Is tipping a cultural thing?

Another three-day workweek went by this weekend, and I would say it went pretty well. The first day was pretty damn short; worked only 3 hours. Friday and Saturday were the usual 11-hour segments that i've come to expect. Usually, the first weekend of the month, as was the case this past weekend, are the weekends where tips do not come as frequent as other weekends. Some say the customers latch on their wallets and become stingy; customers simply reserve these weekends to get in touch with their jewish ancestry (cheap shot). I however think there's something else underneath the surface that results in a higher frequency of stiffs (non-tipping customer). I think that the landscape of customers shifts from a college-student driven customer base on regular weeks to one that includes more locals (and by locals, i mean those who live in low-income homes). In fact, this is so frequent, we have a name for this weekend: welfare weekend.

I got to thinking, is tipping a cultural thing, or is it a luxury for higher income persons? I mean, do people tip simply because they have it, or is it an issue of etiquette that different cultures react to differently?

For a long time, I thought it was the former; customers who live in better homes are in better financial status are more prone to tip than others. Over the course of the last 4 years, however, I have noticed that this might not be the case. I have been to affluent customers who on a regular basis do not tip. I have also been to trailer park homes and get like $4 tip every time I get there (although this didn't happen as often as I would like). There seems to be another characteristic that more accurately predicts whether one tips or not: Culture.