Challenging behaviours (CBs) are a common issue amongst individuals with Fragile X Syndrome (FXS). The aim of the present thesis was to further understanding of this issue, through exploring physiological factors which may have a motivative influence upon the operant learning of CBs in this group. Analysis of prior literature highlighted that CBs were most commonly negatively reinforced amongst males with FXS. This may reflect an elevated motivation to escape from stressors, associated with atypical stimulus-bound arousal. Accordingly, prior data suggest autonomic hyperarousal and a systematic literature review suggested that the hypothalamic-pituitary-adrenal axis may be implicated in males with FXS. However, the relationship between arousal and escape-maintained CB had not previously been explored, and so was addressed through two empirical studies. In the initial study, CBs were observed in a natural environment, alongside explorations of circadian rhythmicity of salivary arousal measures (cortisol and ?-amylase) in boys with FXS and unaffected siblings. Whilst between-group differences were apparent in arousal measures, there were no associations with observed behaviours in the FXS group. In a subsequent study, behaviour and physiological responding were measured in response to a structured demand assessment, amongst individuals with intellectual disability and males with FXS. Despite between-group differences in behaviour, no differences in physiological responding or physiology-behaviour relationships were observed. A final exploratory study of parental reports of the behavioural and emotional timecourse of instances of CBs was conducted, in order to guide future research. Together the results suggest that initial hypotheses were overly simplistic and that a broad range of aspects of the FXS phenotype must be accounted for when explaining CBs in this group. Implications for future research and practice are discussed
