Photography is capable of recording most of the emotional and action expressions of an individual, although not necessarily in the sense of the nineteenth-century investigators of human behavior. In their accounts of expressive behavior, such pioneers as Wundt and Darwin used sketches and still photographs to illustrate body posture, gesture and facial expression, but failed altogether to take into account such considerations as those of social context and the role and position of the human observer. It is well to keep in mind that any kind of observation of behavior – with the exception of behavior observed through one-way screens – occurs in two-person or group situations. The very fact of being observed changes, through feedback, the actions and emotions of the observed individual; actions formerly intended for self-consumption then become a statement to others.
Juergen Ruesch and Weldon Kees, Nonverbal Communication, Berkeley and Los Angeles, University of California Press, 1956