And the digital uprising will make for another shift.
I am thinking of how many times a person is photographed in his/her lifetime, and how that plays into the "pose" taken by the photographed. In the late 1800's and early 1900's, how many times would the average person be photographed in their lifetime -- twice, ten times, perhaps 20 times.
By the mid-1900's, one's life in the USA would be documented from birth to death...limited somewhat by the need to load film and have it developed. Every major event and most minor ones we are asked to stand for a recording of the event..."Blow out the candles, dear." "Stand by the Yosemite sign, everyone!" etc. It becomes a learned behavior...this getting into the ritual position for the paricular event.
Now since the late 1900's and the coming of the digital snapshot and now the camera phone, we are photographed weekly if not more. A dominate position these days seem to be the wide-angle arm-length self/portrait/small group shot.