Darwin had a problem, and he knew it. Natural selection is supposed to weed out costly traits — so why does the peacock drag around a five-foot tail that slows him down, burns calories, and practically waves a flag at predators? Darwin's answer split evolutionary theory in two: alongside selection for survival, there is selection for mating success. That second engine is what this unit is about, and it's also where evolutionary psychology gets both its most cited findings and its most heated arguments.
Darwin's puzzle and the two engines of sexual selection
A trait doesn't have to help you live longer to spread through a population — it only has to help you reproduce more. If a costly, even dangerous trait reliably gets you more mates or better mates, it can be favored anyway, because reproductive success is the currency evolution actually pays in. Darwin proposed two distinct mechanisms that produce this pattern, and your slides likely just named them — worth pulling apart here, because they work through different social routes.
Intersexual selection
Selection driven by mate choice — one sex evolves preferences, and the other sex evolves traits that satisfy them. The peacock's tail exists because, generation after generation, peahens preferred males with bigger, more symmetrical, more colorful tails. The male ornament and the female preference co-evolve together.
Intrasexual selection
Selection driven by competition within a sex for access to mates, independent of what the other sex prefers. Antlers, elaborate horns, and body size in many species evolved because males use them to fight or intimidate rival males — the trait wins by beating competitors, not by being chosen.
Both mechanisms can operate in the same species at once, and both show up in the human mate-preference literature you're about to read — sometimes tangled together, which is part of why interpreting that literature is harder than it first looks.
Why would the sexes want different things? Parental investment theory
Trivers's (1972) parental investment theory gives sexual selection its logic in humans and elsewhere. The core claim: whichever sex invests more time, energy, and risk per offspring should be more choosy about mates, because a bad choice costs them more. The sex that invests less can afford to compete more freely for mating opportunities, because the cost of a failed attempt is lower.
◆ The claim is about averages, not individuals
Parental investment theory predicts population-level tendencies, not a rule for any one person. It says nothing about what any individual man or woman wants, values, or does. Keep that distinction in mind for everything that follows — it's the difference between a defensible evolutionary hypothesis and an overreaching one.
Buss (1989): mate preferences across 37 cultures
Buss (1989) tested parental-investment predictions directly, surveying thousands of people across 37 cultures on what they wanted in a long-term mate. The study reported some patterns consistent with the theory — for instance, average differences in the weight men and women placed on certain traits when rating mate preferences, patterns that showed up across a wide range of cultures and were often framed as evidence for evolved, universal mating psychology.
That's the finding your slides likely named. What they may not have had room to unpack are the caveats that responsible reporting of this study requires — and they matter as much as the headline result.
◆ The caveats that belong right next to the finding
First, the overlap between men and women on every trait dwarfed the average difference — knowing someone's sex tells you very little about their individual preferences. Second, the size and even direction of some patterns shifted with culture, and later work links this to women's economic power and independence: where women have more economic autonomy, some of the classic sex differences shrink. Third, the data are self-report and correlational — what people say they want in a survey is not the same as who they actually choose, and the design can't establish that evolution, rather than culture or economics, caused the pattern. Treat the finding as real but partial, not as proof of a fixed, universal design.
The jealousy debate: a live disagreement, not a settled fact
If parental investment theory predicts different mating strategies for men and women, does it also predict different jealousy triggers? Buss et al. (1992) argued yes: because paternity is never 100% certain for a man but maternity always is for a woman, men should be more distressed by a partner's sexual infidelity (risk of investing in another man's offspring), while women — who risk losing a partner's investment and resources — should be more distressed by emotional infidelity. Their forced-choice data supported this pattern.
DeSteno and Salovey (1996) pushed back hard, and this is a genuine scientific disagreement worth sitting with rather than resolving too quickly.
◆ Update: is the sex difference in jealousy real, or an artifact?
DeSteno and Salovey (1996) and other researchers argued that the sex difference Buss et al. (1992) reported depends heavily on how the question is asked. Forced-choice formats — pick sexual infidelity or emotional infidelity, one or the other — can manufacture or exaggerate a difference that shrinks substantially, or in some studies nearly disappears, when people instead rate their distress on continuous scales for each type of infidelity separately. The debate isn't fully settled either direction: some researchers still defend an evolved-module account, others favor cognitive or social explanations for why the forced-choice format produces the pattern it does. The honest summary for a psychology course: this is an active empirical argument, not a closed case.
The science of beauty: what makes a face attractive, and why it matters more than people admit
Two well-supported cues to facial attractiveness are averageness (faces closer to the population average tend to be rated as more attractive) and symmetry (more bilaterally symmetrical faces tend to be rated as more attractive). Both have been proposed as cues that may signal underlying health or developmental stability, though how directly attractiveness judgments track actual health remains a topic researchers continue to study.
But the more consequential finding for everyday life isn't about what makes a face attractive — it's about what we do once we've decided someone is. Dion, Berscheid, and Walster (1972) documented the halo effect for physical attractiveness, memorably summarized as "what is beautiful is good": people spontaneously assume attractive individuals also have other positive traits — more competent, more sociable, more successful — traits that have nothing logically to do with facial symmetry.
◆ Langlois et al. (2000): the "looks don't matter" maxim is itself a myth
You've probably heard some version of "looks don't really matter, it's what's inside that counts." Langlois et al.'s (2000) meta-analysis — pooling results across many studies — found that beauty stereotypes are not a small or inconsistent effect. They are real, replicable, and consequential: attractiveness shapes how people are judged and treated across social, academic, and professional contexts. The maxim that appearance is irrelevant is, ironically, the actual myth. That doesn't make it fair — it makes it worth knowing about, precisely so you can notice the bias operating rather than pretend it isn't there.
The myth
"Science proves men are wired to want youth and looks, and women are wired to want status and resources — case closed."
What's actually true
The research finds some average patterns, but the overlap between men and women is large, the patterns shift with culture and women's economic independence, and key findings (like the jealousy split) are actively disputed by other researchers using different methods. The pop-science version compresses a nuanced, contested literature into a tidy certainty the data don't support.
Cuteness as adaptation: the baby schema
Attraction research usually focuses on mate choice, but the same evolutionary logic — a trait that recruits a behavioral response from others — shows up somewhere else entirely: caregiving. Konrad Lorenz proposed the baby schema (kindchenschema): a cluster of features — a large head relative to body, a high and prominent forehead, big eyes set low on the face, round cheeks, small nose and chin — that reliably triggers caregiving and protective responses in adult observers, not just toward human infants but toward anything that resembles them (which is a large part of why round-faced, big-eyed cartoon characters and animals strike people as "cute").
Bodhi says
Notice the pattern across this whole unit: an evolutionary account explains why a trait or preference might exist in general, but it almost never tells you what any specific person will do. Baby-schema cuteness doesn't mean every adult is compelled to caretake; mate-preference averages don't mean every man or woman fits the average. Evolutionary psychology is at its best explaining population-level tendencies — and at its weakest when it's stretched to explain individuals.
The other lens: social-role theory
Not every psychologist agrees that mate-preference patterns reflect evolved mating psychology at all. Eagly and Wood's (1999) social-role (biosocial) theory offers a genuine alternative: many of the "sex differences" reported in mate-preference research, they argue, arise from the different social and economic roles men and women have historically occupied — roles shaped by division of labor, access to resources, and social structure — rather than from evolved psychological mechanisms specific to mating.
On this account, if women historically had less direct access to economic resources, valuing a partner's earning capacity would be a rational response to that structural position, not evidence of an evolved preference module. And the finding that mate-preference patterns shift as women's economic power increases (noted above with Buss's data) is exactly what social-role theory would predict.
◆ Two lenses, and the data rarely settle it cleanly
Evolutionary psychology and social-role theory aren't always mutually exclusive — evolved general-purpose learning mechanisms could operate through social roles, for instance — but they make different core claims about why mate-preference patterns exist, and the same cross-cultural correlational data get cited by both sides. This is normal science: competing theories, overlapping evidence, and no single study that cleanly adjudicates between them. Hold both explanations in mind rather than picking a side by instinct.
The one thing to carry out of this unit
Sexual selection gives you a real, well-supported explanation for why some traits exist that natural selection alone can't account for — and it genuinely illuminates parts of human attraction, jealousy, and even why we find round-faced things cute. But every specific claim in this literature — the mate-preference averages, the jealousy split, the beauty stereotypes — comes with caveats, competing explanations, or active scientific disagreement attached. The discipline of good evolutionary psychology isn't reciting the headline finding; it's holding the finding and its caveats in the same sentence.
References
Buss, D. M. (1989). Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures. Behavioral and Brain Sciences, 12(1), 1–49.
Buss, D. M., Larsen, R. J., Westen, D., & Semmelroth, J. (1992). Sex differences in jealousy: Evolution, physiology, and psychology. Psychological Science, 3(4), 251–255.
DeSteno, D. A., & Salovey, P. (1996). Evolutionary origins of sex differences in jealousy? Questioning the "fitness" of the model. Psychological Science, 7(6), 367–372.
Dion, K., Berscheid, E., & Walster, E. (1972). What is beautiful is good. Journal of Personality and Social Psychology, 24(3), 285–290.
Eagly, A. H., & Wood, W. (1999). The origins of sex differences in human behavior: Evolved dispositions versus social roles. American Psychologist, 54(6), 408–423.
Langlois, J. H., Kalakanis, L., Rubenstein, A. J., Larson, A., Hallam, M., & Smoot, M. (2000). Maxims or myths of beauty? A meta-analytic and theoretical review. Psychological Bulletin, 126(3), 390–423.
Trivers, R. L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man, 1871–1971 (pp. 136–179). Aldine.