Originally shutter speeds were very, very long (and that lasted decades for more budget equipment), so smiling was generally a bad idea as people couldn't hold

Originally shutter speeds were very, very long (and that lasted decades for more budget equipment), so smiling was generally a bad idea as people couldn't hold their smiles for so long and would just be blurred weirdly. That probably influenced how people thought photos should be posed for, even when shorter and more reasonable shutter speeds were the norm.