antigen neuroligin-4Y (NLGN4Y), important for synapse formation (X and Y versions differ). Mother's body remembers prior sons, perturbing development. Multiple pathways to gay/straight; strong evidence prenatal testosterone matters for women, mother's body for men.
Older sisters or younger siblings don't matter. Younger brothers don't affect odds. Independent of social factors like absent father or sisters. Challenges stereotypes like lack of male influence or too many sisters causing gay orientation—biology shows opposite.
Heterozygous CAH carriers (one functional, one non-functional copy): 1 in 12 prevalence in both males and females. Common due to multiple mutation sites in adrenal steroid pathway; carriers often asymptomatic or subtle, reproduce fine. Full CAH (two copies) causes no adrenal steroid production, gets medical attention. Carriers statistically more frequent in professions/sports needing long-duration stress tolerance; possible heterozygote advantage like sickle cell in malaria areas.
Proposed (but not done) embryonic finch head swaps: male brain on female body, etc., to dissociate gonad/brain sex effects. Background: Male birds sing, females don't. Chick/quail embryo transplants done by Nicole Luran: distinguishable cells result in hybrids like white leghorn with quail brown feathers.
Gynandromorphs: Half female (gyn), half male (andro) birds, e.g., cardinals split red male side vs. dull female. Birds have mosaic sex chromosomes (ZW system). Two embryos fuse early: one side male genes, other female. Brains show lateralized sex differences (e.g., HVC larger on male side). Gonadal hormones not in charge; brain makes local hormones (testosterone, estrogen) based on its genetic sex, driving differentiation cell-autonomously. Unlike mammals, allows mosaicism.
In bodybuilding/gym culture, high-dose synthetic androgens (anabolic steroids) as natural experiment. Different forms affect psychology differently. Trenbolone causes aggression and, in self-reported heterosexual males, desire for sex with males (or females), associated with hypersexuality. Seems reversible on cessation. Important variables exist.
If the adult brain is still plastic at the level of the hypothalamus to androgens later in life. Millions of men take testosterone replacement safely with doctor's support, not boosting to superphysiological range. This is a naturally occurring experiment with few controls and only anecdotal reports, suggesting the adult hypothalamus remains androgen sensitive, driving powerful changes in behavior and perception.
Julian Davidson at Stanford's physiology department conducted double-blind, placebo-controlled studies on men who lost testes due to accident or cancer. Neither the men nor physicians knew if they received placebo or testosterone. Men on testosterone reported feeling better, more energetic, higher libido, and overall better. Joke: men always knew they were getting testosterone because they felt so much better. Confirms testosterone effects on adult brain, even in elderly prostate cancer cases.
Testosterone diminishes with age but rates highly variable. Scatter plots show some in 70s/80s with levels similar to 20s/30s. Recent talk of dropping testosterone rates, but older studies show tremendous variation. Absolute numbers don't dictate how people feel. Example: CEO of successful entertainment company had testosterone in low 300s (low end of reference range) but felt great—vigor, libido, enthusiasm for life. Advice: if it ain't broke, leave it alone. Low normal not a problem; more might aromatize to estrogen and feel worse. Humans are lousy research subjects due to variability.
Decline in men's testosterone much more gradual than women's menopause. Incredible variation across subjects; statistical trend but can't predict individual's levels at 71.
Meme from MMA guy (Russian/Eastern): "Send two-three years Dagestan and forget." Originally about intense wrestling training for kids. Co-opted for videos of effeminate boys dancing/theater transitioning to Dagestan training, implying early plasticity for masculinization of stereotypically male behaviors.
Born 1975, Northern California near Stanford. Pediatrician told mom for boy Andrew: 1) Don't let him ride motorcycles. 2) Don't let him drink soda (too much sugar). 3) Don't let him do theater. Implication: boys in theater more likely to become gay. Gave this advice to every male parent. Highlights past vs. present beliefs that activities bias sexual orientation. Brain plastic to androgens; behaviors impact androgens. Younger brain more plastic, but limits exist.
No strong data for social correlates of sexual orientation (e.g., missing dad, overly coddling mom, dismissive dad). Data weak/strange—expect to find something if social influence strong. Culture/family influence other male-like behaviors. Persistent biological sex difference: rough-and-tumble play across species. Males together: more physical activity than females. Mixed groups (monkeys/rats): intermediate play—girls calm boys, boys ramp girls.
Born in Ozarks, Springfield, Missouri (southern Missouri/northern Arkansas hill country like Appalachians). Working-class family; no one in mother's generation finished high school. Always reading, loved school. Supportive family. Raised by grandparents (construction worker grandpa, food processing plant grandma; ~3rd grade education). Lucky breaks key.
Junior year, volunteered in Central High School library. Librarian showed College Board book (first year listing all US colleges). Read financial aid sections; Yale's shortest: "committed to making sure everyone admitted gets financial aid needed." Grandfather died; income only social security. Applied early admission (junior year)—no. Regular senior year—interview with Dr. John Ferguson (OBGYN, family friend of French teacher Mrs. Fiser who babysat him). Strong recommendation; got in, full financial aid (family contribute $0).
Mixed feelings but benefited immensely. Attitude: they've made a mistake, won't screw up. Hungry to learn art, literature, sciences from smarter people/students. Drove from Springfield; parked downtown (expensive). Roommates: two from prep schools (Phillips Andover vs. Exeter), one generational wealth name. First night: they smoked dope (new to him; Ozarks judge gave life sentence for possession 1972). Avoided to not risk expulsion. Took max courses (5 + lab), learned neuroscience via comparative psychology, Linda Uphouse's physiological psychology.
Seized opportunities; luck finds the prepared. Theme of exploration. Breedlove inspires following exciting ideas, not careerist planning. Name "Breedlove" helped but work matters. Thanks for precise languaging on complex topics (hormones/sex behavior/sexual orientation) to clarify known/unknown.
More kids = more belief in nature (Steve Glickman study). Daughter Tessa (two older brothers), partner Cindy Jordan (neuroscientist): rarely wore dresses/skirts (impractical lab). Tessa insisted on dresses, walked in mom's shoes. One boy made guns from anything (no toy guns in Berkeley era), loved wheeled toys. Even monkeys: males prefer wheeled toys, females dolls (Melissa Hines).
Writing book on biology of sexual orientation. Six of 11 chapters done; first draft by fall, then reviewing. Expect to buy copy.
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