14. What Motivates Us: Sex

Scientific concepts, discoveries, and nature phenomena mentioned

Time vs. importance of sex (behavioral/cultural data)

• Polls often report sex as the favorite activity, but survey estimates suggest average time spent on sex is about 4 minutes and 3 seconds per day (excluding many related/adjacent activities like flirting, thinking about sex, aftermath, etc.).
• Despite low time investment, sex is framed as central to downstream human outcomes, such as:
  • Marriage
  • Family and children
  • Competition and aggression
  • Cultural and creative life

Evolutionary puzzle: why sexual reproduction evolved

• Sexual reproduction with two sexes is described as “bizarre” from a Darwinian viewpoint because it effectively discards half the genes each generation compared with cloning.
• The lecture frames sex evolution as an ongoing mystery (though it is not the main focus of the talk).

Moral vs. biological explanations

• Biologists are described as tying sex (at least partly) to gene-spreading adaptation, raising the question of whether non-procreative sex is “unnatural” and therefore “wrong.”
• The speaker argues that biological facts don’t directly determine moral norms.
• The lecture also stresses that genes are not automatically “inevitable”:
  • Genetic causes differ from environmental causes.
  • But neither category necessarily implies immutability.

Parental Investment Theory (Robert Trivers)

Definition

• Parental investment: any parental investment that increases offspring survival probability at the cost of the parent’s ability to invest in other offspring.

Key mechanistic claim

• Females typically have higher parental investment because they produce larger eggs and/or must carry/protect them.
• Males often have lower parental investment (small gametes like sperm), enabling potential fertilization of multiple females.

Predicted downstream effects

• Male–male competition for reproductive access tends to increase, often leading to:
  • Larger body size
  • Weapons/displays for fighting or attracting
• Female choosiness increases because females gain more by selecting mates whose traits maximize offspring survival.
• Different reproductive strategies can reverse asymmetries in species where roles flip.

Evidence/examples across species illustrating parental investment predictions

• Pipefish (sex-role reversal)

  • Males carry eggs in a pouch (higher investment), leading to predictions such as:
    • Larger females
    • Increased female–female competition
    • Females competing for male attention

• Penguins (March of the Penguins)

  • Used to motivate that when parental investment is more equal, sexes should be more similar in size/aggression.
  • The lecture claims penguin sexes are about the same size.

• Elephant seals

  • Large male-biased size dimorphism attributed to intense harem competition (“winner take all”).

• Gibbons

  • Similar body sizes attributed to monogamous parenting and more shared investment.

Human sexual dimorphism and competition

• Human cultures are described as serially monogamous rather than lifelong mating.
• Males are ~15% larger on average than females.
• Evidence for intermediate sperm competition:
  • Men have testicles larger than those of gorillas/gibbons but smaller than chimpanzees, interpreted as reflecting differing degrees of sperm competition.
• Aggression differences presented as average trends:
  • Men are described as more physically violent in utero, childhood, and adulthood.
  • Prisons and killings are said to be male-dominated on average.
  • Testosterone is described as increasing aggressiveness in humans and other primates.

Sexual choosiness and mate variety

Sex differences in receptivity to sex

• Parental investment theory predicts:
  • Males should be more receptive to anonymous sex than females.

Cross-cultural/behavioral indicators discussed

• Prostitution interest portrayed as predominantly male.
• Pornography described as universal, with examples including carved depictions and even “monkey porn” experiments.
• “Coolidge Effect”
  • Explained through an anecdote about President Calvin Coolidge and hens/one rooster, used as a concept for increased sexual interest with novelty.

Quantitative preferences (survey-style measures)

• Examples include desired number of partners over:
  • next month,
  • next year(s),
  • lifetime.
• Reported differences:
  • Women generally lower than men.
• Hypothetical approach studies (campus/actors) suggest patterns such as:
  • Women more often accept dating with a very attractive man but far less often accept apartment/sex requests.
  • Men show different patterns, including some refusals followed by apologetic reasons.

Same-sex behavior as a “cleaner” measure of desire (with caveats)

• The lecture argues that comparing heterosexual behavior can be complicated by compromise between both partners’ preferences.
• Therefore, it treats same-sex behavior/desire as a more direct window, though the data are said to be messy.
• Claim:
  • Gay/lesbian data generally suggest lesbians are more monogamous than gay men.
  • Older studies pre-AIDS are described as finding high male partner counts.

Sexual attractiveness: cross-cultural mate preference universals

• Large cross-cultural datasets are cited (e.g., 10,000 people across 37 countries) showing:
  • Both sexes value kindness and intelligence (highly).
  • Females more strongly weight status/power and mate qualities tied to investment and offspring.
  • Males more strongly weight cues related to a partner’s ability to have children (fertility window).
• Age/status reasoning:
  • For males, age differences are framed as more about status and physical strength than fertility (except later declines).

Physical beauty as signaling youth and health

Beauty cues described include:

• Youth/sexual maturity
  • e.g., large eyes, full lips, smooth/tight skin
• Health
  • absence of deformities, clear eyes, unblemished skin, intact teeth
• Averageness
  • average faces are proposed as attractive because they minimize deviations caused by developmental/health disruptions.
  • composite/morphed “average face” studies are described as making faces look more attractive progressively.
• Babies reportedly prefer more average faces, interpreted as partly hard-wired rather than purely cultural.

Ovulation-linked preference for masculinized/attractive faces

• A study logic is described: women prefer a more attractive (“feminized” prototype manipulation vs average) face when women are ovulating.
• The lecture claims replication across multiple labs.

Nature vs nurture debate in sex differences

• The lecture expresses skepticism that all sex differences are biological, suggesting social mechanisms such as:
  • Gendered treatment of infants (blue/pink labeling studies)
  • Name-based biases in evaluations of applications/emails (e.g., “John Smith” vs “Joan Smith”)
  • Gender self-segregation in childhood (boys with boys; girls with girls from ~ages 4–11), proposed to amplify differences
• Discussed sex differences in empathy/social cognition:
  • Simon Baron-Cohen’s thesis (from The Essential Difference): males are by nature less empathetic/more socially impaired.
  • A link between testosterone and sociality is mentioned.
  • Autism-spectrum and related disorders are described as more prevalent in men (autism, Asperger’s, conduct disorder, psychopathy), with Baron-Cohen’s “mild autism” slogan.

Controversial claim about sex differences in math/science aptitude variability

• Larry Summers is mentioned for a controversial argument that women are underrepresented due to differences in “intrinsic aptitude,” especially greater male variability (more extreme low and high ability).
• The lecture points to a debate on The Edge between:
  • Steve Pinker
  • Liz Spelke
• The talk does not adjudicate the pros/cons.

Sexual orientation origins and the “exclusive homosexuality” evolutionary puzzle

Prevalence (high uncertainty)

• Approximate prevalence claims are given (noted as very uncertain due to measurement issues):
  • ~98% of women attracted to men
  • ~96% of men attracted to women (numbers approximate)

Orientation vs behavior

• Orientation is distinguished from behavior:
  • Someone may have same-sex sex without being exclusively oriented.

Evolutionary puzzle

• Exclusive homosexuality is framed as an evolutionary puzzle because such traits would seemingly reduce reproductive success and thus be expected to be selected against.

Genetic predisposition evidence

• Twin/adoption-style reasoning is described:
  • Identical twins show high concordance for homosexuality but not 100%.
  • This suggests genetic influence but also non-genetic (possibly prenatal) factors.

Cross-species homosexuality question

• Many animals show same-sex sexual behavior, but it remains unclear whether any species shows exclusive homosexual orientation comparable to humans.

---

Researchers, authors, and sources featured

• James Gleick (author of a book summarizing sex-time/poll discussion)
• Professor Paul Bloom (lecturer)
• Steve Pinker (How the Mind Works)
• Richard Dawkins (quote about genetic vs environmental inevitability)
• Robert Trivers (“Parental Investment” theory)
• Eleanor Maccoby (proposal about childhood group segregation amplifying aggression/behavior differences)
• Simon Baron-Cohen (The Essential Difference; autism/empathy-related claims)
• Liz Spelke (infant cognition researcher; referenced in debate)
• Larry Summers (referenced for controversy about sex differences in science/engineering “variability”)
• Calvin Coolidge (used in the “Coolidge Effect” anecdote)
• Sasha Baron Cohen (mentioned as more famous cousin trivia; not treated as a scientific source here)
• Duke University (research group mentioned in relation to “monkey porn” experiment)
• The Edge (Harvard debate video) (between Pinker and Spelke)
• President of/associated Harvard (mentioned via Larry Summers as institutional context)
• Movie: “March of the Penguins” (used as an example source; not a researcher)