| Summary: | Agent-based decision-making usually relies upon game theoretic principles that are 'rational' i.e. decision-making is purely mathematical based on utilities such as the wealth of an agent. In the context of public goods games, such reasoning can often lead to non-optimal, destructive outcomes for both individuals and the total system, as shown in many scenarios from game theory. This thesis considers how the use of \textit{emotions} can impact upon decision-making and social interactions amongst agents in the iterated Prisoner's Dilemma game by modelling emotions in a functional manner. The background to the thesis is first presented in chapters 2 and 3 where the argument for emotions being included in agent-based decision-making, and evidence to support this proposition, is outlined. Various philosophical issues are also considered such as: do emotions directly motivate an agent's intentional behaviour and, is an agent's decision-making still rational if emotions are used? The framework developed to allow for modelling of emotions in agents is then discussed in chapters 4 and 5 where major psychological models of emotion and computational implementations thereof are discussed. Finally chapters 6 to 8 present extensive investigations into how the emotions modelled using the framework affect social interactions amongst agents in the context described above. As of yet, this topic has been relatively unexplored by computer science and there is space for novel, interesting contributions to be made, these contributions are outlined below. In chapter 6 the emotions of \textit{anger} and \textit{gratitude} are modelled and their effects upon social interactions are analysed. In particular, I look at whether agents endowed with these emotions offer any improvement upon the success of agents using with the ``tit-for-tat'' strategy when playing against other leading strategies from Axelrod's famous computer tournament. How these emotions affect rates of cooperation/defection and the fairness of individual scores is considered along with why they do so. This investigation is furthered in chapter 7 where \textit{admiration} is modelled and an investigation is performed into what emotional characters are selected for under different initial conditions and why. This examination provides a discussion regarding what emotional social norms emerge in a population when agents admire the individual success of others. Two salient questions are asked: is it is the case that emotional characters which promote the total wealth of the system are selected for as an emergent property and, do different initial conditions affect the emotional characteristics selected for?. Finally, chapter 8 extends chapter 7 by modelling \textit{hope} and enquires as to how particular emotional character populations (after a complete social norm has been established) deal with destabilisation of cooperation cycles due to periodic defection. The performance of agents endowed with differing emotional characters are again tested under different initial conditions and specific behavioural features of particular emotional characters are considered. In doing this I comment upon how different emotional characters deal with periodic defection and what the best approach is both in context of an agent's individual score and the total score of the system.
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