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.