Any Social Historians about ?

imho

1) When it started photography was a rare phenomenon and people used to think they had to be on their best behaviour. They would have to make an appointment with the photographer and dress up in their finest.It wasn't a case of people taking snaps all the time.

When cameras became available to the hoi-polloi the sense of occasion was no longer relevant and it became a more natural process.

2) Rock and Roll... you can't show off your quiff with a flat cap.

Thanks clouseau. Both explanations sound plausible and would make  the change in both habits broadly contemporaneous - c. the early sixties.
The casualisation of life has proceeded apace since then...possibly a little  too much as some (including me) would see it.

Andy Capp even wears his flat cap in the bath tub.

Helps him to go with the Flo ?

Doesn't want to get his hair wet.

In the very early days of photography, exposure times were very long so smiling was out along with any sort of movement.

It was also common to photograph deceased children posing as if they were alive.

Difficult to get them to grin much.

It was also common to photograph deceased children posing as if they were alive.

At first sight, that seems ghoulish but of course in most cases the families would not have had photos taken while the child was still alive.

A modern equivalent might be those requests on reddit or other social media for someone to photoshop out the breathing, infusion and catheter tubes from my child's last photo.

it took about five seconds to capture the image in victorian times. keep grinning? a static statue pose was required, and if you look at old films of workers from the twenties they were always larking about. They seemed more joyful than the people of today who need a skinful to get there.