Human Sexual Behavior I. 101m Robert Sapolsky video.

Part 1 of 3 in a broad biological survey of the nature of human sexual behavior from neuroendocrinology to evolutionary history in the context provided by the animal world. If you want to understand some of the differences and similarities of the sexes, read my detailed notes below.

15. Human Sexual Behavior I

Plan for the rest of the course: look at the behavior from an ethological perspective. Then we will look at the timeline of what led to the behavior (from the big picture evolutionary and environmental influences all way to the neurology that triggered the behavior). Here is Sapolsky's proximity of influence "timeline" for behavior:

evolution of species, peptides including the ecology and environmental situation, genetics of individual, perinatal biology & environment, culture, chronic and acute hormonal situation, the releasing stimuli, and the neurobiology that leads to the behavior.

Everything to the end (the right) depends on everything that comes before it (to the left). Proximal & distal explanations of behavior: to maximize the number of genes to pass on to the next generation (most distal) or for pleasure (proximal). With sex the driving forces are very proximal: close to the behavior on the behavior timeline. This lecture focuses on the neurobiology of sex and just starts discussing the releasing stimuli of sex at the end.

Fixed action patterns (the ethologists answer to the question: What is the behavior?) of sexual behavior across species tend to be strongly conserved within the species but tend to be very different between species.

The conservation of species reproductive specificity is due to the releasing stimulus of each mate providing the fixed action pattern for the other: this reinforcing interplay provides reproductive selectivity for those organisms whose sexual selection is effectively coordinated.

The lingo for discussing sexual behavior:
libido: referring to sexual arousal and motivation; horniness
The following three words are preferred:
attractivity: how attractive an individual is to another
proceptivity: the active behaviors that are done in response to being attracted by another
receptivity: how receptive an individual is to the interest of another

Martha McClintoch quote "studying female rat sexual behavior in a cage is like trying to study the swimming behavior of a dolphin in a bathtub" Female rats will engage in proceptive courting behaviors if they are not confined to a cage (dogma used to be that rat females are very passive, receptive).

Mathematical biologist Joel Cohen proposed in the 1980s (, using the method of randomized response originally proposed by Fiddler and Kleinknect in 1977 to determine actual patterns of behavior despite the potentially embarrassing nature of some questions.

Fertility does not depend on female orgasm. So why did it evolve? Monkeys and Apes have female orgasm. [Note: these studies do not receive federal grant money.] Theories: Female orgasm may facilitate fertilization (facilitation). Sperm motility is improved with vaginal secretions which are enhanced in female orgasm (not proven). Other theories: orgasm exhausts the female, so she lays down so sperm don't need to swim against gravity. Reinforcement theory: it feels good, so increases the behavior. But most studies have shown no relationship between fertility and propensity to orgasm. Female orgasm appears to play no role in reproductive success. Why are clitoral orgasms more easily brought about than vaginal orgasm? Sapolsky: it may be a spandrel! Male reproductive success is improved by orgasm and females may carry along that physiology as evolutionary baggage. Similarly nipples are spandrels in males.

What isn't unique to humans?
Non-reproductive sex: bonobo chimps, cetaceans like dolphins. In most species, the endocrinology of ovulation makes sex pleasurable (for both female and male). In humans, females do not need higher estrogen levels to have "tactile responsiveness to sexually arousing stimuli" (however, it enhances the effect).
Foreplay: bonobos have more patience with foreplay than do humans
Homosexuality & Masturbation: not just in zoos, ethological studies have documented it in other species.
Fantasy: difficult to know, but Sapolsky tells of a low-ranking juvenile baboon who masturbated after the hottest baboon female walked by (could be pheromone induced)

What is unique to humans?
Egalitarian sex: in humans, unlike marmoset monkeys, all adults are allowed to reproduce
Sex in private: in no other species is most sexual behavior done out of sight of others
Men sometimes psychopathologically confusing sexual behavior with violence

Marriage: all human cultures have some form of it: more than 90% of people wind up in each culture's equivalent of a permanent stable relationship. Even though the majority of human cultures have been polygamous, nonetheless, the vast majority of individuals have been in monogamous relationship. But there is a lot less monogamy than you might think. Alfred Kinsey questionnaires and other research shows that humans have high degrees of social monogamy but much less sexual monogamy. Between 10 and 40% of children do not have the father they are supposed to have.

Romance is viewed as a relatively new cultural invention. Sapolsky suggests a couple of centuries old. Sapolsky asserts that the notion that romance and passion should persist throughout a lifetime is only about 30-50 years old in most westernized cultures. Do you believe this? Doesn't "The Tale of Genji" and Dante's "Commedia" refute the hypothesis? Is this an arrogance of modern scholarship or does the historical record support the thesis? Wikipedia doesn't shed much light on the subject: Plato's Symposium has an interesting speech by Aristophanes which suggests to me that the passion of romance was strong in Greek understanding.

Across cultures the average duration of marriage is 2-4 years (roughly corresponds to the inter-birth interval). Humans tend to be serial monogamists.

Compared to many animals, in humans sex is relatively "normal". Many species are hermaphroditic (individuals with male and female sex organs), opportunistic sex changing species (fish), parthenogenesis (reproduction without fertilization: some snakes reproduce parthenogenetically only after sex with a male whose sperm is not used)

The neurobiology of sexual behavior is heavily centered in the limbic system.

Klüver–Bucy syndrome: brain damage to the limbic system causing sex with inanimate objects, and other behavior changes.

The ventral medial hypothalamus is implicated in female sexual behavior: if stimulated you get the behavior of an ovulating female, damage it and sexual behavior goes away. It is the location of estrogen and progesterone receptors. The midbrain also appears to be involved in female sexual behavior. The lordosis reflex (female back-arching effect in hamsters) indicates special spinal pathways that do not exist in males.

The medial preoptic area is the location of testosterone and androgen receptors and is implicated in male sexual behavior (especially performance). The amygdala is involved in male sexual motivation (in addition to its role in fear, anxiety, and aggression). This may be why it is far more likely for male humans to confuse sexuality with aggression.

The autonomic nervous system is involved in penile erection (parasympathetic nervous system which is involved in growth, repair, & vegetative functions) and ejaculation (sympathetic nervous system which is involved in arousal and the fight or flight reflex). The same mechanism is behind clitoral erections in females.

Vascular v. muscular erections: vascular erections are caused by increased blood flow to the penis and blocking its escape leading to vascular engorgement. In muscular erections (such as in rodents) there is the erector levi muscle. Vascular erections last longer, muscular ones can happen quicker. Both methods use the same autonomic physiology.

In females it takes longer for the sympathetic nervous system to return to baseline after arousal (whether an orgasm or a fight).

There are many regions of the brain with sexual dimorphism (difference in size depending on gender) both in the size of a nuclei (center of neuron cell bodies where the nucleus is situated) and the number of axons (the part of a neuron that "connects" to other neurons) between centers. The INAH3 (third interstitial nucleus of the anterior hypothalamus) is a small nucleus in the hypothalamus which is about twice the size in men than women.

But much of the neuroanatomy is the same in males and females. Orgasm is the transition from parasympathetic to sympathetic nervous system activity. Premature ejaculation is when the transition from parasympathetic to sympathetic is too rapid. Women tend to have the more difficulty transitioning from parasympathetic to sympathetic (so, difficulty in reaching orgasm).

The neurobiology of anticipation, pleasure, & reward is the same in both sexes. The mesolimbic pathway is a dopaminergic pathway in the brain. The VTA (ventral tegmental area) sends a big dopaminergic projection to the NAcc (Nucleus accumbens) which then passes it all around the brain. If this pathway is depleted of dopamine, there will be a diminution of proceptivity. This frequently happens in clinical depression. Dopamine is about the anticipation of reward and then fueling the behavior needed to get the reward. If the reward is only presented half the time (intermittent reinforcement), then dopamine rises even higher than when it is a sure thing. "Maybe" is highly reinforcing! Brain scans of people presented with pornography activate the dopaminergic pathways (Sapolsky implies that males respond more than females).

The effects are subtle: if a guy sees an attractive to him female, the dopaminergic pathway activates if her eyes are directed towards him (not if she is looking elsewhere). Is this why we avoid eye contact? If the guy rates the woman as unattractive, his dopaminergic pathways activate if she is looking away. What is that about?

D1 & D2 are two dopamine receptor subtypes. In pair-bonded species (rodents), after mating the D2 receptors are down-regulated and the D1 receptors are up-regulated. D2 seems to reward mating (the attachment) and D1 rewards pair-bonding (faithfulness). In humans one study found a very small effect between the D1/D2 ratio and more stable, longer lasting relationships.

In a brain scan study, seeing a picture of one's beloved activates the dopaminergic pathways if they have been together for 2½ years, but if they were together for 5 years there is dopaminergic response instead in the activation of the AC (anterior cingulate) which is associated with empathy and comfort.

Sapolsky's college dormmate said "a relationship is the price you pay for the anticipation of it". Is dopamine the basis for this?

In general, the neurobiology of homosexuality works the same: just switch the gender of one of the individuals. What does that mean?

The role of the frontal cortex: regulating behavior, impulse control

Gratification postponement plays a large role in sexual behavior. The frontal cortex teaches you appropriate sexual behavior, it keeps you from doing things you will regret afterwards. Cortical damage can cause highly inappropriate sexual behavior (an 80 year old guy with stroke damage to his prefrontal cortex raped an 80 year old woman). Sometimes (e.g., in ungulates = hooved animal) the frontal cortex is involved in getting you to engage in terrifying courtship behaviors. Mostly the frontal cortex is about reigning in sexual behavior that would be inappropriate: it is changing the context in which the fixed action patterns occur.

What are the hormonal responses to sexual behavior?

In females, having sex increases secretion of progesterone-derived hormones (reinforcing the pleasure), testosterone and other androgens (females have only 5% of the levels of males, but levels rise after sex), oxytocin (a hormone, neurotransmitter, and a neuromodulator). In females, androgens (produced in the adrenal gland) are involved in mediating sexual motivation (arousal). In human females, removal of the adrenal glands tends to reduce sexual motivation; give them replacement androgens and their sexual arousal returns. Oxytocin plays a central role in forming attachments. Ocytocin aerosols in the nose makes them more trusting (they are more likely to agree with an argument and play games more cooperatively). Hence the new field of neuromarketing. Woah, is that OK: oxytocin coming out of my TV set before an ad comes on? Oxytocin's main role appears to be for nursing behavior. Hence the theory that monogamy may be a descendant of the neurobiology of mother-offspring attachment.

In males, having sex increases testosterone, vasopressin (also a neuromodulator). The evidence that high testosterone levels make males more sexually active is non-existent. Vasopressin is a neuromodulator: it is to males as oxytocin is to females. Vasopressin is critical for males to form a pair-bond. In monogamous species, the vasopressin receptor gene is expressed on neurons that release dopamine. A gene transfer study in voles showed that polygamous males become monagamous after getting the "right" genes. Those males with more receptors form pair-bonds faster. In monkeys, Marmosets pair-bond (and they have the gene), Rhesus monkeys (tournament species, polygamous, they have the "right" genes for that). Chimps have the polygamous vasopressin receptor gene. Bonobos have the monagamous gene version, but they are polygamous! In humans, the gene is about half-way between the polygamous & monagamous versions. Two studies in humans show that the monagamous version correlates to a high chance of getting married, the marriage is likely to last longer, and both partners are more likely to rate the marriage as stable and happy (but it is a small effect). In families with autism there are mutations on the vasopressin gene where subjects show very little attachment to other humans.

Oxytocin & vasopressin are associated with social attachment in animals & humans.

Neurobiology of sexual orientation

INAH3 is twice the size in males in other species & humans. Simon LeVay showed that the INAH3 in gay men is half the size as in heterosexual men (about the same size as in heterosexual women). The study has been replicated and the effect is distinct. Since the brains were taken from AIDS patients, that may confound the effect. Dick Swaab found the region adjacent to INAH3 is twice the size in women and gay men than in straight men. Swaab's findings were condemned by the gay community wheareas openly gay LeVay's findings were heralded. Gay pride: "the only thing small about me is the size of my sexually dimorphic nucleus". LeVay's 1991 paper in the journal Science "the most influential science journal in this country", influenced the debate about gays in the military during the 1992 presidential election (the paper was held for a few months to maximize its impact on the election).

The digit ratio is the size ratio between the length of the second and the fourth fingers. Gay men tend to have the finger length ratio of straight women rather than straight men. Wikipedia discusses various studies at including this one in the prestigious journal PNAS

Sapolsky mentions that the otoacoustic reflex is a vibration in the ear that differs by sex and sexual orientation, but Wikipedia's description and Sapolsky's may not agree (cf.

Sapolsky implies that these effects are due to the prenatal hormonal environment. He also mentions that gay women show similar effects for digit lengths and ear vibrations.

Neurobiology of transsexuality, 1:24:40-1:29:45

Being homosexual was considered a psychopathology until the 1970s. Transsexuality is still considered a psychopathology by the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders.

The central subdivision of the bed nucleus of the stria terminalis is reliably twice as large in men as in women except in a study of transsexuals where they had the size normally associated with the sex that they insisted they were. So do transexuals have the wrong bodies for their brain? Wikipedia suggests the story isn't as clear as Sapolsky presents it ( Sapolsky is referring to another Swaab study published in Nature (, but Wikipedia and the Nature article appear to involve only six subjects. Why does he think it is a strong effect? When the penis is removed due to cancer, there is often phantom penile sensation. But transgendered individuals do not report phantom penile sensation.

Releasing stimuli for sexual behavior

Male turkeys depend on visual stimuli (styrofoam turkey experiments). Rhesus monkeys are visually attracted to females in estrus. Humans are also highly visual in our sexual responsiveness. Male humans are more responsive than females shown by brain-imaging studies showing more dopaminergic response as well as an effect in the amygdala to visually arousing material. Tactile responsiveness is also well established. Some tactile stimuli are more arousing than others (erogenous zones). Tactile responses vary depending on hormone levels. In women, ovulation enhances tactile responsiveness. In men, more testosterone enhances tactile responsiveness (castration reduces responsiveness).

Sex pheromones are generated only when the testosterone (male) or estrogen (female) levels are high enough. Pheromones tend to be the breakdown products of sex hormones (androgens in males; estrogens in females). Olfactory receptors can detect remnants of the sex hormones. Perfumes have typically been made from the sweat of male animals. Chanel #5 is made from "whipped male Abyssinian cats". Synthetic perfumes tend to be made from synthetic versions of androgens. Attractive to females, but not so much to males. Olfactory communication tells the species, the gender, the relative strength of their sex hormones, it tells about their health, if they are afraid, and how related they are. Men without testosterone and women without estrogen will not be able to detect the sex pheromones that are present. Women can detect male pheromones better when they are ovulating. Gay men can detect the smell of gay men better than straight men or women.