What Can Evolutionary Theory Tell us About Family Life?

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What Can Evolutionary Theory Tell us About Family
Life?

• We are descendants of an unbroken line of
  successful reproducers
• So what?
• People dont act like they are maximizing
  reproduction
• Maybe our brains have hi-jacked our utility
  functions to serve its pleasure centers

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• How else to explain the demographic transition?
• Though people care about more than reproduction,
  these concerns may be calibrated so as to serve
  long term reproductive interest.
• Samuelson-SwinkelsNature the principle, animals
  the agents

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• How else to explain the demographic transition?
• What kind of preferences would Nature choose for
  people designed to reproduce most successfully
  over many generations.
• Samuelson-SwinkelsNature the principle, animals
  the agents
• Alan RogersLove or Money. People care about
  other things than reproduction, but these
  concerns may be calibrated so as to serve long
  term reproductive interest.

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Applications of evolutionary hypothesis to family
matters

• How would evolution have us treat siblings,
  cousins, and other relatives.
• Genetics of parent offspring conflictmammalian
  weaning conflict, biological rotten kid theorem?
  Trivers teenage rebels
• Intertemporal preferences, current for future
  goods, reproduction
• Conflict and cooperation between spouses

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Measuring reproductive success What is fitness?

• First cutexpected number of offspring who
  survive to adulthood
• Probably betterexpected number of grandchildren
  or maybe great grandchildren
• Sometimes not all children equal. Dynamic
  programming needed to value them. Primogeniture
  example, commoners and titles

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• Kin selection and Hamiltons rule
• Degree of relatedness ½ for sibs, ¼ for
  half-sibs, 1/8 for cousins, ½ for kids, ¼ for
  grandkids, etc.
• Hamiltons benefit cost test. Bbenefit to
  recipient, C cost to you, kdegree of
  relatedness.
• Do it if and only if kBgtC
• Maximize inclusive fitness like a utility
  function that weighs number of relatives
  descendants by their degree of relatedness

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Conflict and Cooperation between parents

• To its two fitness-maximizing parents, a child is
  what economists call a public good
• Childs well-being jointly consumed
• Economists worry about free-rider problem in
  public goods provision.

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Foragers at the campfire

• Alice and Bob subsist on berries and share warmth
  of a common fire
• Alice is relatively good at berry-picking and Bob
  is relatively good at wood-gathering
• With no explicit agreement, expected
  equilibrium---Alice gathers no wood. Bob gathers
  some wood and some berries.
• Does Bob gather enough wood?

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• Bob gathers wood to the point where extra warmth
  from an hours wood-gathering is worth less to
  him than extra berries from hours berry-picking.
• When Bob gathers this much wood, Alice doesnt
  want to gather any more.
• But both could be better off if Alice bribed Bob
  with berries to gather more wood.
• But this requires bargaining and if neither knows
  the others preferences, outcome neednt be
  efficient

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Concord at the Campfire

• Suppose that Alice and Bob real concern is only
  for the size of the fire
• Each values berries only as an instrument to
  giving them strength to build the fire bigger
• Then both will agree on exactly how much berries
  each should pick and how much firewood each
  should gather.

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• Bob specializes in wood-gathering and Alice
  gathers some wood, some berries.
• Alice gives Bob some berries to keep up his
  strength for wood-gathering.
• Alice and Bob agree perfectly on how much berries
  Alice should pick and how much she should give
  Bob.
• No bargaining needed.

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Monogamy and Domestic Harmony

• Are monogamous couples of fitness maximizers
  like Alice and Bob the fire maximizers?
• If the only children that either will have are by
  the other, then whatever actions increase the
  number of descendants of one increase the number
  of descendants of the other.
• Environment for selfless love?

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The In-law Problem

• Even in the Eden of perfect monogamy, the snake
  of conflict can be found.
• Hamiltons kin selection theory. Each spouse
  values his or her siblings and other relatives,
  but the sibling of one is unrelated to the other.
  
• Hence inclusive fitness functions conflict
• Conflict returns, though muted.

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More trouble in Eden

• High death rates of people of fertile age
• Remarriages of widows and widowers likely
• And adultery
• Philandering and cuckoldry. (Hawkes, Rogers, and
  Charnovthe males dilemma
• Good provider, health genes story
• Rape

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Conflict of interest

• Children by previous marriage dont get good care
  from non-related spouse
• Daly and Wilson Homicide
• Evidence of bad outcomes for children without
  resident biological father (Comanor and Phillips,
  Kelly Bedard and Heather Antecol)

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Physical evidence of mating historyComparing
primates

• Correlation across species between size
  difference of sexes and degree of polygyny
• Claimin polygynous species there is a bigger
  return to males fighting other males. Size
  advantage yields more mates
• Examplesgorillas and orangsgibbonschimps and
  bonobos

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Whose are bigger?

• Chimps are highly promiscuous and sexes differ by
  only about 6
• Gorillas are not promiscuous, (though
  polygynous). Females stay with same male
• The testes test
• Gorillas, chimps, humans?

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So where do humans place?

• By these physical tests, humans come out as
  moderately promiscuous and moderately polygynous
• In accord with what is known about modern
  hunter-gatherers

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Conflict over Birth Intervals?

• ArgumentChildren are much more expensive to
  women than to men. Therefore we should expect
  women to want fewer children than their husbands
  do
• Hold onWhat about Alice and Bob the
  fire-builders? Wont monogamists want the same
  thing?

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• Difference in interests arise with inlaw problem,
  early death of one spouse, adultery and rape.
• What are these differences?
• Differential values of previous children relative
  to new one.
• Differential values of potential future children
  relative to present new one
• Fidelity threshold and contributions to mans
  sisters children

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• Battle between mother and fetus for resources is
  a battle between fathers genes and mothersDavid
  Haig
• With high cuckoldry probabilities, men will tend
  to want shorter birth intervals than their wives

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A partial explanation for reduced birth rates?

• Tug-of-war model between sexes over birth rates.
  Each sets a target beyond its preferred point.
• As womens social power rises, so does their
  influence in household decisions.

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Competing Theory

• Becker et al attempt to explain demographic
  transition by relative prices
• But this doesnt account well for the fact that
  with large increases in income, number of
  children diminish. Are they inferior goods?
  Wouldnt evolutionary theory suggest that they
  are normal goods?

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Leverage in Tug-of-War

• Perhaps the major explanator of changes in birth
  rates is not changes in prices and incomes faced
  by a single rational decision-maker
• But rather a shift in bargaining power from those
  who favor higher birth rates to those who favor
  lower birth rates.