• Term was coined by
Robert Trivers (1970’s).
• Refers to the offering
and receiving of
support, but at a cost to
• 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
when fitness is
compromised in these
Three Forms of Selection
• Group selection
• Kin 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
• 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
• 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).
• 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).
• Most commonly used model
in studying reciprocal
• Prisoner’s Dilemma.
• Actors in this model follow
the conditions of the
• 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
• 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
• 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