Expert Commentary

Migration, immigration and demographic science: Longer-term perspectives

2016 study in Proceedings of the National Academy of Sciences (PNAS) that provides one of the most comprehensive analyses to date of the many fluid dimensions of population movement across societies.


Whether it is the flow of displaced persons to Europe from the Middle East and North Africa or the ebbs and flows of Mexicans across America’s southern border, issues of migration, immigration and population movement controls are often politically radioactive.

Yet these policy debates — from building walls on borders to limiting quotas for refugees — frequently focus public attention on shorter-term issues and miss the wider patterns that may be even more consequential for societies over time. Operating largely outside of the latest policy controversies, there is a well-established discipline of social science that makes demographic projections based on long-term factors and assesses how the resources and economic needs of society align with changing patterns.

At both the global and national level in the United States, future projections all point to a much larger population, suggesting looming resource and infrastructure needs — and potential emerging areas of conflict, as much of this growth happens in cities across the world — that will require policy planning at all levels. The United Nations’ 2015 World Population Prospects (WPP) report states that over the next 15 years, the global population is expected to increase by more than 1 billion, “reaching 8.5 billion in 2030, and to increase further to 9.7 billion in 2050 and 11.2 billion by 2100.” Further, the report notes, “During 2015-2050, half of the world’s population growth is expected to be concentrated in nine countries: India, Nigeria, Pakistan, Democratic Republic of the Congo, Ethiopia, United Republic of Tanzania, United States of America, Indonesia and Uganda, listed according to the size of their contribution to the total growth.” In a 2015 report, the U.S. Census Bureau projects continued population growth domestically: “Between 2014 and 2060, the U.S. population is projected to increase from 319 million to 417 million, reaching 400 million in 2051.”

Birth rates and patterns of longevity may be fairly predictable over time, but migration is a factor that requires perhaps even more careful analysis. There are a variety of “push” and “pull” factors — from educational and economic opportunity to war and drought — that interact dynamically and sometimes unpredictably to incentivize or limit cross-border migration. A 2016 study published in the Proceedings of the National Academy of Sciences (PNAS), “Probabilistic Population Projections with Migration Uncertainty,” provides one of the most comprehensive analyses to date of the many fluid dimensions of population movement across societies. The study’s authors, Jonathan J. Azose, Hana Ševčíková, and Adrian E. Raftery — all affiliated with the University of Washington — note that “in many countries, migration is a substantial driver of population change,” and thus properly assigning and analyzing uncertainty in projections is crucial to making well-founded forecasts.

The researchers note that their migration model “does not specify that projected migration must respect current migration quotas or that current countries of net in-migration should remain net receivers.” They choose this analytical direction because “quotas do change over time, as is happening with European Union quotas in the wake of Syria’s refugee crisis.”

The study’s findings include:

  • The impact of migration on population levels often varies depending on the level of development within a country. Compared to the United States and Europe, where migration levels matter greatly, “in many developing countries net migration makes up only a small fraction of total population change.” Indeed, “when this is the case, the contribution of uncertainty in migration to uncertainty in population change is dwarfed by the contribution of uncertainty in fertility and mortality.”
  • The United Nations’ projections for the United States “have net migration counts remaining constant at their most recent levels until 2050, then declining linearly to zero in 2150.” This may contribute to inaccurate projections, as “historical trends in the United States data show that although it is often realistic to project that net migration will not change much from one time period to the next, there are occasionally large deviations from this trend.” When projections account for these type of variations, “we see that incorporating uncertainty due to migration roughly doubles the width of predictive intervals for population.”
  • The case of Germany illustrates the potential impact of migration: “Germany’s 80% prediction interval for the 5-year period 2015–2020 has an upper bound on net migration of 2.2 million, substantially higher than the 1.25 million observed in the previous 5-year period, 2010–2015. Although this bound seems high relative to recent history, surpassing it in 2015–2020 remains a possibility, in large part due to an influx of Syrian and other refugees. From January through November of 2015, Germany registered 965,000 migrants, with 484,000 from Syria alone.”
  • Within Europe, however, there are structural factors that make it likely Germany may not remain the dominant country by population. “By 2060, France and the United Kingdom are both more likely than not to be more populous than Germany, and by 2075 Germany has only one chance in eight of being the most populous.”
  • Overall, the model developed in the study suggests a “substantial increase in uncertainty about the populations of Europe and Northern America, with very little change to uncertainty about the population of Africa, Asia, and the world as a whole.”

“One common trend in historical migration data is that a refugee out-migration is often followed by a large return migration,” the researchers conclude. “An ideal migration model would be able to replicate this phenomenon. Such refugee movements are hard to model and especially hard to predict, but finding a way to include them might improve our migration model.”

Related research: The composition of America’s population and related policy implications — for example, greater longevity and a “graying” of the population — has also seen sustained research and analysis. For example, see the 2015 Daedalus paper “Labor-Force Participation, Policies & Practices in an Aging America: Adaptation Essential for a Healthy & Resilient Population”; and see “An Aging Population And Growing Disease Burden Will Require A Large And Specialized Health Care Workforce By 2025,” a 2013 study published in Health Affairs.

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