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Evolution of Parochial Altruism by Multilevel Selection

Here you find a piece of software that helps estimating the probability of fixation for discriminate and indiscriminate altruists in a prisoner's dilemma game. Please see the details on the paper.

You can download the software here.

Editorial Information

García Julián and van den Bergh, Jeroen. Evolution of parochial altruism by multilevel selection. Evolution and Human Behavior 32 Pages 277-287. (2011) Download the article


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.


Here's the list of parameters in the order they appear in the software, and how they relate to the model in the paper.

Parameter Model Simulation Recommended range
Frequency of group conflict κ AverageFrequencyOfGroupsInConflict 0.0 - 0.1
Benefit (in the PD game) b B Any value greater than c
Cost (in the PD game) c C Positive value (or zero for snow-drift game)
Group size n GroupSize Greater than 2
Probability of ingroup interaction α IngroupProbability 0.0 - 1.0
Probability of migration λ MigrationProbability 0.0 - 1.0
Incumbent (or mutant) strategy Mutant Either altruist or parochialist
Number of groups m NumberOfGroups Greater than 3
Seed for random number generation Seed Any value
Intensity of selection w SelectionIntensity For weak selection 0.1
Re-shuffle groups every generation ShuffleGroupsEveryGeneration Set to true, if you want to avoid assortment
Splitting probability q SplittingProbability Between 0 and 1.0 (default is 0.01)
Steepness of winning probability curve z Steepness Between 0 and 1.0 (default is 0.5)

For a more detailed description on these values please refer to section 4.2 in the paper.

Assortment, Group Conflict or Both

Only assortment

For only assortment please set the frequency of group conflict (κ) to 0. And ShuffleGroupsEveryGeneration to false.

Assortment and Group conflict

For assortment and group conflict please set the frequency of group conflict (κ) to a positive value. And ShuffleGroupsEveryGeneration to false.

Only Group conflict

For only group conflict please set the frequency of group conflict (κ) to a positive value. And ShuffleGroupsEveryGeneration to true.