Mating Systems

Mating Systems #

• Monogamy: 1 woman 1 man — rarest in humans
• Polyandry: 1 woman multiple men — most common (mild polygyny)
• Polygyny: multiple women 1 man
• Polygynandry: multiple women, multiple men

Default system for humans: serial monogamy (can remarry) #

• polygyny acceptable but uncommon
• more wealthy (older, successful) men → more polygynous (more success for women vs. marrying poor)

Example: Kisigi pastoralists in Kenya #

• forcibly moved by Europeans, studied marriages & land ownership
• default polygyny, with some monogamy
• as wealth increases, monogamous wives have lower reproductive success vs polygyny
  • direct relationship b/tw acres owned, # of wives
  • small # of successful males have most of the children (up to 80)

Variance in reproductive success #

Farmers > herders > hunter-gatherers (accumulation of material wealth ⇒ more inequality)

Based on polygyny threshold model, hunter-gatherers don’t reach tipping pt. of wealth

Agriculture changes balance of economy + labor: farming women contribute small amt. of calories

Physical Adaptations #

• Average reproductive success:
  • 11–13 m–f in hunter gatherers
  • 13–26.5 m–f in farmers
  • emperors: 49–400 children
  • (early agricultural — empire evolution)

Evolutionary History: monogamy #

• unknown common mating ancestor (chimps, gorillas use different methods)
  • polygynandry: used to protect children from infanticide
  • selection for larger testes size (sperm competition)
  • large physical difference b/tw males, females

Human morphology: #

• small testes (not evidence for polygyny)
• low levels of sexual dimorphism (males slightly larger)
• presence of female orgasm as honest signal for pair bond
• lack of penile spine / baculum (additionally in neanderthals)

(evidence for monogamy / mild polygyny)

Conclusions from morphology: #

• low reproductive variance in females
• selection for long-term care providers (behavioral traits, wealth)
• short-term mating (good genes hypothesis) for diversifying offspring
• male strategy: maximize number of mates in short term (no cost of child care)
  • find long term wife to ensure paternity

Economics of Reproduction #

To increase reproductive success:

• women prefer older, more successful/wealthy males with feminine features (lower testosterone)
  • masculine faces associated w/ coldness, aggression
• men prefer younger, less experienced females → neoteny: preference for features from earlier developmental stage
  • higher proportion desire multiple partners compared to women

Supernormal stimulus: exaggerated features with little/residual value

• increase competitiveness in mate selection
• example: small waists (corsets), large hips + fat deposition pattern indicates fertility
  • gluteofemoral fat is honest signal that improves neural development for fetus (increased cognitive scores after birth)

Baby schema: baby proportions

Choosing Wealth #

• material wealth: goods, housing, land, money (not prevalent in forager societies)
• embodied wealth: health, age, knowledge, strength, intelligence
  • when maximizing embodied wealth, most foragers marry older since they need to learn hunting first
• relational wealth: family size, alliances (social capital)

Family Planning #

Most family sizes constrained by economic concerns

• ex. !Kung desert living: 1 child every 4 yrs, up to 3-4
  • must travel long distances in heat w/ children — can only carry one child
  • interbirth interval inversely related to weight of nuts foraged

Purpose: reduce infant mortality. Birth interval is variable

• Hadza in East Africa: more abundant food, children can be left at camp
  • infant care by grandmothers, self sufficient/weaned by 5
  • Result: much lower interbirth interval, higher fertility
• 18th–19th century Krummhörn, Germany: poor women have shorter lives, more children ← for poor women, having more children decreases health and lifespan
• Gabra pastoralists: wealth measured in camels + livestock, must pay bride price → family size proportional to # camels
  • 7-8 camels ⇒ 1 child

Weird Marriages #

• in Western cultures, marrying cousins is taboo
  • this is the outlier: most cultures (75%) have marriage of cousins
• first cousin offspring have greater chance of success

Other WEIRD traits #

• Bilateral descent (28% of societies) ← considered equally descendant of both parents
• Few within-family marriage (25%)
• Monogamous (15%)
• Nuclear family (8%)
• Neolocal residence (5%) — new couples make separate household

Only 0.7% of societies have all 5 traits

Most human societies: #

• arranged marriage

• 1 in 10 marry a relative

• property owned by family, not individual

• spouses are paid for (dowry, brideprice)

• patrilocal/matrilocal (spouse lives w/ groom or bride’s family)

• patrilineal: oldest men arrange for powerful alliances

Segmentary lineage: links to: #

• immediate family (minimal segment)
• cousins (minor)
• extended family (major)
• common ancestor (maximal lineage)