In 2012, some 4.9 million Puerto Ricans lived in one of the 50 US states or the US capital, while the island had a population of 3.5 million, according to the study out this week.
Furthermore, Puerto Rico lost another 144,000 people between 2010 and 2013.
"We are seeing the biggest outmigration from Puerto Rico to the US that we've seen since at least the 1950s," during the US economic boom, said study author Mark Hugo Lopez.
"It looks like the island is on a long run cycle of population decline."
- Unprecedented population drop -
The United States seized Puerto Rico from Spain in a 1898 war.
Residents of the Caribbean island are US citizens, serve in the military and have US passports, but cannot vote in US presidential elections.
The self-governing territory's sole representative in the US Congress is a non-voting delegate in the House of Representatives. However, island residents pay no federal income tax.
Two parties dominate the political scene -- one favoring statehood, and the other seeking the island's commonwealth association with the United States.
A third, smaller party favors independence.
The two main parties have almost equal representation in the legislature, and have clashed fiercely over how to improve the anemic economy.
The massive outflow of population coincided with an economic crisis that began in 2006, when a federal tax exemption expired on local manufacturing, including food products, chemicals and machinery.
Unemployment jumped to 13.1 percent in June of that year on the island, according to official figures, more than double the 6.1 percent at the same time on the US mainland.
The exodus "has intensified," said Jorge Duany, a Florida International University professor and expert on Puerto Rican migration.
Between 2000 and 2010, there was an unprecedented 2.2 percent population drop after nearly a century of growth, Duany told AFP.
Forty-two percent of those who leave Puerto Rico say they are looking for a job, according to Pew Hispanic.
Census figures suggest that the depopulation trend will continue at least until mid-century.
As a result, the island's population is aging, decreasing the productive population and forcing an ever smaller number of working people to fund the tax and retirement system.
In 2010, the percentage of the population older than 65 in Puerto Rico was higher than the United States and every Latin American country except Cuba, Duany said.
Neither does the drop in the birth rate help. On average, there are 1.7 children per woman on the island, below the level of natural reproduction.
"At that level, it's impossible for the Puerto Rican population to grow," Duany said.
- Vicious cycle -
The hemorrhage not only includes professionals but also blue-collar workers and unskilled laborers, said Kurt Birson, researcher Center for Puerto Rican Studies at Hunter College in New York.
"They contribute to the decline of the economy," Birson said.
A report by the Institute of Statistics of Puerto Rico said that a quarter of the island's gross domestic product was lost between 2006 and 2011 because of the exodus.
"This is a vicious cycle," said Edwin Melendez, head of the Center of Puerto Rican Studies at Hunter College.
"If the population drops, there is less demand, and if there is less demand, there is less economic activity, and less work, and people leave."
Melendez suggested the situation would "eventually" get resolved, but when that would happen and how many people would remain is unclear.
The population movement has also changed parts of the United States, especially Orlando, Florida, which has become the main destination for Puerto Ricans.
Orlando, home to major amusement parks, is expected by the end of the decade to surpass New York as being the home to the largest concentration of Puerto Ricans on the mainland