Environmental psychologists make a distinction between the physical measurement of density (how many people are in a particular space) and crowding (the psychological feeling of not having enough space). Usually, high density is needed for the phenomenon of crowding, which makes people experience sensory overload, a lack of control, and rising stress and anxiety. However, some psychologists see crowding as neutral rather than invariably negative, and believe that people's moods and behaviors intensify as density increases. So if an individual is looking forward to a concert, say, the feeling of crowding enhances their enjoyment of the performance. But if they are dreading an event, crowding will make the person's experience of it even worse. Put into a community setting, crowding may accentuate the dominant behavior ─ an aggressive group may turn violent as density rises. Conversely, creating positive social spaces such as parks and pedestrian areas in high-density urban environments may help to lift the general mood and defuse tension.