A new study explores the custom of tipping in the United States.

Many Americans give tips to people who perform a job for them, especially workers in the service industry. They pay a little extra to people cutting their hair, driving them across town or serving them a meal at a restaurant.

The new survey was done for the website CreditCards.com.

Princeton Survey Research Associates questioned over 1,000 individuals. The subjects were asked about how and when they offer tips.

The researchers found that men give bigger tips to restaurant workers than women. It also found people allied with the Republican Party give bigger tips than supporters of the Democratic Party. And people who live in the northern U.S. are more generous with tips than those living in the South.

The survey found that people who earn $50,000 or more a year give bigger tips than those who earn less than $50,000. One likely reason: the wealthier Americans have more money to spend.

The report also said that when getting a haircut, 67 percent always tip the person doing the cutting, while 12 percent never do. In a coffee shop, 29 percent always tip the person preparing their coffee, while 30 percent never do.

And when staying at a hotel, 27 percent always tip the housekeeping crew and 31 percent never do.

Michael Lynn is a professor of consumer behavior and marketing at the Cornell University School of Hotel Administration in New York. In other words, he knows a lot about the issue of tipping.

In earlier surveys, Lynn said, he found mixed results on the generosity of men and women when it comes to tipping. In some studies, men gave bigger tips than women. In others, women gave more money.

Lynn’s own research found that men give bigger tips when the restaurant server is a woman and women give more when the server is a man.

Lynn said that in any survey dealing with human generosity, people can say they give more than they really do. There is no way of finding out if people answering a researcher’s questions are telling the truth.

Creditcards.com spoke to one server at a restaurant in Virginia. She said, “All of the really big tips I’ve gotten have been from men, and some of the really bad ones have been from groups of women. I think sometimes men tip more because they’re trying to impress someone.”

Tips are very important to service employees. People who work at restaurants often get most of their earnings from tips.

Nationwide, Americans generally tip between 15 to 20 percent at restaurants and leave from $2 to $10 a day for the people who clean their hotel rooms. Tips of 15 percent are also common for people who drive taxis.

In 2015, Conde Nast Traveler magazine published a report on tipping around the world. Here are some of its findings about restaurants:

In Nigeria, a 10 percent tip is enough if a service charge has not already been added to the cost of a restaurant meal. In Brazil, no tip is required because a 10 percent service charge is generally added on restaurant bills.

In Cambodia, add $1 US dollar for the server. In Indonesia, a 10 percent tip is added to the bill. But diners often leave a little extra money.

In Vietnam, a service charge is not usually added to the restaurant bill. If that’s the case, you should add a 10 percent tip, and more if you use a credit card.

But here is something to keep in mind. If you are able to give the person serving you a tip or a bigger one than what is expected, your generosity will be welcome. Many service workers do not make much money, and even an extra dollar or two can make a difference in their lives.