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The literature has struggled to reach an acceptable definition for the construct of impulsivity. Different models have variously suggested that the crux of the impulsivity construct lies in lack of planning, difficulties in concentration, failure of inhibition or attraction to sensation or immediate reward. More recently, research has suggested that impulsivity is increased in conditions of extreme affect, either positive or negative. The development of a novel measure of affective impulsivity, termed the UPPS-P Impulsive Behaviour Questionnaire, has enabled quantitative investigation of the construct. In spite of difficulties in defining impulsivity, there is a broad consensus as to the clinical utility of the construct. It has been shown to be highly predictive of a wide range of so-called impulsive behaviours, which include suicide, binge eating and substance use. At the same time, impulsivity is also an important trans-diagnostic construct. Trait impulsivity is elevated in many psychiatric disorders, including borderline personality disorder and attention deficit hyperactivity disorder. There is also strong evidence that both state and trait impulsivity are elevated in bipolar disorder, and that this elevation may explain a number of common comorbidities and behaviours found within the disorder. The thesis begins with a review of the literature, examining six key perspectives on impulsivity and their corresponding measures within Chapter One. Chapter Three also explores the construct and measurement of emotion-mediated, affective impulsivity. The literature linking impulsivity to behaviour in both clinical and non-clinical samples is reviewed within Chapter Two. Within Chapter Four the evidence supporting continuum models of psychiatric disorder is introduced, with particular discussion of the bipolar spectrum. Chapter Five brings together the preceding chapters, reviewing the literature supporting an interaction between psychiatric disorder, affect, impulsivity and behaviour. The first study of this thesis, a cross-sectional questionnaire study which can be found in Chapter Six, looked to extend these findings by exploring the presence and role of impulsivity in a wider bipolar spectrum sample. Experience of extreme positive mood state, as measured by the Mood Disorder Questionnaire, was found to relate to increased trait impulsivity and elevated engagement in a range of impulsive behaviour. A structural equation model demonstrated that impulsivity could be separated into cognitive and affective components, with affective impulsivity a key predictor of impulsive behaviour. Affective impulsivity was also shown to moderate the relationship between extreme mood experience and impulsive behaviour. The second study, found in Chapter Seven, used semi-structured interviews and thematic analysis to develop an understanding of the individual’s experience of impulsivity and impulsive behaviour. The themes arising from these data (preparatory set, influence of the environment and others, intense emotional state, (lack of) agency, premonitory urge, reflexive action and sensation seeking) suggested that context plays an important role in the prediction of impulsive behaviour, with the physical and social environment of the individual an important factor in their behaviour. Results also supported the validity of the affective impulsivity construct. Individuals described their emotional state as having a large impact upon their decision to act impulsively, either using behaviour as a tool to moderate their affective state or experiencing cognitive difficulties as a result of it. Finally, data suggested a link between impulsivity and loss of control. Models were developed which indicate the presence of a ‘risky mood state’ in which individuals feel unable to control or predict their behaviour. Together the studies highlight the complexity of impulsivity, and the importance of a wide variety of other factors related to the construct. This is particularly true of affective state, which was seen through both studies to have a large impact on both the occurrence and experience of impulsivity and impulsive behaviour. The studies support the validity of the bipolar spectrum, with similar experiences found in our sample as in clinical samples. The finding that impulsivity is related to a wide range of high-risk behaviours, and that many of these behaviours are ego-syntonic, has important implications for the treatment of impulsivity.