Lecture 14: What Motivates Us - Sex, Attraction and Orientation

What this lecture covers

This lecture applies evolutionary reasoning to sex, asking why males and females differ psychologically and physically, what people find attractive, and why some people are exclusively attracted to their own sex. Paul Bloom opens by addressing two ethical worries head-on: that an evolutionary account of sex makes non-reproductive sex "unnatural" or morally wrong, and that a biological explanation for a trait means it is fixed and unchangeable. Both worries, he argues, rest on mistaken inferences from biology to morality or inevitability.

After watching, you can explain Robert Trivers's parental investment theory and what it predicts about size, aggression, and mate choosiness across species, describe cross-cultural findings on human mate preference and physical attractiveness, and summarize why exclusive homosexuality remains an open evolutionary puzzle rather than a settled question.

Key ideas

• Naturalistic and moralistic fallacies: Bloom argues that what is evolutionarily "natural" does not determine what is morally right, and that a trait having a genetic cause does not mean it is impossible to change.
• Parental investment theory (Trivers): the sex that invests more in offspring (typically females, due to costly eggs and internal gestation) becomes the more selective sex, while the lower-investing sex (typically males) competes more intensely for mating access.
• Sex differences follow from investment asymmetry: species where investment is unequal show size and aggression differences between the sexes; species with more equal investment, like gibbons and penguins, show little or no such difference.
• Human patterns fit the theory loosely: humans show a modest male size advantage, more male aggression and violence, and greater average male interest in anonymous or varied sexual partners, consistent with historically unequal (though not extreme) parental investment.
• Attractiveness signals youth and health: cross-cultural research finds converging preferences for facial features linked to fertility and the absence of illness or deformity, including a strong preference for average, symmetrical faces.
• Social explanations also matter: differential treatment of infants and adults by perceived sex, and childhood gender self-segregation, can create or amplify sex differences independent of biology.
• Sexual orientation is not a choice and not fully genetic: twin studies show a genetic contribution to homosexuality, but the concordance rate between identical twins is far from 100%, meaning other factors, possibly prenatal, are also involved.
• Exclusive homosexuality is an evolutionary puzzle: unlike occasional same-sex behavior, an exclusive lack of interest in reproductive sex is hard to explain as a trait favored by natural selection, and Bloom presents it as an open question rather than a solved one.

Walkthrough

Addressing morality and inevitability on evolution, sex and gender (0:00)

Bloom opens with the mismatch between how much people value sex and how little time they actually spend on it, then previews the lecture's scope: sex differences, attractiveness, and the origins of sexual orientation. Before getting into evolutionary claims, he addresses two objections directly. Quoting Steven Pinker, he argues that a behavior serving reproductive fitness does not make its absence (such as voluntary childlessness or birth control) wrong. Quoting Richard Dawkins, he argues that a genetic cause does not imply a trait is unchangeable, using his own poor eyesight (genetic, easily corrected with glasses) versus cultural attitudes toward body weight (not strongly biological, but hard to shift) as contrasting examples.

Basic sex ed (10:47)

The lecture defines male and female biologically by sex cell size rather than anatomy, then introduces Robert Trivers's parental investment theory: the sex with the higher-investment sex cell (typically eggs) bears a greater cost per offspring and becomes more selective about mates, while the lower-investment sex competes more for mating opportunities. This explains why males are often larger and more aggressive, and why males often evolve elaborate displays to attract choosy females. Bloom shows how the theory correctly predicts reversed patterns in species like pipefish, where males carry the offspring, and near-equal patterns in species like gibbons and Emperor penguins, where both parents invest heavily.

Sex differences among humans (19:22)

Applying the theory to humans, Bloom notes evidence of moderate but real asymmetry: modest average size differences between men and women, higher male aggression across the lifespan (including in the womb and in adulthood), and testicle size suggesting some historical sperm competition. He reviews survey and behavioral evidence that men report wanting more sexual partners and are more willing to accept casual sexual propositions from strangers, while women are far more selective, and connects this to the "Coolidge Effect" idea that males show more interest in novel partners. He notes that comparisons of gay male versus lesbian relationship patterns offer a way to observe sex-typical preferences without the compromise inherent in heterosexual pairings.

Beauty: an average face (32:15)

Bloom turns to mate preference research across cultures, noting that both sexes value kindness and intelligence highly, while women tend to weight status and parenting potential more and men weight fertility cues more. He then focuses on physical beauty, arguing that attractiveness cues signal youth and health: symmetry, clear skin, and intact teeth. He presents research showing that computer-averaged, composite faces are rated as more attractive than most individual faces, and that even infants prefer looking at averaged faces, suggesting the preference is not purely learned. He also discusses how features can be exaggerated beyond average to increase perceived attractiveness, and notes a documented shift in women's preferences across the ovulatory cycle.

Social factors for sex differences (38:25)

Bloom distinguishes sex differences with strong cross-cultural, likely biological support from ones whose causes are less clear, including him expressing personal skepticism about biological explanations for some claimed differences. He discusses evidence that people treat infants and adults differently based on perceived sex (such as differing reactions to identical resumes with male versus female names), and describes childhood gender self-segregation as a process that may amplify small initial differences. He summarizes Simon Baron-Cohen's argument that men are, on average, less empathetic, citing higher male rates of autism-spectrum conditions, and briefly covers the controversy sparked by Larry Summers's remarks on possible sex differences in variability of mathematical aptitude, pointing students to a public debate between Steven Pinker and Elizabeth Spelke for a fuller treatment.

Sexual orientation (45:52)

The lecture closes on sexual orientation, noting that the large majority of people are heterosexual but a stable minority are exclusively attracted to their own sex. Bloom argues orientation is not a conscious choice, given the discrimination gay people often face and the fact that orientation-related patterns appear to be evident before puberty. Twin studies show identical twins have a roughly fifty percent concordance rate for homosexuality, well above chance but far below the 100 percent expected if it were purely genetic, implying additional developmental factors. He frames exclusive homosexuality as a genuine evolutionary puzzle, since it appears difficult to explain as a trait favored by natural selection, and assigns students to propose their own evidence-based account as a reading response.

Before you watch

• This lecture builds on the parental care and reciprocal altruism concepts covered in the two-part lecture on emotions earlier in the course.
• Familiarity with the twin-study logic (monozygotic vs. dizygotic concordance) from the lecture on personality and intelligence is assumed when the lecture discusses the genetics of orientation.
• Basic natural selection concepts (traits persist if they aid reproduction) are used throughout without being re-explained.

Check your understanding

1. According to parental investment theory, why does the higher-investing sex tend to become more selective about mates?
2. How do pipefish and gibbons serve as test cases for parental investment theory, and what does each species show?
3. What two features do attractive faces tend to share across cultures, and what evidence suggests the preference for averaged faces is not purely cultural?
4. What is the difference between a biological cause being "natural" and being "morally right," according to Bloom's use of Pinker's argument?
5. Why does a fifty percent concordance rate for homosexuality in identical twins suggest the trait is not purely genetic?