The effects of density and other situational factors on perceptions of and responses to crowding in classroom learning environments were examined in three separate and concurrent investigations. The first experiment examined the effects of various demographic variables, learning environments and room design variables on crowding. In the second experiment, the effect of two levels of density on classroom achievement was examined in the laboratory. The third experiment examined achievement as a function of class size over a 24-year period in two community school districts. It was concluded from the three experiments that: (1) density is primarily a moderator of arousal; (2) high density can therefore have either positive or negative effects on performance, depending on whether the task contains learned or unlearned components, or both; (3) the same interpretation applies to psychological reactions (affect) to the situation; (4) if perceived violations of personal space accompany high density, the outcome is uniformly negative; and (5) the effects of high density (positive or negative) can be eliminated by diverting a person's attention away from the other people in the room, or by increasing the degree of cooperation between people. (Author/BP)