Global migration rates steady since 1990, high return migration

WU says that Raftery developed these new migration rate estimates along with his former doctoral student Jonathan Azose, a UW affiliate assistant professor of statistics and the paper’s lead author. They applied the pseudo-Bayes method to migration estimates by incorporating elements of other methods, and calibrated their estimates against a relatively reliable model of migration among 31 European countries.

Azose and Raftery tested the accuracy of pseudo-Bayes against a set of reliable migration figures and discovered that its estimates were typically accurate to within a factor of three, better than many existing estimates.

“For the migration field, this level of accuracy is a significant improvement,” said Azose. “Even when you look at data from European countries, it’s not uncommon for a single migration flow to have estimates that differ by a factor of three or more depending on whether the sending or the receiving country collected the data.”

They also discovered that, compared to other approaches, pseudo-Bayes can more accurately account for return migration, in which migrants go back to their home countries.

“Our estimate shows a higher global flow of migrants in large part because it indicates that return migration is much higher than previously thought,” said Azose.

The researchers estimate significantly higher migration rates than prior methods — between 67 million and 87 million migrants over each five-year period from 1990 to 2015. By comparison, one widely used alternative method, which only estimates the minimum rate of migration, calculates just 34 to 46 million migrants in each five-year period. In addition, while the total number of migrants estimated by pseudo-Bayes increased from 1990 to 2015, the migration rate over that period remained relatively stable as a proportion of global population — between 1.1 and 1.3 percent.

Azose and Raftery also broke down migration rates by emigration, return migration and transit migration, in which migrants move between two countries that are not their countries of birth. In general, from 1990 to 2015, more than 60 percent of migration was emigration. Transit migration never topped 9 percent. Return migration accounted for 26 to 31 percent of migrants, more than twice the rate of other migration estimates. That high rate of return migration added up over time. From 1990 to 2015, approximately 45 percent of migrants ultimately returned to their home countries.

“We estimate a rate of return migration that is significantly higher than other methods, but it is also supported by history,” said Raftery. “For example, during the Rwandan genocide in 1994, more than a million migrants left the country, but most returned within three years after the conflict ended.”

The top migration rates that they observed more recently between countries were consistent with ongoing world events. Migration out of Syria, for example, accounted for two of the top three emigration flows from 2010 to 2015.

The researchers would like to refine their method by differentiating between refugees, which are counted by their method, and other types of migrants. In addition, they would like to incorporate data from non-governmental sources, such as social media records, to improve the accuracy of their estimates, as well as develop approaches to forecast future migration.