Legal immigrants in the United States have opted to become American citizens in historically high numbers in the last decade, according to a study published yesterday by the Pew Hispanic Center.

The number of naturalized citizens in the United States population in 2005 was 12.8 million, a record high, the study found. The number reflected not only a growing number of legal immigrants coming to the United States, but also a growing eagerness among them to adopt American citizenship, the study reported.

Of all the immigrants now living legally in the United States, 52 percent are naturalized citizens, while others are permanent residents or have some other immigration status. The last time that citizens made up more than half of all legal immigrants was in 1980, the study found.

The research, which is based primarily on census data, was done by Jeffrey S. Passel, a demographer at the Pew center, a nonpartisan organization based in Washington.

“It is clear that today’s legal immigrants are signing on to a closer relationship with the U.S. than was the case a decade or two ago,” Mr. Passel concluded.

For the first time, European immigrants are no longer the largest group of those who choose to become citizens, according to the study. Over the past decade, they were outnumbered by new citizens who came originally from Latin America or Asia. In the decade before 2005 (the last date when figures were available), nearly 2.4 million immigrants from Latin America became citizens, more than from any other region, the study found.

The willingness of recent immigrants to integrate into American society has been hotly debated as Congress has grappled with changes to the immigration system. To become citizens, legal immigrants must be over 18 and show they have lived continuously in the United States for five years. They have to pass background and health checks, as well as tests to show that they speak basic English and have a minimum knowledge of American laws and history.

There is no limit on the number of eligible immigrants who can become citizens, gaining the right to vote and ensuring they can never be deported. Illegal immigrants, estimated to number about 12 million people, are not eligible to apply for citizenship.

While the number of legal immigrants in the United States increased by 30 percent in the decade after 1995, the study found, the number of naturalized American citizens climbed by 73 percent.

“Ultimately people do want to become full members of the society, and these numbers show that it is happening,” said Doris Meissner, a senior fellow at the Migration Policy Institute, a nonpartisan center in Washington. Ms. Meissner was commissioner of what was then the Immigration and Naturalization Service for seven years in the Clinton administration.

Mexicans showed the biggest increase in naturalizations over the decade, with the number of Mexican immigrants who became citizens increasing by 144 percent. But Mexicans are still less likely to become citizens than many other immigrant groups, the study found.

The report does not explain why naturalization rates have increased. Ms. Meissner said the intensifying political debate, in which many groups have called for deporting illegal immigrants, was a factor. “We know that when immigrants feel threatened, and they feel they might be in danger of losing rights of one kind or another, they are more likely to pursue citizenship,” she said.