Minutes, January 31, 2008

Today Ruud presented the SugarScape simulation model of Epstein and Axtell.
Ruud briefly discussed the underlying philosophical motivation to the work of Epstein and Axtell. The notion of explanation turns out to be especially important. Epstein and Axtell call their approach generative rather than inductive or deductive, and they regard the generation of a given macrostructure from agent-interaction rules at the micro-level as a necessary condition for explanation of the macrostructure. We did not discuss these philosophical issues in much detail. Instead, we discussed the notion of heterogeneity of agents, which according to Epstein is a characteristic feature of agent-based computational models. This seemed to contradict Wolf's claim from two weeks ago that the heterogeneity of agents in his model is quite unique. We agreed that heterogeneity can exist at two
levels: heterogeneity of an agent's parameter values and heterogeneity of an agent's structure (i.e., the equations/algorithms inside an agent). Although these two kinds of heterogeneity are perhaps not fundamentally different, they may provide an explanation for the apparent contradiction between Wolf's claim and Epstein's paper. We further discussed Epstein's statement that any microspecification that generates a macrostructure of interest is a candidate explanation of that macrostructure. According to Epstein the choice between competing candidate explanations of the same macrostructure should be made by comparing the microspecification of each candidate explanation with the results from laboratory experiments. We doubted whether this is always possible. Uzay and Ruud suggested that a preference for simplicity may be another way to choose between competing candidate explanations. Ruud then showed us some simulations of the SugarScape model.
It was not always immediately clear why certain patterns emerged in the simulations, but at least the patterns usually looked quite intriguing. We realized that for each pattern there may be many different parameters on which the formation of the pattern is crucially dependent. Ruud also showed us simulation examples of something that may be regarded as emergent behavior. Ruud further mentioned that apart from economic research agent-based simulations are also useful in sociological research and perhaps in psychological research. At the end of the meeting the discussion moved from the SugarScape model to the spatial proximity model of Thomas Schelling. However, we decided to devote a separate meeting to the discussion of this model.


-- Ludo Waltman