In my experience:

- Older generations tip maximum 15%, and decide what to tip based on service quality

- Millennials tip a flat 20%, regardless of service quality

- People who worked in the service industry will tip 30%, because they know what it's like.

Software nudges (e.g. Square). This scales social behavior very quickly.

I'm not from the US so I didn't really have an idea what to take, so I pulled a figure out of my :) sorry for the confusion!

Social norms.

But, yes, it's creeped up from 15% before tax to 18-20% on the total bill in general.

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.