Milgram, Bickerman and Berkowitz (1969) found that if one person stood in a Manhattan street gazing at a sixth floor window, 20% of pedestrians looked up; if five people stood gazing 80% of people looked up
Argyle, M., & Cook, M. (1976). Gaze and Mutual Gaze. New York: Cambridge University Press.
Milgram, Bickerman & Berkowitz (1969). Note on the drawing power of crowds of different size. Journal of Personality and Social Psychology, 13