Chapter 1-7: Evolutionary Theory and Human Sexual Mating
Evolutionary Theory and Sexual Attraction: Comprehensive Notes
Evolutionary Theory: Core Idea
Evolution = change over time; involves adaptations that arise over thousands to hundreds of thousands of years.
Natural selection (Darwin, 1859): traits that help an individual adapt to the environment increase survival and reproductive success, passing those traits to offspring; over time, the population evolves.
Key population-level mechanism: variation within a population, differential survival, differential reproduction, and inheritance of traits.
Quick example: Darwin’s finches on the Galápagos with different beak shapes adapted to available food sources; long-term differential survival leads to beak-shape changes in the population.
“Survival of the fittest” is a shorthand for differential reproductive success (not just survival).
Darwin’s observations led to a broader concept: sexual selection, which explains traits that seem maladaptive for survival but improve mating success.
Natural Selection: Concrete Examples
Finches on the Galápagos: varying beak sizes/types linked to food availability; long-term changes in beak distributions reflect environmental pressures.
Peppered moth (Biston betularia): industrial soot darkened trees; peppered moths with darker coloration gained survival advantage; after pollution reduced, lighter moths became advantageous again. Example of natural selection acting on coloration with environmental change.
Implication: evolution can proceed rapidly when environmental changes alter selective pressures.
From Natural to Sexual Selection
The peacock paradox: male peacocks have massive, bright plumage that seems to hinder survival (predation risk) but increases mating success, prompting Darwin to develop sexual selection.
Sexual selection focuses on traits that increase mating success, not just survival.
Two main routes of sexual selection:
Intrasexual competition (males competing with males for access to mates; e.g., battles, displays).
Intersexual choice (females choosing mates based on traits that signal quality).
Signals of underlying health or good genes may drive female preference for certain male displays.
The parallel with humans: traits favored in the mating market may reflect evolutionary pressures to maximize reproductive success, though culture adds complexity.
Sexual Selection: How It Works
Definition: traits that give an advantage in mating or reproduction, even if they don’t improve (or even harm) survival.
The Peacock Case (two mechanisms):
Direct male competition for mates (intrasexual competition): physical contests, displays, etc. (Darwin’s “law of battle”).
Female choice (intersexual selection): females select mates based on signals indicating good genes, health, or potential parental investment.
Runaway selection (Fisher): a positive feedback loop where a trait and a female preference for that trait become more pronounced over generations, potentially leading to exaggerated displays.
Sexy son hypothesis (Fisherian extension): choosy females preferentially mating with males with attractive traits produce attractive sons, reinforcing the trait and preference across generations.
Human Implications: Parental Investment and Mate Choice
Parental Investment Theory (Trivers, 1972): the sex that invests more in offspring (female in most mammals, including humans) will be choosier about mates; the less investing sex will engage more in competition for access to mates.
In humans (and many mammals), females bear higher reproductive costs (pregnancy, lengthy caregiving, resource demands), leading to greater selectivity in mate choice.
Exceptions in nature (illustrative):
Mormon cricket, pipefish/seahorse have reversed or intensified parental investment by males, affecting mate choice dynamics.
Beewolves in some social insects and other species show different parental roles that influence who is choosy.
Implications for humans: cultural and individual differences interact with evolved pressures; mating behavior cannot be reduced to biology alone.
Three evolved criteria in evolutionary psychology for female mate choice (simplified):
1) Good genes (genetic health of offspring).
2) Resources (offspring support, provisioning, safety).
3) Parental investment (certainty of involvement in offspring care).In current-day terms, these translate into preferences over facial features, body traits, personality, status, and potential reliability of parental investment.
How This Applies to Real-World Attractiveness (Human-Focused Overview)
We carry both natural and sexual selection pressures; culture can modify or override simple biological predictions.
The lecture frames attraction as a composite of signals that historically maximized reproductive success while acknowledging modern social and contraceptive contexts.
Face and Body: Attractiveness Cues in Humans
Facial features and attraction hinge on symmetry and cues of genetic health.
Symmetry and fluctuating asymmetry (FA):
Symmetry = similarity of the left and right sides of facial features.
FA = deviation from perfect bilateral symmetry; higher symmetry is generally preferred and linked to developmental health.
Measurement intuition: compare left-right features (eye width, ear length, etc.); a self-contained quick method: take a frontal photo, split, and compare halves.
Why symmetry matters (hypothesized): developmental stability; fewer developmental perturbations (genetic deviations, inbreeding, mutations, environmental stress in the womb) yield more symmetry; symmetry signals robust health and genetic quality.
Evidence across species and contexts: symmetrical faces correlate with attractiveness in humans, hunter-gatherers, and even nonhuman primates.
Masculine vs Feminine Facial Features and Attractiveness
Masculine features (e.g., strong jaw, prominent brow) reflect higher prenatal testosterone exposure; often signal genetic health via a trade-off with immune function.
Female preferences for masculine faces are context-dependent:
Generally, both sexes often favor more feminine features, but
Women may prefer more masculine faces during peak fertility (mid-cycle) due to potential for strong genes, despite trade-offs in warmth/affection signals.
Across populations, there is substantial variability in how much masculine features are preferred, likely due to personality attributions (dominance, strength, protection) and warmth, nurturance, and kindness signals that also matter for long-term partnerships.
Summary: overall, feminized faces are generally found more attractive, but there is variability in women’s preference for masculine features that may peak during certain reproductive phases.
Self-Similarity, Kinship Cues, and Attractiveness
Self-similarity effects in heterosexual pairing:
In non-sexual contexts, similarity can increase perceived attractiveness due to kinship cues and potential genetic overlap.
In sexual contexts, dissimilarity is favored to maximize genetic diversity and offspring immune system vigor.
Implication: genetic dissimilarity between mates is often preferred to reduce the risk of inbreeding and boost offspring health.
Scent, Voice, and Odor: Genetic Health Signals
Body odor as a source of information about genetic health (MHC, major histocompatibility complex):
MHC dissimilarity between partners is associated with more attractive body odor to potential mates.
Studies involve men wearing un-scented shirts, controlled diets, and no scented products; women rate shirt odors for attractiveness, pleasantness, and intensity.
Result: women tend to prefer the scent of men whose MHC genes are dissimilar to their own, supporting the idea that mate choice can be guided by hidden genetic signals in scent.
MHC and immune system diversity: offspring benefit from diverse MHC genes, improving disease resistance; this provides a rationale for MHC-based scent preferences.
Symmetry and scent: symmetry cues may also be detectable via odor in some studies, linking physical and chemical signals of health.
Thornhill and colleagues emphasize that humans may unconsciously evaluate potential mates through scent cues that reveal immune system compatibility and genetic diversity.
Voice and Body Cues: What Signals Attractiveness
Voice: deeper male voices are generally preferred, signaling higher testosterone, which correlates with certain health and maturation cues.
Testosterone and health: higher testosterone during development associates with masculine facial features; however, it also relates to immune function trade-offs.
Waist-to-hip ratio (WHR) and overall body morphology:
For men, a waist-to-hip ratio around 0.9 is associated with better health indicators and longevity.
Height: many studies show a preference for taller men; tall men are perceived as more dominant and have higher social/economic advantages in various domains.
Body shape: a V-shaped torso (broad shoulders and narrow waist) correlates with earlier sexual activity and more sexual partners; however, excessive muscle emphasis (as portrayed in magazines) may not align with actual female preferences.
Cultural and media influence: public ideals (e.g., extreme muscularity) may distort perceptions of what is actually attractive.
Mate Copying and Social Information in Choice
Mate copying: women may use social cues to evaluate prospective mates, such as observing a man with an attractive partner or a past relationship history.
Findings:
Men who have been in relationships or who are seen with attractive partners are often rated as more attractive by other women, suggesting social proof or preapproval effects.
This provides an ecological cue: if others find someone appealing, that appeal may be reinforced by observed mate pairing.
Implication: social context and perceived popularity can influence mate choice beyond the individual’s own traits.
Practical and Ethical Considerations for Interpreting Attraction Research
Caution in applying evolutionary explanations to human behavior: humans have culture, learning, and conscious choice that can override or modulate raw biological pressures.
Do not reduce all human dating/romantic behavior to biology alone; individual differences, personal history, and social environment are powerful modulators.
Recognize the diversity of motivations for sex and attraction described in research (from altruistic to revenge to playful to seeking health signals); not all motivations map cleanly onto a single model.
Summary of Key Formulas and Numerical References
Waist-to-Hip Ratio (WHR):
ext{WHR} = rac{W}{H}
Ideal/typical value associated with good health signals around ext{WHR} \,\approx\, 0.9
Male height preference in personal ads (empirical finding):
P( ext{prefers height} \ge 6'0") \approx 0.80
Maternal/paternal investment concepts:
Parental Investment Theory (Trivers, 1972): the sex with greater parental investment is more selective; the other sex engages more in intersexual competition for mates.
Symmetry measures (conceptual):
Symmetry score concept: lower fluctuating asymmetry (FA) indicates higher symmetry; FA can be quantified as a function of left-right deviations across features, e.g.,
FA = \frac{1}{n}\sum{i=1}^n |Li - Ri| where $Li$ and $R_i$ are left-right measurements of feature $i$.
MHC-based scent preference (conceptual):
Dissimilarity measure for immune genes between two individuals can be represented as
D{\text{MHC}} = 1 - \text{sim}(\text{MHC}1, \text{MHC}_2)
Runaway and sexy-son dynamics (conceptual references):
Fisherian runaway: positive feedback loop between a trait and female preference, leading to exaggerated traits.
Sexy-son hypothesis: daughters of choosy females tend to have attractive sons, reinforcing the trait and preference over generations.
Connections to Foundational Principles and Real-World Relevance
Foundational principles:
Variation, inheritance, differential reproduction, and environmental pressure drive evolution (Darwin).
Sexual selection explains traits that enhance mating success, sometimes at odds with survival advantages.
Parental investment clarifies sex differences in mate choice and competitive behavior.
Real-world relevance:
Understanding attraction can inform dating behavior, social interactions, and even public discussions about gender norms.
The research emphasizes that attractiveness judgments are multi-faceted, context-dependent, and embedded in cultural narratives.
Caution against simplistic, stereotype-based conclusions about gendered behavior; individual differences and situational factors are substantial.
Quick Recap: Key Takeaways
Evolutionary theory explains how traits linked to survival and reproduction become common over long timescales; sexual selection adds another axis by which traits evolve due to reproductive success.
In humans, female parental investment creates stronger selective pressures on mate choice, while males often experience intense competition for mating opportunities.
Attractiveness signals in humans—facial symmetry, masculine vs feminine features, scent (MHC dissimilarity), voice, height, and body shape—are interpreted as indicators of genetic health, resource potential, and parental investment capability.
The interplay of biology and culture means that while evolution provides a framework, human attraction is influenced by personal experiences, societal norms, and individual differences.
Bonus Quiz and Next Steps
Be prepared to discuss how symmetry, MHC dissimilarity, and parental investment theory interact to shape mate choice.
Reflect on how modern contraception and social norms might modify or dampen some evolutionary pressures in contemporary life.
Consider ethical considerations when discussing human mating research and how to present findings responsibly.