Why do people have sex

Pathogen Defense Theory of Sex

An evolutionary framework proposing that sexual reproduction evolved primarily as a defense against rapidly evolving pathogens, viruses, and parasites that reproduce millions of times faster than complex hosts; scrambling genes randomly every generation ensures offspring inherit unique immune defenses, as demonstrated by New Zealand mud snails reproducing sexually in parasite-dense shallow waters but asexually in parasite-scarce deep waters (Slide 3-4 / 6:23 / 11:20 / 12:56 / p. 238).

Anisogamy

The fundamental biological disparity in gamete size and energy investment between sexes, where males produce billions of small, energetically cheap, mobile gametes (sperm) throughout life, whereas females produce a limited reserve of large, nutrient-rich, energetically expensive gametes (eggs), creating an initial reproductive inequality (Slide 5-6 / 14:01 / 15:05 / p. 238).

Obligatory Parental Investment

The biological requirement that females invest vastly greater physical resources into each offspring than males, including nutrient-dense ova, nine months of internal gestation, and one to two years of metabolic lactation costing 625 calories daily, whereas male obligatory investment requires only minutes of time and sperm (Slide 6, 13 / 16:09 / 31:19 / p. 238-239).

Continuum of Mating Strategies

The spectrum of human romantic and sexual relationships categorized by the degree of investment of time, energy, attention, and resources, ranging from short-term uncommitted encounters to long-term committed pair bonds (Slide 7 / 24:06 / p. 234).

Sexual Strategies Theory

David Buss and David Schmitt's evolutionary framework postulating that while both men and women evolved long-term mating strategies to successfully raise altricial offspring, men on average evolved a greater psychological interest in short-term mating due to asymmetric obligatory parental investment and higher potential reproductive rate (Slide 11-15 / 29:32 / p. 238-240).

Bateman's Principle

The evolutionary principle stating that male reproductive success increases directly and linearly with the number of sexual partners acquired, whereas female reproductive success is strictly capped by biological gestation and lactation constraints regardless of partner quantity (Slide 14 / 32:30 / p. 238).

Clark and Hatfield Campus Experiment

A landmark 1989 field study where male and female confederates approached college students with casual offers to go out, go to an apartment, or go to bed; approximately 50% of both sexes agreed to a date, but 75% of men versus 0% of women agreed to immediate sex, an asymmetry replicated across multiple global studies (Slide 18-19 / 45:13 / 49:32 / p. 239).

Sexual Regret Asymmetry

Andrew Galperin's empirical finding that men and women experience distinct patterns of sexual regret due to evolutionary cost asymmetries, with men predominantly regretting sexual inactions and missed mating opportunities, whereas women predominantly regret sexual actions and hasty involvement with uncommitted partners (Slide 16-17 / 38:03 / 39:49 / p. 239).

Four Whys of Long-Term Mating

Tinbergen's four analytical lenses applied to human pair bonding: Mechanism (feeling love and romantic attraction), Ontogeny (growing mutual psychological dependence over time), Function (cooperating to pay the 2.5 million calorie cost of raising helpless human offspring), and Phylogeny (recurrent independent evolution of monogamy across birds and select mammals) (Slide 2, 9 / 3:22 / 25:38 / p. 233-235, 245).

Waist-to-Hip Ratio Attractiveness

The cross-cultural preference where men judge women with a waist-to-hip ratio of approximately 0.7 as most physically attractive, a morphological cue linked directly to youth, optimal estrogen distribution, higher fertility, and overall metabolic health (p. 237-238).

Sternberg's Triangular Model of Love

A theoretical model postulating that love is composed of three distinct components: passion (physiological arousal and sexual longing), intimacy (feelings of closeness, warmth, and emotional sharing), and decision/commitment (the choice to love another and maintain that bond long-term) (p. 233).

Passionate Love versus Companionate Love

The foundational distinction between passionate love, defined as an intense, emotionally turbulent state of longing for union with another, and companionate love, defined as the deep affection, tenderness, and trust felt for those with whom our lives are intertwined (p. 234).

Nurturant Love versus Attachment Love

Subtypes of companionate love where nurturant love describes the protective, caring feelings parents feel toward offspring, whereas attachment love describes the desire to be protected and cared for by another person (p. 235).

Sociosexual Orientation

An individual difference trait measuring a person's preference for restricted sex (requiring deep emotional attachment and commitment) versus unrestricted sex (casual sex without love), assessed via the Sociosexual Orientation Inventory (p. 240).

Two-Factor Theory of Love

The emotion framework proposing that romantic love consists of non-specific physiological arousal combined with an environmental cognitive label, where arousal from external stressors like shaky bridges or exercise boosts attraction to potential partners (p. 242).

Attachment Styles

Individual styles of romantic attachment derived from childhood infant-caregiver bonds, categorized as secure (confident in partner love), anxious/ambivalent (obsessive fear of abandonment and high emotional turbulence), or avoidant (uncomfortable with closeness and withdrawing under stress) (p. 246).

Erotomania

A rare clinical psychiatric disorder characterized by an unshakeable, delusional belief that one is passionately loved by another person, usually an older individual of higher social status (p. 248).

Exchange versus Communal Orientation

A relationship orientation distinction where exchange-oriented individuals strictly track costs, benefits, and obligations, whereas communal-oriented individuals provide benefits freely based on partner needs without keeping tally (p. 255).

Forms of Marriage Systems

Cultural marital structures including monogamy (one spouse), polygamy (multiple spouses), polygyny (one husband with multiple wives, common in societies with steep resource hierarchies), and polyandry (one wife with multiple husbands, often brothers sharing scarce land) (p. 254).

Equity Rule versus Need-Based Rule

Social exchange principles governing relationships where the equity rule demands benefits proportional to inputs in neutral or transactional bonds, whereas the need-based rule dictates providing assistance based solely on partner need in loving marriages (p. 255).

Twofold Cost of Sex

The significant evolutionary cost of sexual reproduction compared to asexual cloning, arising because sexual organisms reproduce at half the rate of cloners and must expend massive energy and time locating, attracting, and securing mates (Slide 2 / 5:37).

Cavendish Banana Extinction Threat

A lecture illustration highlighting the vulnerability of asexual clones, where all commercial Cavendish bananas are genetically identical clones currently facing wiped-out extinction by a single fungal disease due to zero genetic variation (10:49).

Human Inter-birth Interval

The average temporal gap between consecutive offspring births, which spans 6 years in wild chimpanzees but only 3.5 years in ancestral humans, forcing human mothers to nurse and provision multiple dependent offspring at once (28:19).

Sex-Positive Polyamorous Rocket Ship

A thought experiment created by the professor to demonstrate Bateman's principle, illustrating that sending 1 woman with 4 men on a space mission yields a maximum of 1 or 2 babies, whereas sending 1 man with 4 women can yield up to 4 babies (Slide 14 / 33:19).

Bowerbird Courtship Display

A video demonstration of extreme male courtship display where male bowerbirds meticulously sweep courts, build arches, flash iridescent eye patches, and execute precise dances to win approval from choosy females (53:04).

Coded Human Flirting Displays

The professor's observation that unlike animals displaying explicit sexual requests, humans utilize subtle, coded flirtatious communication such as jacket compliments or chivalry to signal sexual interest while maintaining social plausible deniability (1:00:01).

Non-Human Animal Homosexuality and Inversion

Lecture examples of animal sexual orientation and gender identity variations, including a female chimp behaving in male social roles in Frans de Waal's research and a male penguin pair stealing an egg to co-parent in a Denmark zoo (17:15 / 21:24).

Spandrel Chin Evolutionary Byproduct

An evolutionary byproduct example explained in supplementary video material where the human chin developed not for an adaptive function, but as a structural byproduct of facial flattening and molar retraction during human evolution (WIRED Video / Slide).

Atavistic Extra Nipples

A developmental phenomenon where extra nipples or anatomical throwbacks appear along the embryonic nipple line due to rare genetic recombination, reflecting ancestral mammalian traits (WIRED Video / 58:00).

Vestigial Plantaris Muscle

An anatomical structure in the lower leg that has shrunk into a functionless tendon in modern humans due to bipedal locomotion, despite being fully functional in great apes (WIRED Video / 62:00).

Why is long-term monogamous mating considered biologically rare among mammals yet universal in humans?

Long-term monogamous pair bonding occurs in only about 5% of mammalian species, yet it is a universal human mating strategy found across all historical eras and cultures because human offspring require immense biparental investment to survive (Slide 8 / 24:26).

What is the total estimated caloric cost for a mother to carry, give birth to, and nurse one human child?

Biologists Aiello and Key estimate that birthing and nursing one child costs approximately 2.5 million total calories, with daily lactation requiring 625 extra calories (equivalent to 1.1 Big Macs per day, or 4,400 Big Macs across the nursing period) (Slide 10 / 27:07).

How does brain energy expenditure in human newborns compare to adult brain energy usage?

In adult humans, the brain represents 2% of body weight but consumes 20% of daily energy expenditure, whereas at birth, an infant's brain consumes roughly 50% of total daily energy expenditure (Slide 9 / 27:03).

What percentage of the US population identifies as gay or lesbian, and why does exclusive same-sex orientation pose an evolutionary puzzle?

Approximately 1.6% of the US population identifies as exclusively gay or lesbian, posing an evolutionary puzzle because exclusive same-sex preference seemingly reduces direct reproductive output despite strong natural selection against non-reproducing heritable traits (18:23).

How did relationship status affect Danish men's and women's willingness to accept casual sex offers in Hald & Høgh-Olesen's (2010) study?

In the Danish replication, 59% of single men accepted a stranger's offer of casual sex compared to 18% of men in relationships, while 0% of single women and only 1 out of 23 women in a relationship agreed (21:34 / 49:43).