Why do you tip in a restaurant? And how do you decide how much to hand over? Even if you're fairly certain that you only ever base your tip on the level of service you receive, the research would seem to demonstrate that you're actually likely to make a choice based on how your waiter or waitress looks too.
A new study by Voucherbox.co.uk asked people what - aside from service - they would pay more for. Some 23% said they would pay a premium for staff with a nice smile, 20% said they would tip an attractive server more money than an unattractive one, and one in ten said they'd even be prepared to tip more if their waiter or waitress smelled nice.
The things most likely to reduce a tip were dirty fingernails - which a third said would lead them to pay less, this was followed by scruffiness at 30% and greasy hair at 23%.
The study found that people claimed the figure or build of the waiter or waitress didn't influence their tips, but separate research seems to show that this isn't the whole story.
What we pay more for
Mike Lynn, a professor at the Cornell Hotel School, has written research on tipping that found that blonde waitresses get better tips than brunettes, slim women get better tips than larger women, and those with bigger breasts get better tips. Women in their 30s also get better tips than waitresses who are any older or younger.
In fact, any physical attribute that makes a person seem more or less attractive to a diner will influence the tip they receive. A study in the Journal of Hospitality and Tourism Research even found that men tipped waitresses more if they were wearing red.
We're also prone to tipping more if we think this attractive server likes us. The Voucherbox research found that 16% were prepared to tip more if their server flirted with them, and separate work done by Lynn found that if a server touched the customer on the shoulder when they handed them the bill, they were more likely to tip better.
Lynn also went even further. He looked into the difference that the quality of service makes to the size of the tip, and found that just 4% of the variability of the tips was due to the quality of the service.
Meanwhile, a study in the Journal of Economic Psychology found that a server who is considered 'strikingly beautiful' can earn around £855 more a year in tips than someone less good looking.
But what do you think? What influences you when you leave a tip? Let us know in the comments.