Interesting but not true. We pose because we want to look a certain way - a harmless vanity of smiling, presenting what we think is our best put-together appear

Interesting but not true. We pose because we want to look a certain way - a harmless vanity of smiling, presenting what we think is our best put-together appearance. Portrait and fashion photographers choreograph the poses of their subjects as well, as part of the process of creating an image that meets the expectations and needs of the audience. There's also the need to light the subject according to aesthetics, and because the lights don't move with the subject, we hold the subject still in relationship to the light. Another aesthetic choice relates to depth-of-field and the in-focus/out-of-focus planes, and again that's something that's easier done with the subject is still. One could argue that certain static portrait poses are distantly derived from the look of portraits produced in the early days of photography, and that look is a cultural expectation. There's plenty of portrait photography that does not replicate 19th century stylistic elements.