What Can Evolutionary Theory Tell us About Family Life?

• We are descendants of an unbroken line of successful reproducers

• So what?

• People don’t act like they are maximizing reproduction

• Maybe our brains have hi-jacked our utility functions to serve its pleasure centers

Programmed to reproduce?

• What kind of preferences would Nature choose for people designed to reproduce most successfully over many generations.

• Alan Rogers—Love or Money. People care about other things than reproduction, but these concerns may be calibrated so as to serve long term reproductive interest.

• Samuelson-Swinkels—Nature the principle, animals the agents.

Applications of evolutionary hypothesis to family matters

• How would evolution have us treat siblings, cousins, and other relatives.

• Genetics of parent offspring conflict—mammalian weaning conflict, biological rotten kid theorem? Trivers’ teenage rebels

• Intertemporal preferences, current for future goods, reproduction

• Conflict and cooperation between spouses

Measuring reproductive successWhat is fitness?

• First cut—expected number of offspring who survive to adulthood

• Probably better—expected number of grandchildren or maybe great grandchildren

• Sometimes not all children equal. Dynamic programming needed to value them. Primogeniture example, commoners and titles

Kin Selection & Hamilton’s Rule

• Degree of relatedness= ½ for sibs, ¼ for half-sibs, 1/8 for cousins, ½ for kids, ¼ for grandkids, etc.

• Hamilton’s benefit cost test. B=benefit to recipient, C= cost to you, k=degree of relatedness

• Do it if and only if kB>C• Inclusive fitness—utility function that weighs

relatives’ descendants by their relatedness

Conflict and Cooperation between parents

• To its two fitness-maximizing parents, a child is what economists call a public good

• Child’s well-being jointly consumed

• Economists worry about free-rider problem in public goods provision.

An Arboreal Allegory

• Alice and Bob collect fruit and berries• and share warmth of a common fire• Alice is good at fire-building and wood-

gathering. Bob hasn’t figured it out, but huddles next to Alice’s fire.

• Alice divides her time between gathering food gathering wood.

• Bob collects food but not wood.

Trading Food for Fire

• Alice gathers wood until extra warmth from an hour’s wood-gathering is worth less to him than extra berries from hour’s berry-picking.

• Bob leaves morsels of food for Alice. The “income effect” of these gifts causes Alice to build a bigger fire.

• Alice’s MRS equals MC.

Does Alice Collect enough wood?

• Not with lump sum gifts. Efficiency would require MRSA +MRSB =MC.

• Efficiency might be arranged with a contract in their repeated relationship, but contract would need to be monitored. This may not be easy.

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 agree on the amount of berries each should pick and the amount of firewood each should gather.

No Bargaining Needed

• Alice specializes in wood-gathering and Bob gathers berries.

• Bob gives Alice berries to keep up her strength for wood-gathering.

• Alice and Bob agree perfectly on how much berries Alice should pick and how much she should give Bob.

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?

The In-law Problem

• Even in the Eden of perfect monogamy, the snake of conflict can be found.

• Hamilton’s 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.

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 Charnov—the male’s dilemma• Good provider, health genes story• Rape

Conflict of interest

• Children by previous marriage don’t 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)

Physical evidence of mating history—Comparing primates

• Correlation across species between size difference of sexes and degree of polygyny

• Claim—in polygynous species there is a bigger return to males fighting other males. Size advantage yields more mates

• Examples:gorillas and orangs—gibbons—chimps and bonobos

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?

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

Conflict over Birth Intervals?

• Argument—Children are much more expensive to women than to men. Therefore we should expect women to want fewer children than their husbands do

• Hold on—What about Alice and Bob the fire-builders? Won’t monogamists want the same thing?

• 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 man’s sister’s children

• Battle between mother and fetus for resources is a battle between father’s genes and mothers—David Haig

• With high cuckoldry probabilities, men will tend to want shorter birth intervals than their wives

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 women’s social power rises, so does their influence in household decisions.

Competing Theory

• Becker et al attempt to explain demographic transition by relative prices

• But this doesn’t account well for the fact that with large increases in income, number of children diminish. Are they inferior goods? Wouldn’t evolutionary theory suggest that they are normal goods?

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.