Dalvin Brown | USA TODAY

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Dominic Arizona Bonuccelli

Corrections and Clarifications: An earlier version of this story misidentified Daniel Post Senning.

Millennials have been notoriously credited with taking down bar soap, breakfast cereal, diamonds and homeownership.

What's next? It could be tipping.

Nearly two-thirds of millennials typically tip below the standard 20 percent suggestion for tips at restaurants, while about half of adults older than 38 say they tip less than 20 percent when eating out, according to a new Creditcards.com study.

The study also revealed that 10 percent of millennials say they "skimp out" entirely, often leaving nothing for a server when dining out.

“The restaurant tipping thing surprises me,” said Daniel Post Senning, spokesperson for the Emily Post Institute. “That’s a pretty firm social contract in America.”

But restaurants aren’t the only places where millennials are choosing to forgo the tip. Drivers for services such as Uber, Lyft and taxi drivers are left high and dry by 18 percent of millennials, even when presented with preset tipping options on the ride-hailing apps.

“When it comes to Uber and Lyft, I don’t tip,” said Jade X, a 26-year-old hotel worker in New York City. “Sometimes I take an Uber that’s $60. It’s not as feasible to tip 20 percent.” X added that if she does decide to tip, $1 to $3 is typical for Uber and Lyft rides.

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Millennials may not be used to tipping in certain situations because the way people spend money – through the internet and mobile devices – has changed so rapidly in recent years.

"Millennials use a lot of services that didn’t have options of tipping like shopping online. Uber did not have a tipping option at all until recently. That shifted the norms toward tipping less or not tipping at all,” said Ravi Dhar, director of the Center for Customer Insights at the Yale School of Management.

“Some restaurants nowadays use tablets for payment. When you use tech as an interface rather than directly interacting with the waiter or a person face to face, it can affect the tipping," Dhar said.

Dhar, who studies consumer behavior, said older people typically have more wealth and disposable income, which he suspects is one reason why young adults don’t tip as much as their elders.

“When you don’t make a lot of money your financial margin of error is very small, so even if you wanted to, you may not be able to financially,” CreditCards.com’s senior industry analyst Matt Schulz said.

Still, some millennials are adamant about tipping – even sometimes above the widely accepted 20 percent threshold.

“I’m a big believer in if you can’t afford to tip well, you can’t afford to eat out,” said Samra Ward, 26, who works with people with disabilities in Athens, Georgia. “Growing up, my mom was in the service industry. To me, it’s such an important job. I tip at least 20 percent for in-person or delivery – higher if they’re accommodating to my food allergies.”

But not all millennials agree with the 20 percent rule in every circumstance.

“I reluctantly end up leaving behind $2 or $3,” when dissatisfied with service, said Karissa Both, a home school educator in Ridgland, Mississippi. “In my experience with traveling and living abroad, tipping is used as a direct reflection of how you view the service. Here in the U.S., we’re expected to pick-up the restaurant owner’s slack for not paying their workers fair wages. That’s not how it should be.”

Both added that she believes the traditional tipping system sometimes infringes on her freedom to express her dissatisfaction.

But Senning, a co-host on the Awesome Etiquette Podcast, thinks not leaving a tip is almost never a good idea. When you don't tip at restaurants, "you’re essentially underpaying someone,” he said.

It's also not the best way to let a worker know they didn't meet your expectations.

“You don’t communicate your displeasure through not tipping,” Senning said. If you feel that you shouldn't leave a tip then it's probably something you should address with the manager, he said.

“The root of the word gratuity is the same as the word gratitude," Senning said. The more you can approach tipping from that place, the better off it is for everyone. It’s a way to show appreciation for the person who’s helping to provide an experience."