How much should you tip? It’s one of life’s great mysteries.

But the majority of Americans say they tip 15% or less for a typical meal at a sit-down restaurant, according to a wide-ranging new poll on tipping attitudes from Pew Research Center. The poll surveyed nearly 12,000 people.

Fifty-seven percent of US adults surveyed told Pew they tip 15% or less for an average sit-down meal. Twenty-two percent said they leave a 20% tip.

Tip amounts vary by income, age and other factors, the poll found. Young adults are slightly more likely to tip more generously than older Americans, who tend to be more stingy with tips.

Whether Americans actually are tipping more frequently, if not more generously, is difficult to say, according to Pew. There’s no official data on how many businesses rely on tips to compensate their workers, or what share of workers are regularly tipped.

Tipping amounts have also grown over time.

According to a 1922 edition of Emily Post’s etiquette book, the rule was 10% for a meal in a “first class hotel,” with a 25-cent minimum. By the latter part of the 20th century, a typical restaurant tip was 15%.

Tipping 15% to 20% at restaurants became standard because of a cycle of competition among customers, etiquette researchers say. Many people tip to gain social approval or with the expectation of better service. As tip levels increase, other customers start tipping more to avoid any losses in status or risk poorer service.

But few people know what to do. And a changing tipping landscape — with the option to tip now suggested on touch screens at coffee shops, fast-casual restaurant chains, and other new businesses — has made it even harder.

Seventy-two percent of people told Pew that tipping is expected in more places today than it was five years ago. But only about a third said it was extremely or very easy to know whether or how much to tip for different types of services.

“Even as Americans say they’re being asked to tip more often, relatively few have a great deal of confidence about when and how to do so,” Pew said.

The public is more likely to oppose than favor suggested tip amounts on screens. Forty percent of people oppose businesses suggesting tip amounts to their customers on screens.

Americans’ tipping behaviors also vary by business. More than 90% of people say they always or often leave a tip at sit-down restaurants, 78% when getting a haircut, 76% for food delivery and 61% taking a taxi or ride-share.

Relatively few Americans always or often tip when buying a coffee — 25% — or when eating at a fast casual restaurant, such as a Chipotle or Sweetgreen.