Scientists have learned a lot about human mobility—from traffic to migration patterns—in recent years, yet there's a significant limitation. Most data concerns how we move over the course of hours or days—months or years if we're lucky. Now, researchers using an unusual data set—Korean marriage records in conjunction with clan place names—have opened the door to studying migration over the course of centuries.
Modern life comes with many ways to track our movements in the short term. Traffic cameras can measure how many cars pass through different intersections, and researchers have managed to trace short-term migration using cell phone data. But if you want to follow migration patterns over a few hundred years or so—say, the rural-to-urban migration that has taken place in the United States over the last century—you're generally out of luck. In most places, there's just not enough data.
Diffusion alone couldn't explain Korea's migration patterns—based on their estimates from the jokbo data, it would have taken about 67,000 years for clans to be as geographically mixed as they are today.
Korea is an exception, according to Sang Hoon Lee and other researchers in Korea, Sweden, the United Kingdom, and the United States. Families there keep genealogical records, called jokbo, that describe births and, more importantly, marriages dating back hundreds of years (though the recordscontain no details about where someone was born or lived). That makes marriage records important for two reasons. First, families are subdivided into clans according to their geographic origins—Lee, for example, is a member of the "Lee from Hakseong" clan, and fellow author Beom Jun Kim is a member of the "Kim from Gimhae" clan. Second, brides customarily moved from their homes to join grooms in theirs.
Assuming a kind of gravitational pull between clans—brides should be more likely to marry grooms from larger clans and also clans closer to their own homes—the researchers could get a better handle on where the clans were and how they moved over time. Fitting that model to data, the team discovered that physical distance actually had little to do with migration. Instead, clans seemed to diffuse outward from their place of origin.
To complement that analysis, the team next looked at modern census records, which still record clan names and, therefore, how different clans originally from one area are distributed around the country today. That follow-up made it clear that diffusion alone couldn't explain Korea's migration patterns—based on their estimates from the jokbo data, it would have taken about 67,000 years for clans to be as geographically mixed as they are today. Rather, clans seem to have flowed toward Seoul, a pattern that appears in the data as a correlation between how spread out a clan is and the distance from the capital to its original location.
Writing in the journal Physical Review X (increasingly a home for studies of social structure), the results suggest that diffusion and directed flow, known as convection, could be valuable tools for understanding human migration and especially for comparing migration patterns across countries, times, and cultures.