Why is it that pictures are taken with a subjects shoulders set at a 45 (or-so) degree angle to the camera in the vast majority of portraits?
I am hard pressed to find portraits with the subject head-on, or at 90 degrees.
Since it is clear that 45 degrees is the de facto standard pose for portraiture, what's the science or logic behind this?
There are many, many portraits out there shot "head on" - fashion and magazine photos often use this kind of pose. Frankly, their models can pull it off.
For the majority of people out there, a straight on pose will most often add a certain bit of weight to the face, whereas an angle to the face can reduce "flabbiness". Most people paying for a photo want to reduce their weight on camera.
Additionally, lenses that have significant barrel distortion will have an extra effect on the "flabbiness" when used head on. This effect is more minimal at an angle.
Some people can pull it off, but its really about the lighting. Its a decision you have to make as a photog - what looks best for that person.
EXCELLENT references on the subject really include the recent questions and answers about broad, short, split, butterfly, and Rembrandt lighting (mostly by Jay Lance Photography) -which cover not just the lighting but the facial pose as well.
I'm sorry, but the premise of your question is just false. Most portraits aren't taken at a 45 degree angle, it's not just models, some of the most famous portraits in photography's history have been shot head on.
Look at the cover of any magazine, it's not just models, most politicians, musicians, etc. that are shot head on. You do see the 45 degree angle in a lot of corporate/school/Sears-style portraits, but that's a small subset of photography and not really one you want to emulate.
On the other hand, faces shot at 90 degree angles tend to be rare, because you don't get to see the eyes, nor can you fake eye contact. The eyes are absolutely key in portraiture, leaving them out is extremely hard to pull off well.
