If you search "how much to tip," you'll find articles from many corners of the internet on how to do it correctly. With so much guidance out there, this debate over etiquette is far from settled: There are the recommendations of the traditional route (tip if the service was good) and the super-polite rules (always tip 20 percent) as well as experimental approaches (a game theorist might advise tipping only if you plan on going back). Regardless of what the right amount to tip is, researchers have tried to document the tipping behavior of Americans across the country. Turns out, tipping data is hard to track because cash tips often go unreported. (And sometimes, even having the data doesn't make tipping behavior less bizarre.)

Square, a payment service, tracked credit-card tips and found Alaska, Arkansas, and North Carolina to be the states with the most generous tippers. But Square's data isn't necessarily representative, because it only tracked tips from credit-card transactions, which tend to feature bigger tips than when cash changes hands.

The Frequency of Credit-Card Tips in the U.S., by State

Quartz/Square

Payscale, an online salary-information company, has recently released a report with survey data from 15,000 food-service workers. There are a couple reasons why this data is insightful: Firstly, the Bureau of Labor Statistics doesn't have information specifically regarding tips—it is included in the hourly wage estimation. A 1996 BLS experiment tried to collect data on tips from restaurant workers, and found a high nonresponse rate (40 percent did not respond). Secondly, the IRS estimates that as much as 40 percent of tips go unreported. It's hard to track for an obvious reason: Everyone likes giving and getting tips in cash. Nationally this adds up to as much as $11 billion in unreported (and untaxed) income. So self-reported data, which doesn't come with any tax consequences, might paint a better picture of how much servers are actually getting in tips.