One big change I've observed is the move away from formality to informality. Back in the early 60s when a professional photographer shot myself and my two older brothers, it was all about having a formal line-up and neatly combed hair (see image below).
Sure, there was a the rule of thirds of sorts, but in this particular case, rather than leading your eye to a focal point, it runs right through our heads. Indeed, I'd say this was possibly the first image to inspire me to try photography, though only because I was so appalled at the composition.
Formality was an aesthetic tradition that my father continued as we grew up—lining us up in what we called 'The Three Steps' (see below).
To add to the stiffness of the occassion, we would stand for some minutes with smiles fixed on our faces while our father first checked his Weston light meter then attempted to focus his Argus rangefinder camera. In some respects, it was an experience more like sitting for a daguerreotype than say posing in front of a modern auto-everything DSLR. Of course, I was already the anarchic radical, slyly posing informally with hands in pockets.Today, most domestic portraits tend to be all about the 'casual lifestyle', with people relating (often moving) rather than static posing—like pages lifted from so many popular magazines.
One other big change that I've observed follows on from the first—modern domestic portraits are far more physically intimate. People are placed less shoulder tip to shoulder tip—now they're placed right into each others' faces, leaning, lying on or wrapping around each other (see below).
