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A group of competitive people escaping through an exit could lead to the formation of a deadlock,which significantly increases the evacuation time.Such a phenomenon is called the faster-is-slower effect (FIS) and it has been experimentally verified in different systems of particles flowing through an opening.In this paper,the numerical simulation based on discrete element method (DEM) is adopted to study a group of highly competitive people through an exit of varying widths.The FIS effect is observed for a narrow exit whilst it is not observed for the exit wide enough to accommodate two people through it side-by-side.Experimental validation of such a phenomenon with humans is difficult due to ethical issues.The mouse is a kind of self-driven and soft-body creature and it exhibits selfish behaviour under stressed conditions.Particles flowing through an opening in different systems,such as pedestrian flow,animal flow,silo flow,etc.have similar characteristics.Therefore,experimental study is conducted by driving mice to escape through an exit of different widths at varying levels of stimulus.The escape time through a narrow exit (i.e.,2 cm) increases obviously with the increase of stimulus level but it is quite opposite to a wider exit (i.e.,4 cm).The FIS effect is avoided for an exit wide enough to accommodate two mice passing through it side-by-side.The study illustrates that FIF effect could be effectively prevented for an exit when its width is twice the size of particles.