“Research shows that the more extraverted the personality traits of people in a country, the greater number of service providers they tip and the larger amount they tip,” says Michael Lynn, a professor of food and beverage management at Cornell University who has studied tipping extensively. But, he says, that is not the only reason why some countries tip more than others. Social norms, differing wages, and whether service charges are customary also play a huge role.

There’s evidence tipping customs can also spread from place to place. One 2016 study revealed travel to the US is a factor in shaping tipping rates in other countries. “Tipping is economically consequential but at its root it’s a social norm,” says Edward Mansfield, a professor of international relations at the University of Pennsylvania and the author on the study.

Mansfield says that foreign students, business travellers and tourists that come to the US may adopt the practice of tipping and bring it back to their home country. “For countries where there are more people who come to the US as a percentage of the total population of that country for the rate of tipping in their country tends to be higher,” he says.

Lynn says we have several different motivations on an individual level for tipping, including wanting to encourage better service on the next visit, or to reward or please their server or gain social approval. While his surveys have shown that in the US, they are the minority, Lynn says many people also tip to abide by social norms and avoid disapproval.