Sex Basics

I. What Sex Is, What It’s For, and Why Humans Are Unusual

Most pop-science discussions of sex fall into one of two categories. The first is the reductive biological one: sex is for genetic recombination, the rest is hormonal scaffolding to keep organisms reproducing. The second is the consumer one: sex is for individual pleasure and the body is a sensation-producing apparatus. Both pictures miss what may be the more important thing about human sexuality.

To love is to truly know, and to know is to access truth. The truth being the all-encompassing, unfiltered expression of the universe.

With this lens, sex with someone you are emotionally bound to is the act of momentarily dissolving the perceived boundary between self and other and taking another step towards knowing. The boundary between self and not-self, which is biologically negotiable in the ways covered in Connection, Environment, and The Singularity, is briefly more negotiable than usual. Whatever else sex is, it is also this.

Contemporary neuroscience says approximately the same thing in different language. Sexual activity with a bonded partner produces measurable neurochemical signatures (oxytocin, vasopressin, dopamine, serotonin patterns) that reduce activity in regions associated with self-other distinction and increase activity in regions associated with reward, bonding, and relationship-maintenance circuitry. The neuroimaging research on couples in long-term love shows patterns that overlap with both early-stage romantic love and deeper attachment systems, suggesting that sustained pair-bonding sustains some of the boundary-softening effects of early romance. The default mode network, which generates the experience of being a distinct self with autobiographical continuity, shows reduced activity during peak sexual experience, in patterns that bear some resemblance to those seen during meditation, certain psychedelic experiences, and other states of self-transcendence.

If sex is just genetic recombination, the cultural elaboration around it is mostly decoration. If sex is just consumer pleasure, the cultural framing should fade once everyone has the techniques and toys to optimise it for themselves. But if sex is also the dissolution of the boundary between self and another, then the cultural framing is doing real work: it is the structure within which the dissolution happens or fails to happen. Sex in the context of a long-term bonded relationship is qualitatively different from sex outside that context, not because of moralism but because the neurochemistry and the dissolution work differently.

Pleasure is the motivation for sex, and meaning is the goal. Pleasure draws organisms together; meaning is what they make when they get there. Modern culture has substantially collapsed the idea so that pleasure becomes both the motivation and the goal, with predictable consequences for relationship quality, sexual satisfaction across the lifespan, and the broader stability of pair-bonds. The research on what predicts long-term sexual satisfaction broadly supports the older framing: sustained sexual satisfaction tracks the depth of the emotional and meaning-laden bond more than it tracks technique, novelty, or partner number.

The biology evolved to support pair-bonding; the cultural elaboration around love and sex emerged to support the biology; and the materialism attempts to detach sex from bonding, producing predictable patterns of dissatisfaction even in populations that consciously endorse the detached framing.

II. The Hormonal Cascade of Attraction and Pair-Bonding

The neurochemistry of early romantic love has been mapped through the work of Helen Fisher and colleagues at Rutgers University, particularly her Anatomy of Love (1992, revised 2016) and Why We Love (2004). Fisher’s research, including functional MRI work on people in various stages of romantic attachment, identifies three distinct but interacting neurochemical systems:

• Lust, driven primarily by testosterone in both sexes (and modulated by oestrogen in females, by oestrogen-from-testosterone in males). Testosterone increases libido, mate-seeking behaviour, and willingness to incur cost in pursuit of sexual opportunity. Lust is relatively non-specific; the system can be activated by many potential partners.
• Romantic attraction, driven primarily by dopamine and norepinephrine, with reduced serotonin. This is the system responsible for the intoxicating early-stage in-love feeling: obsessive thinking about the partner, energy and elation, decreased need for sleep and food, racing heart, increased focus on the loved person. The caudate nucleus and ventral tegmental area, both key reward-system structures, light up in functional MRI scans of people in early-stage romantic love. The serotonin pattern is interesting and underappreciated: early romantic love shows serotonin patterns that overlap with obsessive-compulsive disorder, which is why being newly in love can feel like a form of pleasant mental illness.
• Attachment, driven primarily by oxytocin and vasopressin. This is the system responsible for the longer-term feelings of trust, calm, security, and bonding that develop with a partner across months and years. Oxytocin is released during sexual activity (particularly orgasm), breastfeeding, and other close physical contact; vasopressin is particularly important in male pair-bonding. The classic comparative work on prairie voles vs montane voles established the role of these neuropeptides in pair-bonding: prairie voles (monogamous) have high densities of oxytocin and vasopressin receptors in reward-related brain regions, while montane voles (promiscuous) have low densities in the same regions. Human variation in vasopressin receptor density predicts variation in relationship stability and partner-bonding patterns.

The three systems interact in complex ways. Lust can occur without romantic attraction or attachment (one-night stands, anonymous sex). Romantic attraction can occur with limited lust (the chaste crush, the love that survives sexual difficulties). Attachment can occur with diminished lust and romantic attraction (the long-married couple whose tenderness has outlasted both the early in-love feeling and frequent sex). Optimal long-term sexual partnerships tend to involve all three systems sustained over time, which is harder to maintain than any single system alone.

• The neurochemistry of the initial encounter: Early courtship involves a dump of attraction-related neurochemistry. Norepinephrine, dopamine, and testosterone flood the system, producing the energy and focus characteristic of new attraction. The hypothalamus secretes oxytocin and vasopressin during sexual contact, building the trust and connection that underlie attachment. The caudate nucleus, one of the brain’s reward centres, activates strongly in response to images of the partner. Serotonin drops, producing the obsessive-thinking pattern characteristic of new love. Combined, the neurochemistry produces something that crosses over clinically with both addiction and obsessive-compulsive disorder. People in early love often can’t eat or sleep well, can’t stop thinking about their partner, and behave in ways they wouldn’t otherwise endorse.
• The pair-bonding consolidation: As a relationship moves from early attraction to partnership, the neurochemistry changes. Dopamine and norepinephrine subside from their early peaks. Oxytocin and vasopressin patterns stabilise. The intoxicating, obsessive quality fades, replaced by the calmer feelings of established attachment. This is sometimes experienced as loss (the early intensity is missed) and sometimes as relief (the obsessive quality was exhausting). Whether it’s experienced as loss or relief substantially predicts what happens next.
• The female courtship physiology: During courtship and arousal, the female body responds to visual, olfactory, and other cues with vasoactive intestinal polypeptide release, which increases blood flow to genital tissues. Plasma seeps through the vaginal walls, producing lubrication. The Bartholin’s glands and cervical glands secrete mucus. The clitoris, which has substantially more erectile tissue than is externally visible (the bulb of the vestibule and crura extend several centimetres internally), engorges with blood. Heart rate and blood pressure rise. Pupils dilate. The full arousal cascade involves the entire endocrine and autonomic nervous systems, not just genital tissues.

III. The 7-Year Itch and the Ovulation Findings

Wheal’s Recapture the Rapture (2021) synthesises a body of research on long-term partnership dynamics that has both important findings and contested empirical claims. Engaging with this material requires distinguishing the two.

• What’s supported: Long-term pair-bonds do show consistent patterns of declining sexual frequency, declining sexual novelty, and increased relationship maintenance work across years. The neurochemistry of sustained attachment differs from that of early attraction, and the change produces predictable subjective experiences. Divorce statistics show elevated risk at certain points in marriage duration, with the 7-year mark being one of several risk-elevated periods.
• What’s contested: The 7-year itch as a discrete biological cycle programmed into long-term pair-bonds has weaker empirical support than popular synthesis sometimes claims. The divorce-rate pattern at 7 years partly reflects accumulated relationship problems reaching a threshold rather than a defined biological clock. Other risk-elevated periods (2 years, after children leave home, retirement transition) also exist and reflect different mechanisms.
• The ovulation research: A research line in the late 1990s and 2000s found apparent shifts in women’s mate preferences across the menstrual cycle, with ovulating women showing increased preference for more masculine, symmetrical, dominant partners. The strippers-tipping study (Miller, Tybur, and Jordan 2007) found that exotic dancers earned more tips during their ovulatory phase, interpreted as evidence of ovulation-linked attractiveness signalling. The rummage-bag-of-pretend-dicks study (Eisenberg et al. 2014) found that women selected slightly larger sizes when ovulating compared to non-ovulating phases.

The replication picture of this body of work is mixed. The most striking findings (large effect sizes in masculine-feature preference shifts, dramatic ovulation-linked behaviour changes) have shown limited replication when tested with larger samples, better hormonal verification (some early studies inferred cycle phase from self-report rather than direct hormone measurement), and pre-registered analyses. A 2018 meta-analysis by Jones, Hahn, and DeBruine found that the most reliable ovulation-related preference shift was for men with cues of behavioural dominance and confidence; the more dramatic findings about facial masculinity, voice pitch, and body symmetry shifts have either not replicated or shown smaller effects than original reports.

Some real ovulation-linked changes in female mate preference exist, but the popular concept that ovulating women become “dangerous, strong, testosterone-laden partner” seekers in dramatic and predictable ways overstates what the better replicated findings support. The magnitude and dramatic specificity of the original claims have not held up.

The Coolidge effect and novelty: Named after a possibly apocryphal exchange between Calvin and Grace Coolidge, the Coolidge effect refers to the renewed sexual interest males show in novel female partners after declining interest in a habituated partner. The effect is well-documented in many mammals, including humans. The mechanism involves dopamine release in response to novelty, which partially explains both the appeal of new partners and the difficulty of sustaining intense sexual attraction within long-term pair-bonds.

• The Coolidge effect doesn’t mean monogamy is impossible or doomed. It means that long-term sexual relationships have to actively work against habituation, which is one of several ways that sustained pair-bonds require maintenance rather than running on autopilot.

IV. The Dual Control Model and the Architecture of Desire

Emily Nagoski, building on the work of John Bancroft and Erick Janssen at the Kinsey Institute, articulated the dual control model of sexual response in her book Come As You Are (2015, revised 2021). The model proposes that sexual response involves two parallel systems: a sexual excitation system (the accelerator) and a sexual inhibition system (the brakes). Both systems operate in both sexes, and individual variation in their relative sensitivity substantially shapes sexual experience.

The accelerator responds to sexual cues, such as visual, tactile, contextual, and fantasy-based. Activating the accelerator increases arousal. The brakes respond to threat cues, such as stress, fatigue, relational conflict, body image concerns, fear of pregnancy, fear of being walked in on, and fear of inadequate performance. Activating the brakes reduces or eliminates arousal even in the presence of strong accelerator activation.

The dual control model resolves several puzzles in sexual response research. Why does desire often increase when stress decreases? Because reducing brake activation matters as much as activating the accelerator. Why do many women find that sexual desire requires the right context rather than spontaneous emergence? Because their brakes are more easily activated than their accelerators in many everyday contexts. Why do many men in long-term relationships find that their wives’ interest in sex is more contingent on context than their own? Because the same accelerator-brake architecture interacts differently with the different circumstances of male and female lived experience.

Nagoski’s broader argument: most cultural framing of female sexual desire assumes a male-typical accelerator-dominant pattern, which produces both unrealistic expectations and unnecessary distress. Many women have responsive desire patterns (desire that emerges in response to a sexual context rather than spontaneously, independently) rather than spontaneous desire patterns. Both are normal. The cultural assumption that spontaneous desire is the only legitimate form has caused substantial relational unhappiness.

Meredith Chivers at Queen’s University has produced foundational research on the discordance between female genital and subjective sexual response. In her laboratory studies, women’s genital arousal (measured via vaginal photoplethysmography) and subjective arousal (self-reported) often correlate weakly. Genital arousal can occur in response to a wide range of sexual stimuli, including stimuli that women don’t consciously find arousing. Subjective arousal is more selective and depends on cognitive and contextual factors. In men, genital and subjective arousal correlate strongly; in women, they often don’t.

Chivers’ work has implications. The assumption that genital response indicates sexual interest or consent is not reliable for women. The reverse assumption that women’s reported low desire reflects physical dysfunction is also often wrong: subjective desire may be low for relational, contextual, or cognitive reasons even when the genital system is functioning normally. The “arousal nonconcordance” is a feature of female sexual response architecture, not a bug.

Jaak Panksepp at Washington State University, before his death in 2017, articulated the affective neuroscience framework that identifies LUST as one of seven primary emotional systems (alongside SEEKING, RAGE, FEAR, CARE, PANIC/GRIEF, and PLAY). Panksepp’s hypothesis places sexuality within the broader architecture of affective neuroscience, where LUST interacts with SEEKING (motivation and approach), CARE (the parental and partnership bonding system), and PLAY. The integration matters because human sexuality at its best involves all four of these systems engaged simultaneously, not just LUST in isolation.

V. Attachment in Adult Sexual Relationships

Johnson’s Emotionally Focused Therapy (EFT), developed in the 1980s onward, treats adult romantic and sexual relationships as attachment relationships of the same kind as parent-child attachment. This implies that sexual difficulties in established relationships often track attachment-related dynamics (security, anxiety, avoidance) rather than sexual technique or chemistry. Couples who feel secure with each other report better sex; couples who feel insecure or avoidant report worse sex. The directionality runs partly in both directions: secure attachment produces better sex, and good sex reinforces secure attachment.

EFT has accumulated substantial outcome research, with effect sizes among the largest in couples therapy. The framework has clinical traction across diverse populations and is supported by the broader attachment research base.

Esther Perel, in her popular books Mating in Captivity (2006) and The State of Affairs (2017), articulated what she calls the desire-intimacy paradox. Perel’s argument: intimacy and erotic desire often work in tension. Intimacy involves merger, vulnerability, predictability, and safety. Desire involves separation, mystery, novelty, and uncertainty. The same conditions that support intimacy (knowing the partner deeply, sharing daily life, raising children together) can erode erotic desire. The same conditions that support desire (mystery, separateness, unpredictability) can erode intimacy.

Perel’s clinical work isn’t strict empirical research but draws on extensive clinical experience and observation across cultures. Her framework hasn’t been formally tested in the same way that Johnson’s EFT has been, but the broader observation is that long-term sexual partnerships have to navigate a real tension between merger and separateness. This connects to the Coolidge effect in biology and is consistent with the broader research on long-term relationship dynamics.

Johnson’s attachment framework and Perel’s desire-intimacy framework aren’t fully reconcilable, but both name real dynamics in long-term sexual partnerships. Secure attachment is foundational (Johnson’s case), and sustained erotic desire within secure attachment requires deliberate attention to the conditions that support desire (Perel’s case).

VI. The Fisher Temperament Inventory

Helen Fisher’s research programme also produced a personality inventory based on the proposal that four neurochemical systems disproportionately shape individual temperament: dopamine (Explorer type), serotonin (Builder type), testosterone (Director type), and oestrogen (Negotiator type).

• The Explorer: novelty-seeking, adventurous, impulsive, optimistic. Dopamine system dominance.
• The Builder: cautious, socially compliant, rule-following, traditional. Serotonin system dominance.
• The Director: analytical, rigorous, direct, sometimes blunt. Testosterone system dominance.
• The Negotiator: pro-social, empathic, intuitive, integrative. Oestrogen system dominance.

Fisher’s framework has been used in research with millions of Chemistry.com users and has produced some interesting patterns: certain type combinations show different relationship outcomes, certain type combinations show different sexual styles, and so on. The framework has popular appeal partly because it provides a neurochemically grounded personality typology.

Fisher’s typology has limited independent replication outside her own research programme. The mapping between specific neurochemical systems and specific personality clusters is not as straightforward in reality; serotonin, dopamine, testosterone, and oestrogen all interact in complex ways with each other and with many other neurochemicals. The popular personality typologies that work neurochemically (this one, the Big Five with its neural correlates, the various temperament frameworks) all simplify a more tangled empirical picture.

Use the Fisher framework as one organisational tool for thinking about partner compatibility and individual differences in sexual style, not as a definitive neurochemical map of personality. Treating it as a starting point for self-reflection is reasonable; treating it as established science is not.

VII. The MDMA Parallel, Masturbation, and Mystical States

• The MDMA parallel: Wheal noted that the MDMA neurochemical profile resembles the neurochemistry of romantic love: serotonin levels rise (boosting mood and heightening perception), oxytocin follows (reducing fear and stress, increasing trust and connectivity), prolactin is released (contributing to post-experience relaxation), and the neurocircuitry of trauma response is temporarily quieted (hypervigilant amygdala and ventromedial prefrontal cortex reset). This is partly why MDMA-assisted therapy for PTSD has shown promising results in clinical trials, including the MAPS-sponsored Phase 3 trials that produced substantial therapeutic effects.
  • The parallel matters for understanding why early romance can be therapeutic for some forms of psychological distress, and why the loss of an early romantic bond can produce withdrawal-like states. The neurochemistry of love and the neurochemistry of certain therapeutic drugs converge on similar territory.
• Masturbation for chronic pain: The original page noted that masturbation is a reasonable intervention for chronic pain. Sexual stimulation and orgasm release endogenous opioids, oxytocin, and other analgesic compounds. Research has documented analgesic effects of orgasm, including for migraine, cluster headache, and some chronic pain syndromes. This isn’t a replacement for clinical pain management, but it is one of several non-pharmacological interventions.
• The Johns Hopkins comparison: A specific claim from the Hopkins psychedelic research group: In a head-to-head comparison between high-dose psilocybin and orgasm for mystical-experience generation, simple sexual stimulation prompted more reported mystical states than psilocybin by about 6 percent. This is one specific finding from one research line, not a robust replicated result, but orgasm, particularly in the context of bonded sexual partnership, produces neurochemical and subjective states that overlap with what other research traditions call mystical experience. 

VIII. Population vs Individual: The Aristotle Fallacy

One of the contributions of Heying & Weinstein’s A Hunter-Gatherer’s Guide to the 21st Century (2021) is the application of Aristotle’s “fallacy of division” to contemporary discussions of sex differences.

The fallacy of division: assuming that a true statement about a population applies to all individuals in that population. “Men are taller than women” is a true statement about averages. It does not mean that all men are taller than all women. Plenty of individual women are taller than plenty of individual men. The population-level pattern is robust; the individual variation around the population means is considerable.

Almost every meaningful claim about sex differences operates at the population level. Men are, on average, more interested in working with things; women are, on average, more interested in working with people. Men are, on average, more physically aggressive; women are, on average, more verbally aggressive. Men are, on average, better at certain visuospatial tasks; women are, on average, better at certain verbal tasks. Each of these statements is empirically supported as a population claim. None of them determines what any individual man or woman will be interested in, capable of, or drawn to.

Discussions of sex differences often collapse this distinction in both directions. The progressive collapse: denying real population-level differences because they don’t apply to all individuals. The conservative collapse: treating population-level differences as predictive for all individuals in ways that constrain individual lives. Both are versions of the fallacy of division. Both produce bad reasoning and worse policy.

Population-level sex differences and individual variation within sexes are legitimate in many domains. Both can be true simultaneously, and treating either as fully determinative is a category error.

The dimensions where Heying & Weinstein highlight the empirical evidence for population differences: Average disease risk profiles (Alzheimer’s, migraine, drug addiction, Parkinson’s, ADHD, anxiety disorders, autoimmune disease), average personality patterns (women are on average more altruistic, trusting, and compliant; more prone to depression than men), average vocational interests (women on average prefer working with people; men on average prefer working with things), average cognitive patterns (women on average better at detail, men on average better at gist; women on average better at certain language tasks, men on average better at certain spatial tasks). 

IX. The Deep History of Sex

• The cost of sexual reproduction: Sexual reproduction is genetically expensive. If you cloned yourself, you would be 100% genetically related to all your offspring. With sex, only 50% of your genes are in each child. Asexual reproduction is the better strategy if the future will look like the past (if conditions that worked for you will work for your clones). Sexual reproduction is the better strategy when conditions are changing, and genetic recombination produces offspring better suited to varying environments. The cost of sex (50% loss of genetic relatedness, plus the cost of finding and convincing a mate) is paid for by the variation that allows adaptation to novel conditions.
• Anisogamy and the origin of sex differences: Sexual reproduction requires that two organisms contribute DNA and that at least one contributes the cellular machinery (cytoplasm, including mitochondria and ribosomes) needed to build a new organism. The cell that provides this is the egg, which is large and sessile, since it carries the heavy machinery. The cell that provides only DNA is sperm or pollen, which is small and mobile. Anisogamy (the evolution of two different gamete sizes) is the foundational sex difference, and almost every subsequent sex difference follows from it.
• The SRY cascade: In humans and most mammals, the Y chromosome carries the SRY gene (Sex-determining Region Y), which initiates the development of testes when activated. The testes secrete testosterone and anti-Müllerian hormone (AMH). AMH causes the regression of the Müllerian ducts (the female reproductive precursors). Testosterone stabilises the Wolffian ducts (the male reproductive precursors) and is converted to dihydrotestosterone (DHT) by 5-alpha-reductase, which virilises the external genitalia. In the absence of SRY (in XX foetuses), the Müllerian ducts develop into the fallopian tubes, uterus, and upper vagina. 
• Sexual dimorphism across domains: Humans are sexually dimorphic across many domains beyond reproduction. Sex differences exist in brain structure, immune function, disease risk and progression, drug metabolism, pain processing, and many other physiological systems. Some of these differences are mediated by hormones; some are mediated by chromosomal differences (genes that escape X-inactivation produce different protein dosages in XX vs XY cells); some are mediated by interactions between the two. The detail on this is in Biological Sex.
• Sex roles and sexual selection: Sexual reproduction generally produces sex roles based on the differential investment in offspring. Females, who carry the larger gamete and (in mammals and birds) typically the greater parental investment, are the “limiting sex” (the resource over which males compete). Males in most species are therefore selected for display, competition, and aggression. Females in most species are selected for choosiness, since the cost of poor mate choice is higher for them.
  • Sex-role reversal occurs in some species where male investment exceeds female investment (some seahorses, jacanas, and other species). In these cases, males become the limiting sex and the usual patterns flip. This shows that sex roles are not determined by sex per se but by relative investment patterns, which in turn are largely determined by the biology of gamete production and parental care.
• Sex change in some species: Several species of fish, plants, and insects show sequential hermaphroditism, in which individuals begin life as one sex and switch to the other at some point. In mammals and birds, with our genetic sex determination, sex change is not biologically possible (chromosomal sex is fixed at conception). Behavioural roles (what humans call gender) are more flexible than biological sex, but the underlying biology is not negotiable in the way that some politically motivated social scientists suggest.

X. Sexual Selection in Humans

Human sexuality is unusual among animals in several respects, and the unusual features point to specific aspects of human evolution.

• Persistent breasts: Human females develop breasts at puberty, and breasts persist throughout the woman’s life regardless of whether she has children or is breastfeeding. In no other primate species do breasts persist when there are no infants to feed. Human breasts function as mammary glands when needed and as sexual ornaments at other times. They are advertisements as much as feeding apparatus.
• Concealed ovulation: Most mammals advertise fertility through visible physical or behavioural changes during the ovulatory window. Humans largely don’t. The concealment serves several functions: it encourages year-round sexual activity rather than seasonal mating, it reduces the certainty males have about specific paternity windows (which has both costs and benefits depending on social structure), and it supports the broader pattern of human sex for bonding rather than primarily for reproduction.
• Year-round sexual receptivity: Most mammals are sexually receptive only during specific reproductive windows. Humans are sexually receptive across the menstrual cycle (with some variation in desire and arousal) and across the year. This supports sex for bonding, pleasure, and pair-maintenance rather than just reproduction.
• Partial sex-role reversal: The standard sex-role pattern across animals is male-male competition and female choice. In humans, we see both: male-male competition exists, but female-female competition also exists, and male choice of partners is also operative. The ornamentation of human women (makeup, jewellery, dress) is consistent with partial sex-role reversal. Women advertise to attract male attention, in addition to men competing for female access. This is an unusual pattern and reflects something specific about human pair-bonding and parental investment.
• Larger penises: Human male penises are considerably larger relative to body size than those of any other great ape. The function is not fully understood. Hypotheses include displacement of rival sperm with the scoup-like head, signalling to females, support for face-to-face sexual positioning (which is more common in humans than in most primates), and sexual selection by female preference.
• Frequent female orgasm: Female orgasm in humans occurs in non-reproductive contexts more frequently than in any other species observed. The function of female orgasm has been debated for decades. Hypotheses include facilitating sperm retention (the “upsuck” hypothesis), supporting pair-bond formation through neurochemistry, and being a byproduct of clitoral development that shares embryological origin with the penis.

Concealed ovulation, abundant recreational sex, permanent female breasts, frequent female orgasm, and larger penises occur in no other species in this combination. Along with bipedal posture and large brain size, sexuality completes the trio of decisive ways in which the human lineage diverged from the other great apes.

XI. Mating Strategies

The work of David Buss at the University of Texas, particularly The Evolution of Desire (1994, updated 2016), established the empirical framework for human mating psychology. Cross-cultural research across dozens of cultures has produced consistent findings:

• Women in essentially every culture studied show a stronger preference for mates with high earning potential or resource-acquisition capacity than do men.
• Men in essentially every culture studied show a stronger preference for mates who are young and physically attractive than do women.
• Both sexes show preferences for kindness, intelligence, and emotional stability in long-term partners.
• The differences are larger for short-term mating contexts than for long-term partnership contexts.

The evolutionary interpretation: Women who might become pregnant benefit from partners who can contribute resources to offspring; men who might father children benefit from partners whose youth and apparent health suggest fertility. These preferences are statistical patterns rather than deterministic constraints on any individual; plenty of people make partner choices that depart from these averages.

Heying & Weinstein build on this evolutionary psychology to articulate three broad reproductive strategies:

1. Long-term partnership and investment: Find a partner, build a life together, invest in offspring and the relationship, and grow old together.
2. Forced reproduction: Rape and other coercive strategies. Universally condemned across cultures; legally and morally indefensible.
3. The short game: Sex without commitment or investment. Multiple short-term partners. No expectation of relationship continuity.

Strategy 1 is broadly the best outcome for individuals, children, women, and society. Strategy 2 is universally condemned. Strategy 3 is the contested one.

Strategy 3: Heying & Weinstein argue that strategy 3, despite being endorsed by “sex-positive” groups, is a worse outcome than strategy 1 for most people most of the time. Their argument: women who pursue strategy 3 are often hoping to attract men interested in strategy 1, while men interested in strategy 3 are often signalling availability for strategy 1 to access strategy-3 partners. The mismatch produces predictable patterns of mutual disappointment and difficulty forming stable partnerships.

The “sex-positive” framing argues that consensual short-term encounters between adults are no one else’s business and serve real human needs (sexual exploration, learning, situational compatibility). The above critique argues that the cultural endorsement of strategy 3 has produced measurable declines in pair-bond stability, marriage formation, and reported relationship satisfaction in populations that have most thoroughly adopted it.

Consensual short-term encounters between adults are not inherently harmful and serve genuine needs for some people in some life circumstances. The aggregate cultural pattern of normalised short-term encounters as the default mating pattern has produced documentable population-level outcomes that include later first marriages, lower marriage rates, declining marital satisfaction, declining birth rates, and elevated reported loneliness across multiple Western populations. The individual practice and the aggregate cultural pattern are different things, and conflating them produces bad conversations.

Hotness vs beauty: Hotness signals short-term sexual availability and fades rapidly with reproductive potential; beauty signals long-term partner quality and fades far more slowly. The cultural shift toward hotness as the dominant aesthetic, particularly through social media, reflects the cultural shift toward strategy 3 as the dominant mating pattern. The Jia Tolentino observation on the “Instagram face” captures part of this dynamic:

The face is “a young face, of course, with poreless skin and plump, high cheekbones. It has catlike eyes and long, cartoonish lashes; it has a small, neat nose and full, lush lips. It looks at you coyly but blankly, as if its owner has taken half a Klonopin and is considering asking you for a private-jet ride to Coachella. The face is distinctly white but ambiguously ethnic.”

The convergence of facial aesthetics through cosmetic surgery, makeup, and digital filtering toward a single “Instagram face” reflects the underlying convergence of sexual signalling toward strategy 3.

XII. Pornography and What It Does

The argument on pornography deserves engagement.

• The reductionism critique: Sex is interactional and emergent. “Having sex” with one person is qualitatively different from “having sex” with another person. The phrase treats sex as if it were like “watching Netflix”: a consumer activity where the specific partner is interchangeable. The reduction misses what makes human sexuality work.
• The “sexual autism” framing: Pornography produces patterns in its users that resemble the diagnostic criteria for autism applied specifically to sexuality: primary attention to sensory input, backgrounded social and emotional communication, repetitive behaviour patterns, atypical sensitivity to sensory inputs, difficulty with novelty and surprise, and inflexible adherence to routine. 
• The empirical research: Pornography use at the population level has been associated with reduced relationship satisfaction, particularly when use is regular and solo, and particularly when the user is in a relationship. Studies have documented changes in sexual response patterns in heavy pornography users, including reduced response to partnered sex and erectile difficulties in younger men (sometimes called porn-induced erectile dysfunction, though the empirical picture on this is mixed). The mainstream sexology research based on pornography effects is genuinely mixed; some research finds modest negative effects, some finds no effects, and some finds context-dependent effects (different for casual vs heavy users, for partnered vs single users, for different content types).
• The “what women are being asked to do” observation: Populations that have come of age on a steady diet of pornography show specific patterns: women in heterosexual relationships are far more likely to report being asked to engage in anal sex, choking, and other practices commonly depicted in pornography, even though survey research suggests most women don’t desire these practices outside of specific situations. This is a documented pattern, not a moral claim. It reflects the educational role pornography has come to play in the absence of other forms of sexual education or modelling.

Pornography exists; people will use it; some uses are problematic, and some are not. The cultural pattern of treating pornography as the primary or only sexual education source produces predictable problems. The hardcore extremes of the anti-pornography position (all pornography is harmful, all use should be eliminated) and the libertine position (pornography use is just consumer choice and has no broader effects) both miss what the empirical research actually shows. Pornography use has real population-level effects; those effects are stronger for heavier use, partnered users, and users in formative developmental windows, and reducing reliance on pornography as a sexual model is broadly beneficial for relationship quality.

XIII. Love and the Becoming-One Question

Love is a state of the emotional mind that causes one to prioritise someone or something external as an extension of self. Love is a matter of intimate inclusion.

The evolutionary origin: Love first evolved between mother and offspring in early mammals, approximately 200 million years ago. The first mammals were egg-layers (the extant monotremes like echidnas and platypus retain this), but all mammals make milk. Lactation produces extended close contact between mother and infant, which requires sustained behavioural commitment from the mother. The neurochemistry that supports this commitment (oxytocin in particular) is the same neurochemistry that later supports pair-bonding between adult mates.

The expansion: Love evolved first between mother and child, then between fathers and children (in species with paternal care), then between adult mates (in pair-bonding species), then between other relatives, then between friends and allies, then eventually to abstractions (love of country, love of God, love of justice, love of truth). All of this is built on the same neurochemical foundation that originally evolved to sustain maternal care of helpless infants.

The pair-bonding extension: Humans are an unusually pair-bonding species among mammals. The biology supports it: high oxytocin and vasopressin receptor densities in reward-related brain regions, hormonal cascades that sustain attachment, and sexual response patterns that include strong bonding components alongside the reproductive ones. The cultural elaboration around love (courtship rituals, marriage institutions, love poetry, romantic narratives) emerged across cultures to support and extend the biology.

The boundary-dissolving function: Sex within a pair-bond does something specific in human neurobiology: it briefly reduces the distinction between self and partner. The neuroimaging shows it; the subjective experience reports it; the cross-cultural literature on love and sex reports it. This is what is meant by “to love is to know, and to know is to access truth.” The truth being accessed is the underlying continuity that the separate appearances of self and other emerge from. Sex with a bonded partner provides one of the more reliable everyday access points to that continuity.

Sexual practices that maximise pleasure while minimising bonding (one-night stands, pornography, casual sex, the cultural defaults that have emerged in the past several decades) miss the boundary-dissolving function. They produce pleasure without producing the deeper experience. The result is people who report having a lot of sex but feeling chronically alone, sexually active populations with declining sexual satisfaction, and cultural patterns that produce the recognisable contemporary loneliness epidemic alongside saturated sexual content.

Pleasure is the motivation. Meaning is the goal. The biology supports both, but the meaning requires sustained pair-bonding and the meaning-laden context that the human cultural elaboration evolved to provide.

XIV. Mating Systems

• Monogamy: individuals of both sexes have one partner at a time. Includes serial monogamy across the lifespan.
• Polygyny: one male, multiple females. Most common form of polygamy across species and across human cultures historically.
• Polyandry: one female, multiple males. Rare across species and rare in human cultures.
• Promiscuity: both sexes have multiple partners. In humans sometimes called polyamory.

Sexual size dimorphism (the difference in body size between males and females) is a reliable predictor of polygyny across vertebrate species. In strongly polygynous species, males are substantially larger than females, since male-male competition selects for size and aggression. In monogamous species, sex size differences are small. Humans show modest sexual size dimorphism (men are on average about 15% larger and substantially stronger than women), suggesting our ancestors were modestly polygynous rather than strictly monogamous. This is consistent with the cross-cultural anthropological record, which shows most pre-modern human cultures permitted polygyny in some form, even when monogamy was the practical norm for most people.

The case for monogamy: Despite the historical prevalence of polygynous arrangements, monogamy has emerged as the dominant marriage form across most contemporary cultures. The case for monogamy as the superior arrangement has several dimensions:

• Pair-bonding fidelity: Sex ratios in human populations are approximately 1:1. Monogamy is the only mating system that allows nearly every adult to find a partner. Polygynous systems produce sexually frustrated young men with limited reproductive prospects, who are statistically more likely to engage in violence and risky behaviour.
• Child outcomes: Children with two committed parents tend to show better outcomes across multiple dimensions (educational attainment, mental health, life expectancy, relationship stability in adulthood) than children in other arrangements, on average. The effect persists after controlling for many confounders.
• Female bargaining position: In polygynous arrangements, women’s bargaining power tends to be reduced relative to monogamous arrangements, since powerful men can demand multiple partners and women have to compete for access to male resources.
• Reduced male violence: Monogamous societies tend to show lower rates of male violence than polygynous societies, since competition for limited female access drives much male aggression.
• Co-parental investment: Monogamy enables substantial paternal investment in offspring, which is rare in polygynous arrangements. The substantial paternal investment in human children appears to be one of the distinctive features of our species.

The fragility of monogamy: Monogamy is fragile in mammals. Many ostensibly monogamous mammal species (including some bird species traditionally classified as monogamous) actually engage in substantial extra-pair copulation. Human monogamy is partially maintained by cultural institutions (marriage, kinship structures, religious frameworks, legal frameworks) on top of biological foundations that incline toward but don’t strictly enforce monogamy. The collapse of cultural supports for monogamy in modern Western societies produces predictable shifts back toward more polygynous patterns, with serial monogamy and the accumulating reproductive advantages of high-status males being the contemporary expression.

Casual sex and the modern context: The shift toward normalised casual sex in modern Western cultures, supported by birth control technology, has produced a complex pattern. On the positive side: greater sexual freedom, reduced unwanted pregnancy, and reduced exposure to historical risks of female sexual expression. On the more complicated side: declining pair-bond stability, declining marriage rates, less responsibility for choices, declining birth rates, and a pattern where women’s reproductive bargaining power has substantially decreased even as their formal social and economic power has increased. Whoops.

The oxytocin and vasopressin biology matters here. Women’s neurochemistry produces bonding responses to sexual contact, particularly with the same partner repeatedly. Men’s neurochemistry produces bonding responses primarily to extended association rather than primarily to sexual contact. Women in casual sex contexts are often unintentionally activating bonding circuitry that the modern culture tells them to suppress. Men in casual sex contexts can engage without the same bonding activation, since their bonding requires the time and shared experience that casual encounters don’t provide. The asymmetry produces predictable patterns of relational mismatch.

Sexual freedom is genuinely valuable, and the older restrictive frameworks had real costs. The contemporary defaults have produced their own costs, particularly for the populations most thoroughly socialised into them. The interesting question is how to maintain genuine sexual freedom while supporting the cultural infrastructure for sustainable pair-bonds, which is not a question that either the conservative or the progressive framing has answered well.

XV. Cross-Links

The biological details on chromosomal architecture, hormonal profiles, and sexual dimorphism across physiological systems are covered in Biological Sex.

The neurochemistry of desire, arousal, and the relational architecture of long-term sexual satisfaction is covered in Optimizing Pleasure.

The practical interventions for hormonal health, fertility, and sexual function are in Sex Cheatsheet.

The foetal development of sex, intersex conditions, the research on sexual orientation, and contested empirical territories are in The Sex Rabbit Hole.

The pair-bonding biology cross-references the social bonding work in Connection. The endocrine disruption material connects directly to the reproductive health implications covered in The Environmental Rabbit Hole and The Elements. The meaning-laden framing of sexual bonding connects to Finding Meaning and the broader work on what sustains long-term wellbeing.