Men and millennials are the most reluctant tippers, according to a new study from Creditcards.com. Baby Boomers are most likely to tip for services.

Analyst Ted Rossman said an interesting discovery in this survey centers around the frequency of tipping. Men and Millennials don't tip as often but when they do tip, they give more money than women and Baby Boomers.

Women are more likely to tip consistently. Rossman said part of that is because 70% of tipped workers are female. So there is a sympathy factor in this equation.

For Baby Boomers, they are way more likely than Millennials to tip at restaurants, taxi and ride-share drivers, hairstylists, hotel staff and food delivery. Across the board, Baby Boomers tip a lot more regularly. But when Millennials do tip, they are giving 22% on average at restaurants versus only 17% for Baby Boomers.

Tipping at restaurants is the most universal but only 77% of respondents say they always tip at a sit-down restaurant. Rossman found this to be a low percentage. About two-thirds of U.S. adults are always tipping the food delivery person or hairstylist. Taxi and ride-share drivers were 50-50. Hotel housekeepers and coffee shop baristas are only getting tipped a quarter of the time, he added.

Rossman said when the Creditcards.com study was done last year, it asked the question, "What if we did away with tipping all together. Would you be willing to pay more for a restaurant meal if there was no tipping?" Millennials were more likely to say they would pay for for a meal, which debunks the theory that they are cheap.

For a lot of Millennials, there is a social justice component to this. They are rebelling against the system. Rossman said they don't think it's fair that restaurant owners and other business operators are paying their staff less, then expecting customers to make up the difference.

Two services where most respondents don't tip at all are hotel housekeepers and coffee shop baristas. When it comes to hotel housekeepers, Rossman said tipping is low because for many, there is no face-to-face interaction. These people work hard but customers don't see how hard they work. He suggested hotel housekeepers should get at least a $5 a day tip.

Many customers feel insulted when they see the tip jar at a coffee shop. Many don't believe making a cup of coffee is a hard job. But Rossman pointed out that sometimes coffee drinks are more complicated to make than drinks a person would order at a bar. So 50 cents or a dollar tip should cover it.

Most respondents also said they never tip their trash/recycling collectors (70%) or mail carriers (60%) during the holidays. However, more than half of U.S. adults who have teachers or childcare providers give them holiday tips or gifts.

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