The Maria-fueled exodus is really an acceleration of a trend that's been underway for years. Puerto Ricans are US citizens, part of the US labor market, and can take jobs and move wherever they want. And due largely to Puerto Rico's economic crisis and persistently high unemployment, its residents have been leaving in large numbers in recent years.
Pew Research Center estimates that the Puerto Rican population peaked in 2004, and the territory lost 446,000 people between then and 2016, nearly 12 percent of the population. In 2015 alone, 89,000 Puerto Ricans left for the US mainland, and 64,238 of them have not returned. San Juan, the capital, lost about 10 percent of its population in the decade from 2005 to 2015.
The financial crisis and recession hit Puerto Rico slightly before the US as a whole, beginning with the end of tax breaks that Congress had offered manufacturers for locating in Puerto Rico. The breaks had allowed Puerto Rican subsidiaries of US companies to send earnings back to the mainland without paying federal corporate taxes. US politicians widely viewed this as an illegitimate corporate tax giveaway, and in 1996 Congress opted to phase it out gradually over 10 years. In 2006, the full brunt of the breaks' removal hit, and manufacturers, in particular pharmaceutical companies, began closing plants; an estimated 100,000 manufacturing jobs have been lost, with more indirect jobs going away as a result.
The result was a recession, worsened by the US mainland financial crisis and recession in 2007 and 2008.
Meanwhile, federal law had also caused an explosion in the territory's public debt. Puerto Rican debt is "triple exempt" from taxes: Bonds issued by the territory's government are exempt from state/territory-level, municipal, and federal taxation. There's other triple-exempt debt in the US too, but you generally have to live in the place in question to enjoy it; Californians buying California state debt, for instance, get a triple exemption. Puerto Rico's debt, however, is triple-exempt no matter who buys it. That naturally led a lot of people to want to buy it, giving the government a reason to take out lots of debt.
The sudden collapse of the territorial (and national) economy in the late 2000s made repaying that debt tough. That led to pressure for austerity policies that made the economic situation as experienced by ordinary Puerto Ricans even worse. Even now, after the continental US has largely recovered from the Great Recession, unemployment in Puerto Rico is above 10 percent; it peaked at 17 percent in 2010. The Census Bureau estimates the poverty rate at 43.5 percent. In those conditions, relocating to the mainland, which has always enjoyed far higher household incomes than Puerto Rico, starts to look very attractive.
"If you compare it to the historic periods where there's been the greatest levels of migration, like after WWII and in the '80s, the last 10 years are another one of those peaks," Elizabeth Aranda, a professor of sociology at the University of Southern Florida who studies Puerto Rican migrant communities, says. "It might even surpass migration in the 1980s."
Lyman Stone, an economist at the US Department of Agriculture who writes widely on migration issues, has used data on flights into and out of Puerto Rico to estimate migration levels in recent years; some of the flights are tourism, of course, but those are round trips, and the net direction of flights tells us a lot about where people are going.
In 2006 outflows began picking up, with a slight break as the recession made going to the US mainland less receptive. Over the past few years, the outflow has gotten really severe in scale.
Stone estimates that before Hurricane Maria, Puerto Rico was seeing -2 percent net annual migration: About 1 in every 50 people was leaving every year. The resulting fall in population looks not dissimilar to what happened to Ireland during the 1840s famine.
On the flip side, this has contributed to a marked increase in the size of the Puerto Rican diaspora on the American mainland. Meléndez and his colleague Carlos Vargas-Ramos at Hunter's Center for Puerto Rican Studies estimate that in 2003, equal numbers of Puerto Ricans lived in the territory and on the US mainland — about 3.8 million in each. By 2016, the population in the US mainland had grown to 5.4 million, and the population in Puerto Rico had shrunk to under 3.3 million. Today, more than 60 percent of Puerto Ricans live outside the territory.
Much of that increase has happened in New York and Florida, the two states with the largest Puerto Rican populations. The increase in Florida has been particularly striking, where the number of Puerto Ricans (about 1 million) is starting to rival the number of Cubans (1.4 million). But the population has become more dispersed as well. New Jersey, Pennsylvania, Connecticut, and Massachusetts all have sizable Puerto Rican populations, and Illinois, California, and Texas are seeing theirs increase. Indeed, from 2015 to 2016 the Puerto Rican population in Florida and New York actually shrank, despite the national population increasing by more than 70,000 people (the largest increase was in Illinois).