[bobman0330]

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I think the bulk of the reason is sociological. If I had to guess, I would say that the incremental moves are because (as has been suggested) people want to feel 'better' than the norm. If the norm is 15%, then 20% is 'better'. Also, I am sure this is pushed by the perception of what more rich and/or famous people do. I don't think this really goes on at a conscious level though. This seems similar to words and names coming in to and out of fashion with the masses to me. We are, as individuals and cultures, striving by large to be better than the norm, however we perceive better than the norm as being.

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This is the answer. People who leave less than the average tip are stiffs that no one likes. People who leave more than the average tip are generous people who feel for the common man. It's also in the nature of restaurant meals that they are often situations where people want to show how great they are (dates, business lunches, even meals with friends. eating by yourself where there's no one whose opinion you care about to witness the tip is a rarity). So everyone wants to be above average, which is obviously mathematically impossible. When the perceived average was 15%, the real average was probably 20%. Now that the perceived average has caught up to the old reality of 20%, the real average is even higher.

Some people think that a similar effect plays a role in CEO pay, of all things. No board goes around looking for a bargain CEO who will work for less than average. They want a great CEO and they tell their comp consultants to pay him an above-average salary. And the same cycle goes on.