THE calamity brought by Hurricane Maria to Puerto Rico in September led to predictions of an exodus. The island’s troubles have continued: 10% of Puerto Ricans still have no electricity and days ago power cuts in San Juan, the capital, left residents in and around the capital without power for days. The island’s government reckons that by the end of 2018, 200,000 people may have left for the mainland; surveys suggest that many of them will stay away for good.

This would be an acceleration of a trend. Since 2005, when Puerto Rico suffered an economic downturn, Puerto Ricans have been leaving for other parts of America. Between 2005 and 2013 Puerto Rico’s GDP per capita as a proportion of American GDP per capita fell from 59% to 52%. Over that same period, the island lost 5.5% of its population. Pew Research Centre estimates that nearly 500,000 people left over the past decade.

Before then, however, population flows from the island were viscous–although the economic incentives for leaving were high and the barriers low. And arguably they have remained surprisingly low.

Puerto Ricans have had American citizenship since 1917. In the years that have followed, they have remained much poorer than most Americans. In 2015, median household income in Puerto Rico was $18,626 compared to a median income in New York state of $73,854. That fourfold gap within American territory lies between the median household income gap between America as a whole and Brazil (4.4 fold) and Argentina (3.3 fold) as reported by the World Bank. Leaving, meanwhile, was made easier by the presence of Puerto Ricans on the mainland. Nonetheless, net migration was a minor factor in demographic change in Puerto Rico up until the 2005 recession on the island.

The European Union has seen a similar pattern: open borders did not mean an instant exodus from the poorer to richer countries, but an economic downturn did speed up the process. Data from the UN suggests Greece’s emigrant population in the continent of Europe as a whole is less than 4% of the population still in Greece, though it is one of the poorest countries in the European Union, and still in the throes of economic turmoil. For Italy, the proportion living in the rest of Europe is closer to 3% and for Spain it is under 2%. As with Puerto Rico, the global financial crisis did spark more movement. Germany, the strongest and largest economy in the EU, received gross migration flows from the rest of the European Union equal to 5% of its 2006 population between January of that year and September 2014 (the country’s overall population declined regardless).

Why then do people stay in a comparatively poor place when open borders allow them to move to a richer one? It seems that most will not move for money alone. Michèle Belot of Oxford University and Sjef Ederveen of the Ministry of Economic Affairs in the Netherlands looked at patterns of migration between OECD countries and found that “cultural barriers” mattered more than economic considerations. And analysis by Douglas Massey and Fernando Riosmena of Princeton suggests that income differences between America and the country from which a migrant comes plays less of a role in the decision to move than a recent economic downturn at home and having friends and family who have lived in the United States.

The viscosity of emigration flows should be a comfort to Donald Trump and his supporters worried that America will be flooded by migrants from the south. But the impact of crises suggests that if the president wants to reduce the number of economic migrants from Latin America and the Caribbean he should ensure that his administration responds rapidly and generously to downturns and natural disasters. Perhaps he could start at home—with Puerto Rico’s power grid.