Reciprocal Altruism

Elbert LimAnthro 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.

Origin

• 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 societies?

Three Forms of Selection

• Group selection• Kin selection• Reciprocity

selection

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

growth• 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 another.• Success occurs if the fraction of genetic material

preserved within a group is greater than the fraction of genetic material lost.

• 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 altruism.

• 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 occurrence)

• 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 trusted.

– 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 inconclusive.