You're out to dinner. The food was delicious and the service was fine. You decide to leave a big fat tip - why? The answer may not be as simple as you think.

Tipping, psychologists have found, is rarely just about service. Instead, studies have shown tipping can be influenced by psychological reactions to an array of factors ranging from the waiter's choice of words to how they carry themselves while taking orders to the bill's total. Even how much waiters remind customers of themselves can determine how much change they pocket by the end of the night.

"Previous studies have shown that mimicry¹ enhances positive feelings for the mimicker," wrote Rick van Baaren, a social psychology² professor at the University of Nikmegen in the Netherlands, in a recent study in the Journal of Experimental Social Psychology. "These studies indicate that people who are being mimicked³ become more generous toward the person who mimics⁴ them."

To detect the benefit of copying the customer, van Baaren and his colleagues surveyed staff in American-styled restaurants in southern Holland. Among a group of 59 waitstaff, van Baaren requested that half respond to diner's meal orders with a positive phrase such as, "Coming up!"

Those in the other half were instructed to repeat the orders and preferences back to the customers. Van Baaren then compared their take-home. The results were clear - it pays to imitate your customer. The copycat waiters earned almost double the amount of tip than the other group.

Leonard Green and Joel Myerson, psychologists at Washington University in St. Louis, found the generosity⁵ of a tipper may be limited by their bill. After compiling data from nearly 1,000 tips left for waiters, cab drivers and hair stylists, they found that tip percentages in all three areas dropped as customers' bills went up.

In fact, tip percentages appear to plateau when bills topped 0 and a bill for 0 garnered⁶ the worker no bigger percentage tip than a bill for 0.

Why? Green has his theories, including one he attributes to an old Woody Allen saying: "Eighty percent of success in life is showing up."

"That's also a point of tipping," Green says. "You have to give a little extra to the cab driver for being there to pick you up and something to the waiter for being there to serve you. If they weren't there you'd never get any service. So I think part of the idea of a tip is for just being there."

Green explains since everyone would earn the "just being there" tip, it's inevitable⁷ that portion would make up a larger percentage of smaller bills.