Seeing the crowd for the bomber: Spontaneous threat perception from static and randomly moving crowd simulations.

Human observers are often relied upon for monitoring suspicious crowd behavior in both civilian and military contexts. However, little research has examined what individual- and crowd-level variables independently and interactively modulate threat perception among human observers. Five experiments gathered threat estimates while participants viewed static or dynamic crowd simulations. Experiments 1 and 2 used static crowd stimuli and manipulated crowd size (number of entities), crowd density (distance between entities), and historical information about adverse events. Experiments 3-5 used moving crowd stimuli and either fixed (Experiment 3) or dynamic (Experiment 4-5) crowd size and density. Experiments 4 and 5 further examined several individual- and crowd-level parameters subjectively reported by observers as critical to generating risk estimates. Overall, results demonstrated that human observers rely heavily on both crowd size and density cues, but also consider several other cues, such as perceived individual isolation and grouping behavior, when estimating risk levels within a crowd. We also show that reliance on such parameters is highly variable across participants in terms of both directionality and magnitude. Results are discussed within the context of continuing sensor system and modeling efforts, and understanding how threat perception emerges from the observation of intentional agents.