If you come from a country of immigrants, you’re more likely to crack a friendly smile on the street. That’s the conclusion of a new study, which may explain why Americans beam more than their Chinese and Russian counterparts.

Scientists have known for decades that societies have their own unwritten rules about when it’s appropriate to smile, frown, or get angry. These rules are part of a country’s “emotion culture,” the norms that influence how and when people express whether they’re pleased or upset. Researchers often study these differences geographically, finding that the United States and the West tend to be more expressive than China and the East. But those geographical studies overlook the important role migration played in shaping emotion culture, says Paula Niedenthal, a psychologist at the University of Wisconsin, Madison.

Niedenthal first thought about migration’s role while living in France and reading Laura Ingalls Wilder’s famous Little House on the Prairie children’s series. “The characters in the stories are constantly interacting with people who did not come from the same heritage,” she says. “I was struck by the fact that people were Swedish living next to Poles living next to Germans. How did they do that?”

Niedenthal and her colleagues suspected that, over time, countries without many immigrants would agree on rules for how much emotion to show in certain situations. People in those countries might even suppress their true feelings so as not to upset the social pecking order. In Japan, for instance, subordinates use smiles around their bosses to hide feeling upset. For countries with a more diverse past, though, the story would be different. “What we're talking about is a collision of differences in language and emotion culture,” Niedenthal says. People in these melting pots would need to beef up their facial expressions to overcome the language barrier.

To test their hypothesis, the researchers needed a way to measure just how much migration a country has experienced. They hit upon something called historical heterogeneity, which captures the history of a country’s migrations in a single number. It’s a tally of the countries that have contributed more than a tiny percentage (about 0.1%) to the current population. For example, Canada scores a 63, which means that Canada’s current population has largely come from 63 different source countries over the past 500 years. By contrast, China and Japan both score 1.

The researchers compared these numbers with some reanalyzed data from an earlier study of emotional expression. In that work, more than 5000 participants from 32 countries filled out a survey that posed various emotional scenarios. For instance, it asked respondents to imagine being happy with a close friend in public or upset with a female professor in her office. It then asked the participants how they should respond, with options like “show more than you feel” and “hide your feelings by smiling.” When Niedenthal and her colleagues tallied the results, they found that countries with more migration also tended to be more expressive.

Then the team zeroed in on a particular kind of facial expression: the smile. They conducted a new study of 726 people in nine countries, including the United States, Japan, and France. Here, participants were again asked to complete a survey, which inquired what constituted a good reason for someone else to smile. There were options such as “is a happy person,” “wants to sell you something,” and “feels inferior to you.” For each reason to smile, the participants picked from among seven choices, from strongly disagree to strongly agree. The researchers compared the results for each country with their migration numbers. Countries with greater immigration over the past 500 years were more likely to interpret smiles as friendly gestures, whereas those with less migration thought smiles were related to the social hierarchy, the team reported online before print in the Proceedings of the National Academy of Sciences.

Daphna Oyserman, a social psychologist at the University of Southern California in Los Angeles, who was not involved in the new work, praises the study. When asking questions about culture and facial expressions, she says, historical heterogeneity provides “another axis we should be thinking about.” But, she adds, no studies will conclusively explain the reasons for smiling in different cultures.

Niedenthal is now in the early stages of studying emotional expression inside the United States. “The point there would be that different regions of the U.S. have experienced different amounts of long-history migration,” she says. She and her colleagues are working with census data to calculate the migration numbers that she needs to study differences between the Southwest, New England, or other regions. As they work on that, Niedenthal is also developing a way to measure emotional expression without a survey. She hopes to use a camera on Google Glass–style glasses to watch faces and directly measure how much people show their emotions, allowing Niedenthal to see if smiles and frowns are more widely used in certain parts of the United States.