Parochialism

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Contents

Abstract

The evolution of parochial altruism is not well understood. We study this problem by considering a prisoner’s dilemma game with four strategies: altruists who cooperate with everyone; parochialists who only cooperate with members of their own group; traitors who only cooperate with outgroup individuals; and egoists who never cooperate. We develop a model that allows for both assortment and conflict between groups. Individuals discriminate between in- and outgroup members. While assortment and conflict allow for the evolution of both indiscriminate and parochial altruism, discriminate behavior creates an advantage for parochialists over altruists, as the latter waste help on outgroup members. We use computer simulations to study the multilevel selection dynamics. The simulation model describes an ab- sorbing Markov chain. We examine the absorption probabilities of altruists and parochialists. Three model versions are compared, with only assortment, with only group conflict, and with both mechanisms. We find that parochialism is selected for by group conflict as well as assortment. Discrimination allows for cooperation inside groups to withstand regular interactions with outgroup members.

Run the software

You can download the software here.

Once the program is running just click on the big play button to start the fun. Feel free to re-arrange and re-size the windows, zoom-in and out as the program is running.

Screenshot of the simulations

Use Java RunTime 5.0 or newer to run the software. It should be just fine if you already have java installed in your machine, just double click once you have downloaded the file, or choose open from the browser box.

Parameters

Parameter Model Simulation
Frequency of group conflict κ AverageFrequencyOfGroupsInConflict
Benefit (in the PD game) b B
Cost (in the PD game) c C
Group size n GroupSize
Probability of ingroup interaction α IngroupProbability
Probability of migration λ MigrationProbability
Incumbent (or mutant) strategy Mutant
Number of groups m NumberOfGroups
Seed for random number generation Seed
Intensity of selection w SelectionIntensity
Re-shuffle groups every generation ShuffleGroupsEveryGeneration
Splitting probability q SplittingProbability
Steepness of winning probability curve z Steepness

Parameter ranges