1. psychological tension produced in environments of high population density, especially when individuals feel that the amount of space available to them is insufficient for their needs. Crowding may have a damaging effect on mental health and may result in poor performance of complex tasks, stressor aftereffects, and increased physiological stress. In nonhuman animals, crowding can lead to impaired reproduction, decreased life expectancy, and a variety of pathological behaviors. Two key mechanisms underlying crowding are lack of control over social interaction (i.e., privacy) and the deterioration of socially supportive relationships. See also behavioral sink; density.

2. a phenomenon in which the perception of a suprathreshold target is impaired by nearby distractors, reflecting a fundamental limitation on visual spatial resolution. In reading, for example, letter recognition is reduced by crowding from adjacent letters. Also called crowding effect.