Yes, but social norms don’t come out of nowhere. It was 15% for a long time, then it went to 18% (why 18? I can see the jump to 20% as people wanting to do easier math than 18%). Someone somewhere had to have created this new social norm.

> Someone somewhere had to have created this new social norm.

Someone didn't need to do anything and as far as I know, no individual decreed anything. (Although possibly guidebook and etiquette writes have helped contribute.)

It makes some sense with inflation and probably lack of increase in wait staff salaries. Restaurants often show recommended tips and even mandatory service adders for large groups. But, for the most part, it's probably been mostly organic.