Sept. 29, 2008 -- Researchers have proposed a new model for how populations migrate from one country to another, which they say could improve population projections for countries worldwide.
"Our model is not supposed to explain as much as possible of migration," said Marta Roig of the United Nations Population Division in New York. "We are trying to develop a simple model that can be used for all of the countries of the world."
The study used information from 11 countries -- many European countries plus the United States, Canada and Australia -- with sufficient data. Eight of the countries had information on where their populations emigrated to (destinations), and where its immigrants came from (origins), while three of the countries only had information about where people came from.
In all, there were 228 origins and 195 destinations between 1960 and 2004. The researchers used relatively straightforward information, like the population of the origin and destination countries, land area of the countries, and the distance -- as the crow flies -- between origin and destination capital cities.
"We found we could explain with a few relatively simple variables about 60 percent of all of the variability from year to year and country to country," said study lead author Joel E. Cohen of Rockefeller and Columbia Universities in New York.
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The team found that the bigger the population of a country, and the greater its population density, the greater the number of people who left for other countries. Yet at potential destinations, greater population numbers also tended to attract more immigrants. For origins and destinations, population numbers mattered more than population density.
The research appears in current issue of the journal Proceedings of the National Academy of Sciences (PNAS).
"The more people there are at the origin, the more candidates there are for migration," explained Cohen. But doubling the population does not double the number of emigrants. "There's a tradeoff between the attraction of being in a bigger country and having more candidates to depart."
"On the receiving side," he said, "the more people in a potential destination, the higher the inflow of migrants. ... But, to our surprise, the population density of a destination doesn't play a significant role. The migrants don't care. They want to go where there are more people, but they don't care if it is more or less crowded."
"In the future, migration is going to be relatively more important," Cohen added, because fertility rates have declined and survival rates have improved, meaning births and deaths have a smaller effect on a country's annual population growth than in the past.
Countries rely on population projections to plan many aspects of their economies, which is why projections are important. Agencies and governments have sometimes assumed steady migration rates from year to year.
Others have done better at trying to explain the forces behind migration in individual countries, including wages, population age structure or cultural attitudes, Roig said, but these have not been aimed at global projections.
"What we needed is data for all countries in the world, or as many as possible," Roig said.
"The Achilles heel here is the quality of the data," Cohen said. "Different countries have vastly different definitions of what constitutes a migrant." Australia, for example, reported 3,971 migrants to the U.K. in 1998, while the U.K. registered 41,800 Australian arrivals. Australia counted permanent emigrants, while the U.K. counted anyone staying a year or longer.
The team has not yet made any future predictions with the model; that is the next step. The U.N. Population Division is considering using the model, or a future version, in its population projections, Roig said.
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