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The aim of this thesis was to consider the role of appraisals of intrusive thoughts in the development of Obsessive-Compulsive Disorder. A narrative literature review explored the hypothesis that 'normal' intrusive thoughts lie on a continuum with clinical obsessions. The review discussed previous research on intrusive thoughts in nonclinical samples and drew comparisons with characteristics of clinical obsessions. An internet-based empirical investigation employed a randomised controlled trial design in order to test the effectiveness of an intervention based on normalising information in reducing problematic meta-cognitive beliefs. A large sample (N = 148) of young adults (aged 18-20 years) was screened in to the study based on high levels of problematic meta-cognitive beliefs. Participants completed questionnaire measures of meta-cognitive beliefs, obsessive-compulsive symptoms, reactions to intrusive thoughts and experiential avoidance. Participants completed an interactive quiz based on normalising information (experimental condition) or pet information (control condition). Significant reductions in problematic meta-cognitive beliefs and experiential avoidance were observed in both conditions, thus no additional benefit of normalising information was indicated. The implications of these findings are discussed in the context of the potential normalising effects of symptom monitoring. Overall this thesis supports the comparison of 'normal' intrusive thoughts and obsessions and suggests that negative appraisals, such as problematic metacognitive beliefs, may not be the only defining factor in the development of Obsessive-Compulsive Disorder.

Five studies tested a key tenet of the Elaborated Intrusion theory (EI theory; Kavanagh, Andrade & May, 2005), that intrusive thoughts play an active role in craving, as well as the predicted adverse effects of thought suppression and beneficial effects of mindfulness for coping with intrusive thoughts

Associahedra can be realized by taking the convex hull of coordinates derived from binary trees. Similarly, permutahedra can be found using leveled trees. In this paper we will introduce a new type of painted tree, (T ◦ Y)n where n is the number of interior nodes. We create these painted trees by composing binary trees on leveled trees. We define a coordinate system on these trees and take the convex hull of these points. We explore the resulting polytope and prove, using a bijection to tubings, that for n ≤ 4 the poset of the painted face trees with n+1 leaves is isomorphic to the face poset of an n-dimensional polytope, specifically KF1,n, the graph-associahedron for a fan graph, F1,n.