Immigration is reshaping societies around the globe. Barriers erected by wealthier nations have been unable to keep out those from the global South — typically poor, and often desperate — who come searching for work and a better life. While immigrants have often delivered economic benefits to the countries taking them in, they have also shaken the prevailing order and upended the politics of the industrialized world — where the native-born often exaggerate both their numbers and their needs.

Share of population who are immigrants 0 10% 20% 5% 15% 40% 100% Canada 22% Russia 8% U.S. 15% U.K. 13% China 0.1% Germany 15% Libya 12% France 12% India 0.4% Switz. 30% U.A.E. 88% French Guiana 40% Austr. 29% Lowest-income countries are shown with a stripe pattern. Share of population who are immigrants 0 10% 20% 5% 15% 40% 100% Russia 8% U.K. 13% Canada 22% Germany 15% Kazakh. 20% France 12% Switzerland 30% U.S. 15% Italy 10% China 0.1% Spain 13% Libya 12% Saudi India 0.4% Arabia 37% U.A.E. 88% French Guiana 40% Equatorial Guinea 18% Australia 29% Lowest-income countries are shown with a stripe pattern. Share of population who are immigrants 0 10% 20% 5% 15% 40% 100% Russia 8% Canada 22% U.K. 13% Germany 15% Kazakh. 20% France 12% Switz. 30% Italy 10% U.S. 15% Spain 13% China 0.1% Libya 12% Saudi Arabia India 0.4% 37% U.A.E. 88% French Guiana 40% Equatorial Guinea 18% Lowest-income countries are shown with a stripe pattern. Australia 29%

Donald J. Trump’s promise to build a wall to keep Mexicans and Central Americans from crossing the United States’ southern border was central to his successful campaign for the presidency. Antipathy toward immigrants is spreading through Europe, fueling Britons’ desire to leave the European Union, upending Italy’s political establishment and giving the populist Hungarian government of Viktor Orban a fourth term.

Fear of immigrants takes different forms. Immigration from the Middle East and North Africa has led to calls in Europe to prevent its so-called Islamization. In the United States, despite a long history of cultural, religious and ethnic mixing, several studies have concluded that alongside their anger over lost jobs and stagnant wages, many of the non-Hispanic white voters who tipped the presidency to Mr. Trump were motivated by fears that they were losing demographic ground to other groups.

While it is far from a consensus, on both sides of the Atlantic the proposition that immigration amounts to a large-scale threat is gaining ground on the right of the political spectrum.

People perceive there are more immigrants than there really are

A study based on surveys in the United States and a variety of European countries by the economists Alberto Alesina, Armando Miano and Stefanie Stantcheva found that people across the board vastly overstate their immigrant populations.

Actual Perception of share of population who are immigrants share of population who are immigrants United States Avg. in each country Britain Germany France Sweden Italy 0 5% 10% 15% 20% 25% 30% 35% 40% Actual Perception of share of population who are immigrants share of population who are immigrants United States Average in each country Britain Germany France Sweden Italy 0 5% 10% 15% 20% 25% 30% 35% 40% Note: For the United States, the number of actual immigrants includes estimates of the illegal immigrant population | Source: “Immigration and Redistribution,” 2018. Alberto Alesina, Armando Miano and Stefanie Stantcheva (Harvard University)

The overestimates are largest among particular groups: the least educated, workers in low-skill occupations with lots of immigrants, and those on the political right. They overstate the share of immigrants who are Muslim and understate the share of Christians. They underestimate immigrants’ education and overestimate both their poverty rate and their dependence on welfare. Almost a quarter of French respondents, as well as nearly one in five Swedes and about one in seven Americans, think the average immigrant gets twice as much government aid as native residents do. In no country is this true.

Actual Perception of share of immigrants who are unemployed share of immigrants who are unemployed Italy Germany France Sweden Britain United States 0 10% 20% 30% 40% 50% Actual Perception of share of immigrants who are unemployed share of immigrants who are unemployed Italy Germany France Sweden Britain United States 0 10% 20% 30% 40% 50%

“People who are against immigration generate a sense of crisis,” Professor Alesina said. “They create a sense that ‘This is a huge problem; we need a wall.’”

Share of respondents who think the average immigrant gets twice as much government aid as natives do France Sweden Italy United States Britain Germany 0 5% 10% 15% 20% 25% Share of respondents who think the average immigrant gets twice as much government aid as natives do France Sweden Italy United States Britain Germany 0 5% 10% 15% 20% 25%

In any event, the sentiment is eroding support for Europe’s social democratic model as well as for the United States’ more limited social safety net. “Just making people think about immigrants generates a strongly negative reaction in terms of redistribution,” Professors Alesina, Miano and Stantcheva write. This raises a fundamental question. If immigration from the South continues apace, will support for the liberal market democracies with robust social safety nets, which have prevailed in northern countries since the middle of the 20th century, hold in the 21st?

The flow of migration varies around the world

MIGRATION IN 1990 IN MILLIONS NORTH AMERICA EUROPE 28 1 6 1 within the continent 13 4 10 8 2 35 4 2 LATIN AMERICA ASIA 1 13 AFRICA OCEANIA MIGRATION IN 1990 IN MILLIONS 1 within the continent 28 EUROPE NORTH AMERICA 6 1 13 4 10 8 2 LATIN AMERICA ASIA 35 4 2 1 AFRICA OCEANIA 13 MIGRATION IN 1990 IN MILLIONS 1 within the continent 28 EUROPE NORTH AMERICA 6 1 13 4 10 8 2 LATIN AMERICA ASIA 35 4 2 1 AFRICA OCEANIA 13 MIGRATION IN 1990 IN MILLIONS 1.0 within the continent 27.9 NORTH AMERICA 6.2 EUROPE 1.1 13.4 4.4 10.0 8.1 1.8 LATIN AMERICA 4.0 35.4 ASIA 1.8 1.1 AFRICA OCEANIA 13.4 MIGRATION IN 1990 IN MILLIONS 1.0 within the continent 27.9 NORTH AMERICA 6.2 EUROPE 1.1 13.4 4.4 10.0 8.1 1.8 LATIN AMERICA 4.0 35.4 ASIA 1.8 1.1 AFRICA OCEANIA 13.4

Largest diaspora populations in 1990 in millions Russia Germany 1. 12.7 11. 3.3 Afghanistan Kazakhstan 2. 6.7 12. 3.0 India Turkey 3. 6.7 13. 2.5 Ukraine Mozambique 4. 5.5 14. 2.2 Bangladesh Philippines 5. 5.5 15. 2.0 Mexico Portugal 6. 4.4 16. 1.9 China Palestine 7. 4.2 17. 1.8 Britain Belarus 8. 3.8 18. 1.8 Italy United States 9. 3.4 19. 1.7 Pakistan Ethiopia 10. 3.3 20. 1.7 Largest diaspora populations in 1990 in millions Russia Mexico Germany Portugal 1. 12.7 6. 4.4 11. 3.3 16. 1.9 Afghanistan China Kazakhstan Palestine 2. 6.7 7. 4.2 12. 3.0 17. 1.8 India Britain Turkey Belarus 3. 6.7 8. 3.8 13. 2.5 18. 1.8 Ukraine Italy Mozambique United States 4. 5.5 9. 3.4 14. 2.2 19. 1.7 Bangladesh Pakistan Philippines Ethiopia 5. 5.5 10. 3.3 15. 2.0 20. 1.7 Largest diaspora populations in 1990 in millions Russia Mexico Germany Portugal Indonesia 1. 12.7 6. 4.4 11. 3.3 16. 1.9 21. 1.6 Afghanistan China Kazakhstan Palestine Azerbaijan 2. 6.7 7. 4.2 12. 3.0 17. 1.8 22. 1.6 India Britain Turkey Belarus South Korea 3. 6.7 8. 3.8 13. 2.5 18. 1.8 23. 1.6 Ukraine Italy Mozambique United States Morocco 4. 5.5 9. 3.4 14. 2.2 19. 1.7 24. 1.6 Bangladesh Pakistan Philippines Ethiopia Poland 5. 5.5 10. 3.3 15. 2.0 20. 1.7 25. 1.5 Largest diaspora populations in 1990 in millions Russia Mexico Germany Portugal Indonesia Iraq 1. 12.7 6. 4.4 11. 3.3 16. 1.9 21. 1.6 26. 1.5 Afghanistan China Kazakhstan Palestine Azerbaijan Spain 2. 6.7 7. 4.2 12. 3.0 17. 1.8 22. 1.6 27. 1.4 India Britain Turkey Belarus South Korea Uzbekistan 3. 6.7 8. 3.8 13. 2.5 18. 1.8 23. 1.6 28. 1.4 Ukraine Italy Mozambique United States Morocco Egypt 4. 5.5 9. 3.4 14. 2.2 19. 1.7 24. 1.6 29. 1.3 Bangladesh Pakistan Philippines Ethiopia Poland El Salvador 5. 5.5 10. 3.3 15. 2.0 20. 1.7 25. 1.5 30. 1.2 Largest diaspora populations in 1990 in millions Russia Mexico Germany Portugal Indonesia Iraq 1. 12.7 6. 4.4 11. 3.3 16. 1.9 21. 1.6 26. 1.5 Afghanistan China Kazakhstan Palestine Azerbaijan Spain 2. 6.7 7. 4.2 12. 3.0 17. 1.8 22. 1.6 27. 1.4 India Britain Turkey Belarus South Korea Uzbekistan 3. 6.7 8. 3.8 13. 2.5 18. 1.8 23. 1.6 28. 1.4 Ukraine Italy Mozambique United States Morocco Egypt 4. 5.5 9. 3.4 14. 2.2 19. 1.7 24. 1.6 29. 1.3 Bangladesh Pakistan Philippines Ethiopia Poland El Salvador 5. 5.5 10. 3.3 15. 2.0 20. 1.7 25. 1.5 30. 1.2 Note: Immigration flows from unknown origins not shown. | Source: United Nations Department of Economic and Social Affairs, Population Division

Immigration is unlikely to slow down. All over the world, migration has grown sharply over the last quarter-century. In 1990, fewer than seven million Indians lived abroad, according to calculations from the United Nations. By last year, nearly 17 million lived outside of India. The Mexican diaspora increased to 13 million from 4.4 million over the period. China’s rose to 10 million from 4.2 million.

Most migrants from poor countries never make it to the United States or Western Europe, instead moving to other developing countries nearby. A little over half of emigrants from Africa settle in other African countries, while 60 percent of Asian migrants relocate elsewhere in Asia.

Migration increased the most from Latin America

MIGRATION IN 2017 IN MILLIONS NORTH AMERICA EUROPE 8 41 1 1 17 within the continent 21 26 1 5 7 9 3 6 63 3 LATIN AMERICA ASIA 4 3 1 19 1 AFRICA OCEANIA MIGRATION IN 2017 IN MILLIONS 8 41 1 1 NORTH AMERICA EUROPE 17 within the continent 21 26 1 5 7 9 3 LATIN AMERICA 6 63 ASIA 3 4 3 1 AFRICA OCEANIA 19 1 MIGRATION IN 2017 IN MILLIONS 1.2 within the continent 7.6 41.0 1.0 17.2 NORTH AMERICA EUROPE 4.6 20.5 9.3 2.6 26.4 1.4 7.1 1.3 LATIN AMERICA 6.1 63.3 ASIA 4.4 3.1 1.0 3.2 OCEANIA AFRICA 19.4 1.1 MIGRATION IN 2017 IN MILLIONS 1.2 within the continent 7.6 41.0 1.0 17.2 NORTH AMERICA EUROPE 4.6 20.5 9.3 2.6 26.4 1.4 7.1 1.3 LATIN AMERICA 63.3 6.1 ASIA 4.4 3.1 1.0 3.2 OCEANIA AFRICA 19.4 1.1 MIGRATION IN 2017 IN MILLIONS 1.2 within the continent 7.6 41.0 1.0 17.2 NORTH AMERICA EUROPE 4.6 20.5 9.3 2.6 26.4 1.4 7.1 1.3 LATIN AMERICA 6.1 63.3 ASIA 4.4 3.1 1.0 3.2 OCEANIA AFRICA 19.4 1.1

Largest diaspora populations in 2017 in millions India Afghanistan 1. 16.6 11. 4.8 Mexico Poland 2. 13.0 12. 4.7 Russia Indonesia 3. 10.6 13. 4.2 China Germany 4. 10.0 14. 4.2 Bangladesh Kazakhstan 5. 7.5 15. 4.1 Syria Palestine 6. 6.9 16. 3.8 Pakistan Romania 7. 6.0 17. 3.6 Ukraine Turkey 8. 5.9 18. 3.4 Philippines Egypt 9. 5.7 19. 3.4 Britain Italy 10. 4.9 20. 3.0 Largest diaspora populations in 2017 in millions India Syria Afghanistan Palestine 1. 16.6 6. 6.9 11. 4.8 16. 3.8 Mexico Pakistan Poland Romania 2. 13.0 7. 6.0 12. 4.7 17. 3.6 Russia Ukraine Indonesia Turkey 3. 10.6 8. 5.9 13. 4.2 18. 3.4 China Philippines Germany Egypt 4. 10.0 9. 5.7 14. 4.2 19. 3.4 Bangladesh Britain Kazakhstan Italy 5. 7.5 10. 4.9 15. 4.1 20. 3.0 Largest diaspora populations in 2017 in millions India Syria Afghanistan Palestine United States 1. 16.6 6. 6.9 11. 4.8 16. 3.8 21. 3.0 Mexico Pakistan Poland Romania Morocco 2. 13.0 7. 6.0 12. 4.7 17. 3.6 22. 2.9 Russia Ukraine Indonesia Turkey Myanmar 3. 10.6 8. 5.9 13. 4.2 18. 3.4 23. 2.9 China Philippines Germany Egypt Colombia 4. 10.0 9. 5.7 14. 4.2 19. 3.4 24. 2.7 Bangladesh Britain Kazakhstan Italy Vietnam 5. 7.5 10. 4.9 15. 4.1 20. 3.0 25. 2.7 Largest diaspora populations in 2017 in millions India Syria Afghanistan Palestine United States South Korea 2.5 1. 16.6 6. 6.9 11. 4.8 16. 3.8 21. 3.0 26. Mexico Pakistan Poland Romania Morocco Portugal 2. 13.0 7. 6.0 12. 4.7 17. 3.6 22. 2.9 27. 2.3 Russia Ukraine Indonesia Turkey Myanmar France 3. 10.6 8. 5.9 13. 4.2 18. 3.4 23. 2.9 28. 2.2 China Philippines Germany Egypt Colombia Uzbekistan 4. 10.0 9. 5.7 14. 4.2 19. 3.4 24. 2.7 29. 2.0 Bangladesh Britain Kazakhstan Italy Vietnam Somalia 5. 7.5 10. 4.9 15. 4.1 20. 3.0 25. 2.7 30. 2.0 Largest diaspora populations in 2017 in millions India Syria Afghanistan Palestine United States South Korea 2.5 1. 16.6 6. 6.9 11. 4.8 16. 3.8 21. 3.0 26. Mexico Pakistan Poland Romania Morocco Portugal 2. 13.0 7. 6.0 12. 4.7 17. 3.6 22. 2.9 27. 2.3 Russia Ukraine Indonesia Turkey Myanmar France 3. 10.6 8. 5.9 13. 4.2 18. 3.4 23. 2.9 28. 2.2 China Philippines Germany Egypt Colombia Uzbekistan 4. 10.0 9. 5.7 14. 4.2 19. 3.4 24. 2.7 29. 2.0 Bangladesh Britain Kazakhstan Italy Vietnam Somalia 5. 7.5 10. 4.9 15. 4.1 20. 3.0 25. 2.7 30. 2.0 Note: Immigration flows from unknown origins not shown. | Source: United Nations Department of Economic and Social Affairs, Population Division

Immigrant populations have risen sharply in most advanced nations

11.6% More developed regions 7.2% Share of population who are immigrants World avg. 3.4% 2.9% 1.8% 1.7% Less developed regions ’90 ’95 ’00 ’05 ’10 ’17 11.6% More developed regions 7.2% Share of population who are immigrants 3.4% World avg. 2.9% Less developed regions 1.8% 1.7% ’90 ’95 ’00 ’05 ’10 ’17 Source: United Nations Department of Economic and Social Affairs, Population Division

The economic pressures pushing migrants from their homes are unlikely to abate soon. But the patterns of migration from the poor South of people seeking a better life in the rich North are likely to change.

Economic and other forces are driving immigration

While instability in Central America continues to drive people north, the vast traffic of low-skilled immigrants into the United States across the southern border has slowed. The demographic bulge of Mexican men in their teens and early 20s who flocked illegally to the United States from the 1990s until the Great Recession has petered out, in part a consequence of declining Mexican fertility since the 1970s. Today, Mexicans are older, on average. Fewer are willing to take the risk. And Mexico’s economy is in better shape than in the 1980s and 1990s, when repeated crises drove many Mexicans from their homes.

There were fewer undocumented immigrants living in the United States in 2016 than in 2007. Apprehensions along the border with Mexico plummeted last year to their lowest level since 1971. Things may change if, say, Mr. Trump decides to leave the North American Free Trade Agreement, setting off another economic crisis in Mexico. Still, the United States seems more likely to suffer from a lack of immigrants than from a continued surge.

30 % Australia Switzerland 25 Israel New Zealand 20 Canada Ireland Foreign- born share of population Estonia Austria Belgium Spain Sweden Latvia United States Germany France Britain Norway Netherlands Slovenia 10 Italy Denmark Russia Portugal Greece Czech Republic Lithuania 5 Hungary Finland Chile Japan Korea Mexico Poland 0 $10,000 20,000 30,000 40,000 G.D.P. per person 30 % Australia Switzerland 25 Israel New Zealand 20 Canada Foreign-born share of population Ireland Estonia Austria Belgium 15 Spain Sweden Latvia United States Germany France Britain Norway Netherlands Slovenia 10 Italy Denmark Russia Portugal Greece Czech Republic Lithuania 5 Hungary Finland Chile Japan Korea Bulgaria Mexico Slovak Republic Romania Poland 0 $5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 G.D.P. per person 30 % Australia Switzerland 25 Israel New Zealand 20 Canada Foreign-born share of population Ireland Estonia Austria Belgium 15 Spain Sweden Latvia United States Germany France Britain Norway Netherlands Slovenia 10 Italy Denmark Russia Portugal Greece Czech Republic Lithuania 5 Hungary Finland Chile Japan Korea Bulgaria Mexico Slovak Republic Romania Poland 0 $5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 G.D.P. per person 30 % Australia Switzerland 25 Israel New Zealand Canada 20 Ireland Estonia Austria Belgium 15 Spain Sweden Latvia United States Foreign-born share of population Germany France Britain Norway Netherlands Slovenia 10 Italy Denmark Russia Portugal Greece Czech Republic Lithuania 5 Hungary Finland Chile Japan Korea Bulgaria Mexico Slovak Republic Romania Poland 0 $5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 G.D.P. per person Source: Gordon Hanson and Craig McIntosh (University of California, San Diego)

Although immigration into the United States might have passed its high-water mark, other parts of the rich world — Europe, notably — are likely to experience more immigration than they have before.

Consider Africa. As Gordon Hanson and Craig McIntosh of the University of California, San Diego, have noted, immigration across the Mediterranean may soon come to look like the vast flows of people who in the 1990s streamed across the Rio Grande.

The number of migrants from sub-Saharan Africa jumped 25 percent over the first decade of this century and surged 31 percent from 2010 to 2017, according to the Pew Research Center. Persistently high fertility rates across Africa have produced a demographic bulge of young people eager to make a better life across the Mediterranean.

Demand for immigrant labor will probably rise in Europe as its population ages. The number of working-age people is already shrinking in many countries. What’s more, migration from many poor African countries is likely to keep rising even as their economies develop: They will remain poor enough for many of their people to crave a better life elsewhere but will become rich enough for more of them to afford the journey.

Migration peaks in lower-income countries

Research by Michael Clemens of the Center for Global Development finds that incomes per capita in the countries with the largest diaspora populations range roughly from $7,000 to $20,000. Some big African countries -- like Nigeria -- have entered that range.

0.15 migrants per national population 2010 0.12 2000 1990 0.09 More migrants 0.06 0.03 0 $500 $5,000 $50,000 Wealthier countries 0.15 migrants per national population 2010 0.12 2000 1990 0.09 More migrants 0.06 0.03 0 $500 $5,000 $50,000 Wealthier countries Notes: Emigrant stocks are the number of people born in each country residing outside that country, divided by national population. Wealth is measured in G.D.P. per person, in 2005 purchasing-power parity U.S. dollars, plotted logarithmically. | Source: Michael A. Clemens (Center for Global Development)

Then there is the wild card, which could well intensify patterns of migration everywhere: climate change.

Global warming is driving migration

Rising average temperatures are already pushing people from their homes in many middle-income countries, according to research by Cristina Cattaneo and Giovanni Peri, increasing migration from rural areas to urban centers and across borders to other nations. As warming continues in the coming decades, it will probably push people from agricultural areas to urban areas and from the global South to the richer global North.

+0.06 El Salvador Trinidad & Tobago Fiji Cape Verde Nicaragua +0.04 Sao Tome and Principe Romania Comoros Guatemala Dominican Republic Bulgaria +0.02 Haiti Poland Honduras Swaziland Ecuador Sierra Leone TRENDLINE Ivory Coast Peru Bolivia Philippines Iraq South Africa Thailand Mauritania Indonesia Sri Lanka Change in migration Pakistan Solomon Islands India Egypt Mongolia Zimbabwe Colombia Iran Venezuela Papua New Guinea China Syria Senegal Angola Cameroon Congo Costa Rica Saudi Arabia Lebanon Vanuatu Kenya Morocco Uruguay Guinea Djibouti Bhutan Paraguay Botswana Samoa Panama –0.02 Turkey Tunisia Bangladesh Algeria –0.04 Logarithmic scales Belize –0.06 –0.2 0 +0.2 +0.4 +0.6 +0.8 +1.0 +1.2 Change in average temperature St.Vincent & Grenadines +0.08 +0.06 El Salvador Trinidad & Tobago Fiji Cape Verde Nicaragua +0.04 Sao Tome and Principe Romania Comoros Guatemala Dominican Republic Bulgaria +0.02 Haiti Poland Honduras TRENDLINE Swaziland Ecuador Sierra Leone Ivory Coast Peru Bolivia Philippines Iraq South Africa Thailand Mauritania Change in migration Indonesia Sri Lanka Pakistan Solomon Islands India Egypt Mongolia 0 Zimbabwe Colombia Iran Venezuela Papua New Guinea China Syria Senegal Angola Cameroon Congo Costa Rica Saudi Arabia Lebanon Vanuatu Kenya Morocco Uruguay Guinea Djibouti Bhutan Paraguay Botswana Samoa Panama –0.02 Turkey Tunisia Bangladesh Algeria –0.04 Logarithmic scales Belize –0.06 –0.2 0 +0.2 +0.4 +0.6 +0.8 +1.0 +1.2 Change in average temperature St.Vincent & Grenadines +0.08 +0.06 El Salvador Trinidad & Tobago Fiji Cape Verde Nicaragua +0.04 Sao Tome and Principe Romania Comoros Guatemala Dominican Republic Bulgaria +0.02 Haiti Poland Honduras TRENDLINE Swaziland Ecuador Sierra Leone Ivory Coast Peru Bolivia Philippines Iraq South Africa Thailand Mauritania Change in migration Indonesia Sri Lanka Pakistan Solomon Islands India Egypt Mongolia 0 Zimbabwe Colombia Iran Venezuela Papua New Guinea China Syria Senegal Angola Cameroon Congo Costa Rica Saudi Arabia Lebanon Vanuatu Kenya Morocco Uruguay Guinea Djibouti Bhutan Paraguay Botswana Samoa Panama –0.02 Turkey Tunisia Bangladesh Algeria –0.04 Logarithmic scales Belize –0.06 –0.2 0 +0.2 +0.4 +0.6 +0.8 +1.0 +1.2 Change in average temperature St.Vincent & Grenadines +0.08 +0.06 El Salvador Trinidad & Tobago Fiji Cape Verde Nicaragua +0.04 Sao Tome and Principe Romania Comoros Guatemala Dominican Republic Bulgaria +0.02 Haiti Poland Honduras TRENDLINE Swaziland Ecuador Sierra Leone Ivory Coast Peru Bolivia Philippines Iraq South Africa Thailand Mauritania Change in migration Indonesia Sri Lanka Pakistan Solomon Islands India Egypt Mongolia 0 Zimbabwe Colombia Iran Venezuela Papua New Guinea China Syria Senegal Angola Cameroon Congo Costa Rica Saudi Arabia Lebanon Vanuatu Kenya Morocco Uruguay Guinea Djibouti Bhutan Paraguay Botswana Samoa Panama –0.02 Turkey Tunisia Bangladesh Algeria –0.04 Logarithmic scales Belize –0.06 –0.2 0 +0.2 +0.4 +0.6 +0.8 +1.0 +1.2 Change in average temperature Notes: The horizontal axis records the natural logarithm of the average temperatures between 2000 and 1981 minus the natural logarithm of the average temperatures between 1960 and 1980. The vertical axis records the difference between the natural logarithms of the average emigration rates between 1990 and 2000 and the emigration rates between 1970 and 1980. | Source: Cristina Cattaneo (Fondazione Eni Enrico Mattei (FEEM)) and Giovanni Peri (University of California, Davis)

How will the North’s political systems respond to the challenge? Alongside studies suggesting that ethnic diversity can reduce trust and support for social insurance, there is a body of scholarship suggesting that direct contact between people of different ethnicities, nationalities and cultures can breed trust: It’s easier to fear an abstract immigrant you have never seen than one who lives down the block, sends children to the same school as yours and shops at the same store.

The research by Professors Stantcheva, Miano and Alesina suggests that Americans who know an immigrant have more positive perceptions about immigrants’ work ethic, education, dependence on welfare and other behavior. Across the countries in their study, people exposed to positive images of immigrants -- say, about their strong commitment to work -- become much less negative in general about immigration.

Natives’ views can also be manipulated in a negative direction, though, something currently reflected in the politics of many countries. Professor Stantcheva argues that negative portrayals of immigrants in the media could help explain the biased and erroneous views about immigrants’ behavior. And as Professor Alesina notes, “Anti-immigration parties foster these misperceptions in a variety of ways strategically to gain support for their anti-immigration stands.”

But there are already plenty of walls, and they have done little to stop immigration. If rich countries want fewer immigrants, their best shot might be to help poor countries become rich, so that fewer people feel the urge to leave. That would include helping them adapt to climate change, and simply opening up their own markets to developing countries’ exports. “If you want to have fewer immigrants, you would want poorer countries to take advantage of trade,” Professor Alesina said. “The idea that because there is too much immigration you should restrict trade makes no sense.”

What’s more, as Mr. Clemens argues, rich countries should probably start writing new rules and creating new institutions to manage the large immigration flows of the future. They could work to promote new destinations and develop mutually beneficial forms of migration (say, varieties of temporary work visas). They could establish mechanisms to assist vulnerable native-born people, whose jobs might be at stake.