15. Human Sexual Behavior I

What this lecture covers

This lecture launches the second half of the course, introducing the "march to the left" strategy the course will use for every topic from here on: start with the behavior itself, then work backward through neurobiology, environmental triggers, hormones, development, genetics and evolution. Sexual behavior is the first case study, chosen partly because its proximal drivers (pleasure, arousal) dominate over distal ones (gene propagation) more clearly than in almost any other behavior covered in the course.

The lecture surveys what is and is not unique about human sexual behavior compared to other species, works through competing explanations for female orgasm, and then moves into the neurobiology: which limbic and hypothalamic regions differ by sex, how the mesolimbic dopamine system drives anticipation rather than pleasure itself, and how oxytocin and vasopressin shape pair bonding. It closes with the landmark and politically fraught brain-imaging and postmortem studies on the neurobiology of sexual orientation and transsexuality, before beginning a section on sensory releasing stimuli (vision, touch, pheromones) that continues into the next lecture.

Key ideas

• Proximal vs. distal explanations: sexual behavior is driven almost entirely by proximal mechanisms (it feels good) rather than any conscious calculation of reproductive success.
• Attractivity, proceptivity, receptivity: the standard trio of terms describing how attractive an individual is, how actively they pursue a partner, and how open they are to another's advances.
• Female orgasm debate: not required for conception, only weakly linked (if at all) to fertility, and possibly a "spandrel"—a byproduct of shared developmental physiology rather than a direct adaptation, much like male nipples.
• Mesolimbic dopamine system: dopamine rises in anticipation of reward and to fuel goal-directed pursuit of it, not primarily as a response to the reward itself; uncertain ("maybe") reward schedules produce the largest dopamine responses.
• D1/D2 receptor shift: in monogamous species, dopamine D2 receptors are linked to forming a pair bond while a later rise in D1 receptors is linked to maintaining fidelity to it.
• Oxytocin and vasopressin: oxytocin supports attachment and pair bonding in females (and increases trust experimentally), while vasopressin, acting on dopamine neurons, drives pair-bond formation in males; vasopressin receptor gene variants correlate loosely with relationship stability in humans.
• Sex-differentiated brain regions: the ventromedial hypothalamus is more central to female sexual behavior, the medial preoptic area and amygdala to male sexual behavior and motivation.
• INAH3 and sexual orientation: Simon LeVay's postmortem study found this hypothalamic nucleus was on average smaller in gay men than straight men, similar in size to that of women—a finding with major scientific caveats and outsized political impact.

Walkthrough

Course strategy and proximal vs. distal explanations (2:00)

Sapolsky lays out how the rest of the course will treat each new topic: start with the observed behavior, then step backward through brain mechanisms, releasing stimuli, hormone levels, development, and finally genetics and evolution. He uses a Martians-on-Earth joke to introduce the distinction between distal explanations for sex (passing on genes) and proximal ones (it feels good), arguing that sexual behavior is unusually dominated by the proximal, in-the-moment drivers.

Cross-species patterns and human specialties (9:08)

The lecture reviews which sexual behaviors are broadly conserved across vertebrates (pelvic thrusting, lordotic reflexes, ejaculation) versus which are more human-specific. Non-reproductive sex, foreplay and homosexuality turn out to be shared with other species, notably bonobos, while private sex, universally egalitarian mating access, and pathological fusion of sex and violence appear closer to uniquely human. He also covers the ethological methodology point (studying animals in naturalistic settings) using Martha McClintock's female-rat research as an example.

The puzzle of female orgasm (15:15)

Sapolsky presents the central problem: female orgasm is not required for conception, and evidence for it improving fertility (through "facilitation" of sperm motility or post-coital horizontal positioning) is weak. He lays out the heritability data and the spandrel hypothesis, comparing it to why male nipples exist without being adaptive in men.

Neurobiology and sex differences in the brain (32:25)

The lecture connects sexual behavior firmly to the limbic system, starting from early lesion studies (the Klüver-Bucy syndrome) that first tied these structures to emotion and sexual behavior. It details the ventromedial hypothalamus (female sexual behavior, estrogen/progesterone receptors), the medial preoptic area (male sexual performance, androgen receptors), and the amygdala's role in male sexual motivation—drawing a parallel to the amygdala's role in aggression. It also covers the autonomic mechanics of erections (parasympathetic then sympathetic activation) and the INAH nucleus, roughly twice the size in men as in women.

Dopamine, anticipation, and reward uncertainty (42:32)

This section works through the mesolimbic dopamine pathway's role in sexual motivation, including brain-imaging studies on responses to visual sexual stimuli and eye contact. A key experiment shows dopamine rises highest not for guaranteed rewards but for a 50% chance of reward, illustrating why uncertain reinforcement (as in gambling) is so powerfully motivating. The lecture also covers D1/D2 receptor dynamics in pair bonding and a study showing dopamine activation to a partner's photo fades after long-term relationships, replaced by anterior cingulate activation associated with comfort and empathy.

Hormonal responses to sex and pair bonding (1:00:46)

Sapolsky distinguishes hormones that cause sexual behavior from hormones released in response to it: progesterone and androgens rise in females after sex, oxytocin is released and tied to attachment formation, and testosterone rises in males after sex rather than driving it beforehand. He covers vasopressin's role in male pair bonding, including gene-transfer experiments that turned polygamous voles monogamous, and notes that human vasopressin receptor gene variants show only a modest correlation with marital stability.

Neurobiology of sexual orientation and transsexuality (1:13:01)

The lecture covers Simon LeVay's finding that a hypothalamic nucleus (INAH3) is smaller on average in gay men, discussing methodological caveats (small effect size, brain donors who had died of AIDS, no independent replication) and the study's contrasting political reception compared to an earlier, similarly framed finding. It closes with studies on the bed nucleus of the stria terminalis in transsexual individuals, which matched the size typical of the gender they identified with rather than their assigned sex, controlled against hormone-treatment confounds.

Before you watch

• Review the earlier lecture on the limbic system, since this lecture assumes familiarity with the amygdala, hypothalamus, ventral tegmental area and nucleus accumbens.
• Recall last week's material on hormones and the autonomic nervous system (sympathetic vs. parasympathetic), referenced here for erection and orgasm physiology.
• Be comfortable with the course's proximal/distal framework for explaining behavior, introduced in earlier lectures.

Check your understanding

1. What distinguishes a proximal explanation for a behavior from a distal one, and why does the lecture argue sexual behavior leans heavily proximal?
2. What evidence complicates the idea that female orgasm evolved specifically to increase fertility, and what is the "spandrel" alternative?
3. Why does dopamine rise more strongly to a 50% chance of reward than to a guaranteed reward, and what does this suggest about intermittent reinforcement?
4. How do oxytocin and vasopressin differ in their roles in female versus male pair bonding?
5. What are the main methodological caveats in LeVay's INAH3 study on sexual orientation, and why did its political reception differ from an earlier, related finding?