You're out to dinner. The food is delicious and the service is 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 not just about service. Instead, studies have shown that tipping can be affected by psychological reactions to a series of different factors 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.

    “Studies before have shown that mimicry(模仿) brings into positive feelings for the mimicker,” wrote Rick van Baaren, a social psychology professor. “These studies show that people who are being mimicked become more generous toward the person who mimics them.”

    So Rick van Baaren divided 59 waiters into two groups. He requested that half serve with a phrase such as. “ Coming up !” Those in the other hall were instructed to repeat the orders and preferences back to the customers. Rick van Baaren then compared their take-home pay. 'The results were clear—it pays to mimic your customer. The copycat(模仿者) waiters earned almost double the amount of tips to the other group.

    Leonard Green and Joel Myerson, psychologists at Washington University in St. Louis, found the generosity of a tipper maybe limited by his bill. After research on the 1,000 tips left for waiters, cabdrivers, hair stylists, they found tip percentages in these three areas dropped as customers' bills went up. In fact, tip percentages appear to plateau(稳定期) when bills topped $100 and a bill for $200 made the worker gain no bigger percentage tip than a bill for $100.

    “That's also a point of tipping,” Green says. “You have to give a little extra to the cabdriver 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 part of the idea of a tip is for just being there.”