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The standard paradigm for exploring hypothesis testing behaviour is Wason's (1960) rule discovery task, which exists in two variants: the standard single goal (SG) task, and the logically identical dual goal (DG) fonn. Despite the close similarity of the two fonns of the task, the reported success rates in the two variants vary considerably, with approximately 20% of participants successfully solving the SG variant compared to over 60% correctly announcing the rule in the DG fonn. It was this disparity between the patterns of perfonnance across the two versions of the task which fonned the impetus for this thesis, as it was felt that an explanation for the facilitatory effect of DG instructions would lead to insights into the poor performance in the SG form. Several competing contemporary accounts of the effect are introduced, and predictions derived from them empirically tested across a series of seven experiments. Data analyses showed that no single contemporary theory could provide a wholly adequate account of the DG facilitation effect. However, these analyses led to a novel observation: that it is the production of a contrast class triple which appears to be the key predictor of success on the task, and furthennore, that the DG variant of the task promotes the generation of such a triple. Support for the "contrast class" account of the DG effect was provided by direct manipulation of the information provided to participants. A theoretical account of the critical role of contrast class cue information is developed in the thesis by situating the account within a proposed extension to Oaksford and Chater's (1994) "Iterative Counterfactual Model" of hypothesis testing. It is further suggested that rather than providing mutually exclusive accounts of the DG effect, competing theories (e.g., Vallee-Tourangeau et al. 's, 1995, triple heterogeneity theory, and Wharton et al. 's, 1993, information quantity theory) could be subsumed within this new model, which would then reflect a process whereby participants' strategies change and develop over the course of the hypothesis testing session. Finally, it is suggested that findings from this thesis can be accommodated more generally within Evans' (2006) "hypothetical thinking framework", and thereby within contemporary dual process accounts of reasoning.

La presente ricerca mira ad approfondire la natura strategica e gli effetti del ragionamento controfattuale utilizzato in discorsi di natura conflittuale tra avversari politici, con l'obiettivo implicito di persuadere un pubblico di ascoltatori. A questo scopo, sono stati condotti due studi. La prima ricerca quantitativa mira a comprendere quanto un attacco controfattuale possa minare l'immagine dei politici nei confronti dei cittadini, a dispetto dell’importanza dell’orientamento ideologico del pubblico stesso. Nella seconda ricerca quantitativa si sono approfonditi gli effetti di diverse tipologie di attacchi controfattuali sull’immagine dei leader politici; gli attacchi controfattuali sono mossi a diversi livelli di astrazione e sottendono la violazione di una norma di competenza o di integrità da parte del leader target. / This study aims at deepening the strategic nature and the effects of counterfactual reasoning used in dialectical discourses among politicians, whose implicit goal consists in persuading receivers. For this purpose, two studies have been realized. The first quantitative research aims at understanding if the use of counterfactual attacks can harm politicians' image, in spite of the importance of political attitudes of citizens. The second quantitative research studies the effects of different counterfactuals on politicians’ image; these counterfactual attacks (at different levels of abstraction) can evoke the violation of a norm of agency or communion by the leader.