Reciprocal Altruism

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					Reciprocal Altruism

                      Elbert Lim
                      Anthro 179
Reciprocal Altruism

  • Term was coined by
    Robert Trivers (1970’s).
  • Refers to the offering
    and receiving of
    support, but at a cost to
    reproductive fitness.
  • Relatively recent field of
    study: no real work
    done on this subject
    until late 1960’s.

  • Darwin’s theory of
    evolution: survival of
    the reproductively fit
  • How could natural
    selection favor the
    development of
    cooperative societies,
    when fitness is
    compromised in these
Three Forms of Selection

                 • Group selection
                 • Kin selection
                 • Reciprocity
Group Selection

  • Populations of species are divided into islands, or demes.
  • Favors any gene that lowers the likelihood of extinction for the
    deme that it occurs in.
  • Survival is population density-dependent; must stay within an
    optimal range
  • Altruism favored because it prevented explosive population
  • Wynne-Edwards; 1959, 1962
  • Considered the weakest principle of the three because:
      – It fails to address how and why extinction occurs if a population
        lacks the appropriate regulatory controls.
      – Pays little attention to the possibility of subdivision within
        successful populations.
Kin Selection

  • Principle of competition among populations is rejected.
  • Assumes that actors can effectively identify each other from one
  • Success occurs if the fraction of genetic material preserved
    within a group is greater than the fraction of genetic material
  • Exchange of altruistic behavior, as a result, is greater between
    relatives than between unrelated individuals.
  • Hamilton hypothesis: in order for altruistic behavior to occur,
    the benefits, even among kin, must outweigh the cost (I.e. an
    actor is more willing to give up his life to save multiple actors
    over saving just one).
Reciprocity Selection

  • Just as in kin selection, the actors involved must be able to
    identify one another.
  • Most favored principle out of the three.
  • Actors involved in altruistic exchange do not need to be related.
  • Exchange is more likely to take place with actors exhibiting a
    propensity towards reciprocating cooperating (I.e. you’d be
    more likely to buy a round for a generous friend than with the
    miser of the group).
Prisoner’s Dilemma

                 • Most commonly used model
                   in studying reciprocal
                 • Prisoner’s Dilemma.
                 • Actors in this model follow
                   the conditions of the
                   reciprocity model.
                 • Most common strategy used
                   in this problem is Tit-for-Tat.
Tit-for-Tat Strategy (in order of

   • Start off by cooperating, and continue to do so as long as the
     partner does.
   • Defect only when the other actor defects.
   • Go back to cooperating with the partner once cooperation is
     restored by the other actor.
   • When mutual cooperation occurs, go back to cooperating with
     each other.
   • If multiple defection occurs, start defecting as well.
   • Other strategies included:
      – Defecting on the first turn to gauge whether the partner could be
      – Continual defection against the “sucker”.
Shortcomings of the Prisoner’s Dilemma:

   • Does not take into account group formation.
   • Individuals are not capable of choosing partners, as in real life,
     in this model.
   • Factors involved in changing interaction partners, such as
     migration, mutation, etc. are not taken into consideration.
   • Such shortcomings were found as recently as the late 1990’s.
   • Zeggelink’s Social Evolution Model (SEM), published in June
     2000, offers a more inclusive simulation to study, but results,
     such as findings for existence of an ideal population density, are