Purpose The purpose of this paper is to understand the effect of store messiness and human crowding on shoppers' competitive behaviours, in-store hoarding and in-store hiding, through the mediating effect of perceived scarcity and perceived competition. Design/methodology/approach 2 (store messiness: messy x tidy) x 2 (human crowding: high x low) between-subject factorial experiment was conducted online to manipulate retail store atmospheric factors. A total of 154 responses were collected through Amazon MTurk. The hypotheses were analysed using ANOVA and PROCESS (Hayes, 2013) procedure. Findings Results suggest that store messiness and human crowding within a fast-fashion store lead to perception of scarcity and competition that further affects competitive behaviours. When consumers experience store messiness, they are likely to hide merchandise in store, thus making it inaccessible for other consumers. Further, when they experience human crowding in the store, they feel that the products will be gone immediately so they have a tendency to hoard them. Research limitations/implications This study examined the effects of scarcity perception by studying the case of fast-fashion retailers; generalizability needs to be established across different contexts. Practical implications Retailers by manipulating human crowding and store messiness can create a perception of scarcity in their stores, thus enhancing sales. However, they should also pay attention to deviant behaviours such as in-store hoarding and in-store hiding as these behaviours may decrease the store sales. Originality/value This research contributed to the retailing literature by finding a significant relationship between human crowding, store messiness and competitive behaviours through perceived scarcity and competition.