1. Modular Validation of a Social Trust Model 2. 1. Source: Babb, S. E. (1977). Accuracy of Planetary Theories, Particularly for Mars. Isis, 68(3): 426-434. A close approximation between synthetic (i.e. model-derived) and aesthetic (i.e. observed) data is necessary but not sufficient to demonstrate a close approximation between the ‘true’ and model systems. For example, Ptolemy’s Geocentric model of the solar system actually provides a better fit to observed data than does Copernicus’ Heliocentric model (Babb, 1977). Theory “Macro” solar system model“Micro” causes ElementsArrangementMotion PtolemyPlanetsGeocentricEpicycles- KeplerPlanetsHeliocentricCircles- CopernicusPlanetsHeliocentricEllipses- NewtonPlanets (Mass & Distance) (Incidentally) Heliocentric Conic sections Gravitational force EinsteinPlanets (Spacetime) (Incidentally) Heliocentric GeodesicsCurvature of Spacetime A close approximation between synthetic and aesthetic data demonstrates the existence of a plausible model-based description of macro system behaviour. To go beyond description and on to explanation requires specifying the causal processes by which the macro behaviour comes about, often at a more fundamental, micro level. To be considered part of the ‘real’ system, rather than as convenient modelling artefacts, the micro-level model specification is equally in need of comparison to micro-data as the macro system is to macro-data. To be justified by additional data, not just planet- ary motion 4. Gracia-Lazaro et al. (2012). Heterogeneous networks do not promote cooperation when humans play a Prisoner’s Dilemma. PNAS, 109: 12922-12926. Human participants played repeated Prisoner’s Dilemma games for money. Players were symbolically arranged on a 25 X 25 square grid and played with the four players in their von Neumann neighbourhood. Players selected one move (Cooperate or Defect) per round, with this move played against all four neighbours (A). Results showed high levels of cooperation initially, which decayed rapidly to near-asymptote (B). Players showed heterogeneous strategies, with many almost always defecting, very few always cooperating, and most showing behaviour contingent on their neighbours’ (C). B C A 5. The game structure from Gracia-Lazaro et al. (2012) was implemented in Netlogo. The cognitive micro- specification was drawn from other neuropsychological and behavioural experiments using dyadic Prisoner’s Dilemma. The cognitive model included: - Utility maximisation - Expectations of others’ behaviours - Memory of others’ behaviours - Preference for Fairness - Fear of / Intolerance for Defection A variety of agent strategies were implemented, blending elements of the above cognitive model, e.g.: - Utility maximisation - Utility maximisation, discounted by Fairness. - Friendly/wary: first move reflects fairness, subsequent ones reflect expectations of defection. 3. 1. Dyadic Prisoner’s Dilemma Lab Experiments 2. Network Prisoner’s Dilemma Lab Experiments 3. Socially Embedded Prisoner’s Dilemma Experiments & other Trust Data Social trust is a psychosocial phenomenon, inherently a product of interactions between individuals but at the same time based upon individuals’ judgements and / or behaviours. To model and explain macro social trust phenomena we must also model individuals’ cognitive architecture at the micro level. ‘Trust’ covers a very broad and ill-defined set of phenomena at both the macro and micro levels. (cf. solar system model above). The SCID social trust model adopts an explicitly modular approach, taking a narrow but well-evidenced idealisation of the cognitive architecture of trust from lab experiments using the Prisoner’s Dilemma. This architecture will be validated against lab data, then “plugged in” to wider social models. 6. B C Synthetic results showed the expected lack of fit for straightforward utility maximizing strategies, which predict defection for the standard Prisoner’s Dilemma payoffs. Discounting the expected rewards of selfish utility by constant other- regarding fairness preferences resulted in too much cooperation. The friendly/wary strategy proved able to reproduce the initially high but rapidly decaying level of cooperation seen by Gracia-Lazaro et al. (2012). Here, the initial move was based upon a high level of other- regarding fairness, then subsequent moves were based upon a low tolerance for defection and a short memory for others’ behaviour (B). The conditional probability of defection was less well modelled by this strategy (C).