There’s a reason why the premise of American Gods is so alluring: the US is home to a wild and glorious mishmash of gods, folktales, and cultural heritage. One by one, groups from around the world picked up and landed on a new shore, bringing their stories with them.

The mere existence of certain tales can be revealing. They develop and mutate as they get passed from one group to the next, and the best stories are passed on more readily. Understanding the spread of folktales can help us understand cultural evolution more generally, and a paper in this week’s PNAS does just that by combining data on folktales with genetic, geographic, and linguistic information.

Researchers studying cultural evolution use biological evolution as a starting point for their ideas, but they also point out that clear and important differences separate the two types of evolution. The timescales, for instance, are often very different—cultural units can be transmitted between people of the same generation, while powerful ideas (like religions) can spread incredibly quickly and easily. A lot of work in cultural evolution is dedicated to trying to divine the mechanisms that underlie the spread of cultural ideas. How do people choose which ideas to adopt? And how does the spread of ideas compare to the spread of genes?

Migration vs. diffusion

There are different ways for folktales to spread. One is the American Gods-style model: people move and take their stories with them. But stories can also spread without migration, as they get passed among neighboring populations through “cultural diffusion.” For instance, you might be more likely to know what the deal is with St. Patrick and the snakes if you grew up around Boston rather than Albuquerque.

That’s where the genomic data comes in. If stories are diffusing through neighboring populations, you’ll see a similar pattern across genes, stories, and geography: places that are closer together will have similar genes and similar stories; places that are further apart will be more different. But people don’t sit still. A migratory model should show a different pattern: the genes and stories will be strongly linked, but not correlate as strongly with geographic distance.

So, comparing folktale spread with geographic and genomic data can give us insight into the relative importance of migration in cultural diffusion. That’s exactly what these researchers did: they compared the movement of 596 stories with the genetic data of 33 different populations in Eurasia.

The researchers found that geographic scale made a big difference to the spread of stories. At smaller distances—less than 4,000 km—people and stories showed patterns of migrating together. But at larger distances, the diffusion model was the best at explaining the patterns in the data. Overall, linguistic barriers played an important role: the diffusion was often interrupted by language boundaries.

In “Thumbling,” one of the most popular stories in the group, “a couple wishes for a child and is given a tiny boy through supernatural means. The boy is lost and goes on a series of adventures until he is reunited with his parents,” the authors write. It’s a story that’s clearly related to Hans Christian Andersen’s telling of “Thumbelina.” Along with around 15 other extremely popular stories, it’s a particularly strong example of a story spread by migration, rather than diffusion. So, certain stories behave as “partially independent evolutionary units.”

Illuminating human biases

The results suggest that combining genomic and cultural data can be a useful way to understand the spread of cultural ideas—but caution is needed when including geographic scale. Because the patterns look so different at varying geographical distances, the authors say that distance should be thought through carefully when evaluating cultural spread. And at a cross-continental or global scale, they write, things get more than a little shaky: genetic variability at the global level looks so different, and covers such huge historical scales, that it can’t necessarily yield sensible data on more recent cultural processes like the transmission of folktales.

Methods like these can be used in future to figure out some of the mechanisms underlying cultural evolution: what causes some stories to spread like wildfire while others don’t travel as far? Does it have to do with who’s telling the stories or how novel the stories are? Or is it something else to do with the content of the stories themselves? Studies in psychology labs are one way of investigating these ideas, but population-level studies like the one in PNAS provide an important way of testing their ideas against cultural patterns in the real world.

PNAS, 2017. DOI: 10.1073/pnas.1614395114 (About DOIs).