We love our relatives, although sometimes we squabble with them.Why?
The evolutionary logic of kin selection (Hamiltons Rule)
c < rb
c; cost to actorb: benefit to recipientr: genetic relatedness between actor and recipient
Calculating the coefficient of relatedness (r)
r between actor and recipient:
the expected (average) fraction of genes that are identical byvirtue of their family relationship
Kin selection and altruism in non-humans
I sterile workers in hymenoptera
I alarm calls in Beldings ground squirrels
I helpers at the nest in scrub jays
Alloparenting in Florida scrub jays
I Non-breeding jaysmore likely toprovisionclosely-related young
I Removing helperslowered fledglingsurvival; helpersreduced predationand fed young
But do animals do algebra?
No need to calculate Hamiltons Rule. Biased behavior to relativescan involve:
I kin recognition (e.g., via MHC or physical appearance).
I living near kin (if altruistic to nearby individuals)
Kinship and the sociobiology wars
No system of human kinship relations is organized inaccord with the genetic coefficients of relations asknown to sociobiologists (The use and abuse ofbiology, Sahlins, 1977 p. 57)
the Yanomamo. . . discriminated in many subtle andnot-so-subtle ways against non-kin, no matter howthey classified them. (Chagnon, 1979, p. 87)
Biological and classificatory kinship:
Our classificatory kinship system: many relatives called cousin.Some kinship systems, same term for father and fathers brother
Is the psychological distance the same to each of them?
Yanomamo Axe Fight (Chagnon andBugos 1979): In a fight between twogenealogically-overlapping villages, menwere more likely to fight to supporttheir genetic kin.
Biological and classificatory kinship: full and half sibs
Unlike most polygamous families living in Montana, the Salt LakeValley, and northern Mexico, the preferred ideal in Angel Park is tolive together in one united and harmonious household.
(from Jankowiak et al. 2000)
Fictive kinship plays on our evolved emotions
brother, can you spare a dime?
Relationships among co-wives
Polygynously-married wives usually live in separate dwellings.. . . unless the wives are sisters (sororal polygyny).
shared dwelling separate dwellingsororal polygyny 60 (81%) 14 (19%)nonsororal polygyny 96 (32%) 207 (68%)
(from Martin and Voorhies)
Homicide risk at home
Denver (UPI) A just-completed national survey indicates theAmerican home is the most violent place in the country, threesociologists reported Friday. The pioneering study led researchersto conclude that physical violence occurs between family membersmore often than it occurs between any other individuals. . .
Homicide risk at home
??? Doesnt kin selection predict less violence among familymembers ???
Killing requires access. People have a lot of access to familymembers.
Kin selection predicts that, given the same opportunity and access:there will be fewer homicides between blood relatives than others
Kinship predictions selfishness to non-kin
Daly and Wilson looked at homicides in Detroit households (1970).Households included:
I blood relatives (parents, children, sibs)
I non-blood relatives (e.g., spouses, in-laws)
Average resident lived with 3 other people, 60% lived with aspouse, 98 total domestic homicides.
Q: If homicide was randomly distributed within households, howmany domestic homicides would be spousal homicides??
Homicide risk to kin and non-kin
# in % in Expected Actual Relativehousehold household Homicides Homicides Risk
A B=(A/3) C=(B*98) D E=(D/C)Non-Kin
Spouses 0.6 0.20 19.60 65 3.32Other non-kin 0.1 0.03 3.27 11 3.37Total non-kin 0.7 0.23 22.87 76 3.32Kin
Offspring 0.9 0.30 29.40 8 0.27Parents 0.4 0.13 13.07 9 0.69Other kin 1.0 0.33 32.67 5 0.15Total kin 2.3 0.77 75.13 22 0.29
Grand Total 3.0 1.00 98.00 98
(from Daly and Wilson 1988)
Homicide as a conflict assay
Daly and Wilson study homicide because it has an objectivelyobservable outcome.
They want to identify psychological factors that underpin suchconflicts
They dont assume homicide is an adaption.
Kin-directed altruism depends on more than r
Little data showing precise match between altruism and rbecause
I b and c matter (for yearling jays, c depends on territory)
I real r reflects paternity uncertainty
I friends (r = 0) confer other benefits
I reproductive value of the recipient matters
Kin altruism and reproductive valueIntensity of grief at a lost child matches reproductive value of achild in the EEA, not in modern societies:
(from Crawford et al. 1989)
Parent-offspring Conflict: Optimal time for weaning isdifferent for mother and child (why?)
Some mornings I just stayed around and my tears fell and I criedand refused food. That was because I saw him [the brother]nursing, I saw with my eyes the milk spilling out. I thought it wasmine (Shostak 1976)
Weaning among the Ache is an extremely unpleasant experiencefor mothers (and apparently for children), with children screaming,hitting, and throwing tantrums for several weeks. (Hill andHurtado 1996).
Farmer parents explained that the timing of weaning wasimportant because it enabled mothers to leave their youngestchildren at home and thus farm more productively. (Fouts et al.2005).
Parent-offspring Conflict: Implications
picture caption: Are you worriedabout the change in your3-year-olds behavior after yoursecond child?
I Weaning conflicts
I Children regressing (acting more needy) when new babyarrives
I Mom wants children to share with each other more thanthey do
I Maternal-fetal conflict
Parent-offspring conflict in utero (David Haag)
I fetal cells remodel spiral arteries into low-resistance vessels mother cant restrict blood to fetus
I endometrial spiral arteries a countermeasure?
Parent-offspring conflict in utero
Conflicts over nutrients:
I fetal hormones increase output of glucose from liver
I fetal hormones increase insulin resistance in mother (moresugar in blood)
I mother increases insulin production (restricts fetal access)
In a cooperative system, mother should evolve a sensitivity to fetalhormones, not resistence. Yet:
Fetus maternal insulin resistance. Mother insulin.
A sign of conflict. Implies an arms race. (from David Haag).
Mother-daughter conflict in TrinidadWhen mother and daughter live in same household, typically onlyone reproduces. If they are both reproductive, there is more conflictbetween them (from Flinn 1989, describing a rural village in Trinidad)
I We love our family (genes shared by descent) and are morealtruistic to them than to non-kin, other things equal. But. . .
I Family conflicts (parent-child, sibling rivalry, husband-wife)are inevitable because we are not genetically identical.
I Evolutionary theory predicts familial interactions shaped by:I degree of relatednessI costs to actor, benefit to recipientI age (reproductive value) of the recipient