Gossip, Sexual Recombination and the El Farol Bar: modelling the emergence of heterogeneity - Bruce Edmonds

2. Modelling Boundedly Rational Agents using the Evolution of Mental Models

The purpose of the model is to explore some of the possible ways that emergent social structures in an agent community might effect the overall behaviour of that collection of agents.

Since we primarily have humans in mind in this exercise we wish for our software agents to at least capture some of the known qualitative characteristics of humans. In particular we are interested in agents:

who incrementally develop models of their environment;
who develop their models in a parallel manner so that different (and even contrary) models can be brought to bear in different circumstances;
whose mechanisms of learning dominate those of inference;
who are able to identify other agents individually and develop models specifically concerned with those agents;
who have a flexible and expressive internal system of representation, so that they are as little constrained as possible in what model the can develop;
who are able to develop any connection between their communication and their action that is appropriate;
who are able to deal with received communication in what-ever way is best for it;

The agent modelling approach adopted broadly follows [5]. Each agent has a population of mental models, which broadly correspond to alternative models of its world. This population develops in a slow evolutionary manner based on what its past success at gaining utility might be.

Each notional week, the new population of models is produced using a genetic programming (GP) algorithm (Koza 1992). In GP each `gene' is a tree structure, representing a program or other formal expression of arbitrary complexity. A population of such genes is evolved using a version of crossover that swaps randomly selected sub-trees and propagation. Selection of genes for crossover and propagation is done probilistically with a likelihood of selection in proportion to its fitness.

I have slightly modified this here by only using some tree crossover but with a high degree of propagation and also some new random genes introduced each time. Then the best model is selected and used to determine first its communicative action and subsequently whether to go to El Farol's or not. Thus the evolution of mental models is a rough representation of learning.

The cross-over operation is not very realistic but does as a first approximation. For a critique of cross-over and further discussion of the philosophy of agent design for the purposes of the credible modelling of human agents, see [5]. This model of learning fits into the wider framework of modelling economic learning as modelling described in [9].

Gossip, Sexual Recombination and the El Farol Bar: modelling the emergence of heterogeneity - Bruce Edmonds - 17 DEC 97

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