WASHINGTON - Some people sneak back to the table to add more money, some to snatch back a few dollars. Husbands and wives dicker, parents and children argue, all over how much to tip when they eat out.
It's summertime, when people eat out the most, and it's a sure bet that the final course at many a meal will be a debate over what the service was worth.
Ask around, and it seems everyone has a tipping system - and a theory on who's the most generous or stingy.
Marty Mondragon, 40, of Albuquerque, N.M., says his sister calculates 15 percent down to the exact penny. "Just round it," he insists.
Sandra Lawless, 37, of Venice, Fla., said her husband "pretty much does what I tell him - usually 15, 20 percent."
Jessica Bellis and fiance Paul Rogers came out of Cafe Asia on 19th Street in Washington one recent evening and reported tipping 20 percent because it's easy to do the math - take 10 percent and double it.
"Paul will very easily just take 20 percent," Ms. Bellis chimed in, "and I'll be like 'Whoa, whoa, whoa. What are you doing? We don't need to give that much."'
Overall, support for the practice of tipping has grown in recent years, from 55 percent in a 1978 Roper survey to 73 percent in an Associated Press poll.
The poll, conducted by ICR of Media, Pa., finds that younger people are more supportive of tipping than older Americans, and that people in the Northeast are more accepting of the practice than those in other regions. More than three-fourths think a tip should be earned rather than automatic, and more than half say they've stiffed a server because of poor service.
But social researchers say that in reality, the biggest factor in determining who gets the tiptop tips is simply the size of the bill, not the quality of the service.
"Whether it's sunny outside has as big an impact on tipping as the rating on the service," said Michael Lynn, a Cornell University professor who has researched tipping habits for two decades, earning the nickname "Mr. Tipping."
People tend to tip a little more when it's sunny, one study found.
Other oddities gleaned from the annals of tipping research: Servers who squat down at the table to look customers in the eye get about $1 more, servers who smile or touch diners on the shoulder also net more cash, men tip more than women if the server is female, women tip more than men if the server is male, and blacks tip less than whites, although the racial variance declines among blacks who dine out more frequently.
Other studies show that people tip more if their check arrives on a tray decorated with a credit card logo. Writing "thank you" on the check also increases tips, as does bringing a piece of candy with the check - or, even better, two pieces.
Then there's the smiley-face factor.
It turns out that female servers who draw a happy face on the back of the check boost their tips, but male servers who do the same decrease their take.
Bill Marvin, a consultant known as the "Restaurant Doctor," says most people don't reduce tips much for poor service, despite what they may say.
"If people truly tipped honestly, waiters would get a lot better feedback," he said. Instead, he said, diners "tip 15 to 20 percent for terrible service and then never come back."
Waiter Paul Paz, who's put three children through college on his restaurant earnings in Oregon, says there's no predicting tips based on diners' appearances.
"I've been stiffed by businessmen who are talking about multimillion-dollar contracts," said Mr. Paz, who works at Stanford's in Lake Oswego. Then there are groups of elderly women asking for separate checks who end up "throwing money at me."
Peter Chan, a waiter and bartender at Smith & Wollensky steakhouse in downtown Washington, says business people tend to tip better than families or elderly people who "mean well, but they just don't have the money, or they don't know" tipping customs.
The more people drink, Mr. Chan said, the worse they tip.
"They get too drunk and can't add."
Tipping etiquette isn't just fodder for restaurant kibitzing. There are serious legal points at issue.
Just last month, the Supreme Court gave the Internal Revenue Service the OK to use estimates to catch - and tax - unreported restaurant tipping.
The IRS argued that it should be able to estimate the amount of cash tips that servers collect based on the percentage of tips that show up on credit-card slips. Restaurant Fior D'Italia, which has been serving veal and pasta in San Francisco for 116 years, protested that customers who pay cash tend to tip less, some customers leave no tip at all, and some write a high tip on the credit card slip, but ask for cash back.
Part social grace, part business transaction, tipping seems to inspire endless head-scratching and not just over the calculations scribbled on a napkin. Why the fascination?
"You're paying more than you have to for a service," Mr. Lynn of Cornell said. "Where else do you that?"
Tipping tips from etiquette expert Hilka Klinkenberg, founder of Etiquette International in New York:
Some demographic details from the AP poll on tipping, conducted by ICR of Media, Pa. The poll of 1,001 people was taken June 14-18 and has an error margin of plus or minus 3 percentage points, larger for subgroups.
Is tipping fair or unfair?
Young adults were more likely - four in five - to think it's fair to tip people for services performed than adults 65 and over, three in five. People in the Northeast are more accepting of tipping than those in the Midwest.
Does a tip need to be earned?
People who live outside of metropolitan areas were more likely than those who live in metropolitan areas to say tips have to be earned - by 84 percent to 75 percent.
What size tip do you leave in a restaurant?
About one-fourth of those who make less than $50,000 a year said they leave a tip of between 6 percent and 10 percent - more than twice the number of those who made more than $50,000 a year who said they leave that amount. People in the Northeast were twice as likely - almost four in 10 - as those in the South to leave a tip of between 16 percent and 20 percent.
Do men or women tip more?
About one-third of men and women thought men tip more than women. But men and women matched up evenly in the question about what size tip they leave - suggesting the perception that men's tipping more may not match reality.
Have you ever left a restaurant without leaving a tip because of poor service?
Men were more likely than women to leave without tipping, people in the South and West were more likely to leave than people in the Northeast. Blacks were more likely than whites to leave without tipping.