When (and why) did people start saying "Smile!" for pictures?

I was looking at a wall of old, old photos and everyone looks so grim! I realize that early photography involved holding very still for a long time – perhaps smiles were too difficult to maintain? Or do you think because early photos were so rare, they were a serious matter? Now, of course, people yell out the insipid “Smile!”. When did we go from serious and stiff to ‘you have to look happy in your picture’?