Hoarding disorder was recently recognized as a mental disorder. It is characterized by difficulty discarding objects and excessive clutter limiting the functionality of living spaces in the home. Hoarding represents a considerable public and personal health concern, but there is still much that is unknown about its development and maintenance. Furthermore, current psychological treatments provide only modest outcomes. The present dissertation examined two key aspects of
the cognitive-behavioural model of hoarding: information processing (i.e., memory, attention, decision making, categorization; Study 1) and emotional features (i.e., emotional intensity, experiential avoidance; Study 2). Exploratory questions concerning preference for cluttered spaces were also examined (Study 3). Three novel virtual reality (VR) environments were developed to examine these components with improved ecological validity. Two groups (i.e., with hoarding disorder, n = 36; without hoarding disorder, n = 40) similar in age and gender were recruited from the community. In Study 1, participants completed a series of standardized and novel VR memory and decision making tasks, and created a categorization strategy for objects in a messy VR home office. Higher attentional difficulties, poorer category efficiency, and poorer trait, but not state, memory confidence was reported in the hoarding group. Indecisiveness was positively correlated with perfectionism. There was no evidence of memory and decision making impairments specific to the hoarding group. In Study 2, participants engaged in two VR shopping trips following a negative or neutral mood induction. The hoarding group acquired more objects and at a faster rate than the nonhoarding group, and discarded fewer objects following the negative mood induction. There were no group differences on emotional intensity, but the hoarding group demonstrated higher emotional reactivity and experiential avoidance. In Study 3, reactions to different levels of clutter were observed using a VR living room that became progressively more cluttered. There were no differences in subjective or physiological reactivity to increasing clutter levels. The hoarding group reported a preference for slightly more cluttered rooms; however, they also reported higher claustrophobic fear. Results from this research advance our understanding of the cognitive-behavioural components of hoarding and offer implications for future treatment and VR research initiatives.