A new study shows that more and more black men are marrying women of other races. In fact, more than 1 in 5 black men who wed (22 percent) married a nonblack woman in 2008. This compares with about 9 percent of black women, and represents a significant increase for black men — from 15.7 percent in 2000 and 7.9 percent in 1980.

Sociologists said the rate of black men marrying women of other races further reduces the already-shrunken pool of potential partners for black women seeking a black husband.

“When you add in the prison population,” said Prof. Steven Ruggles, director of the Minnesota Population Center, “it pretty well explains the extraordinarily low marriage rates of black women.”

Among all married African-Americans in 2008, 13 percent of men and 6 percent of women had a nonblack spouse. This compares with nearly half of American-born Asians choosing non-Asian spouses.

“The continuing imbalance in the rates for black men and black women could be making it even harder for black women to find a husband,” said Prof. Andrew J. Cherlin, director of the population center at Johns Hopkins University.

The study, to be released Friday by the Pew Research Center, found that intermarriage among Asian, black, Hispanic and white people now accounts for a record 1 in 6 new marriages in the United States. Tellingly, blacks and whites remain the least-common variety of interracial pairing. Still, black-white unions make up 1 in 60 new marriages today, compared with fewer than 1 in 1,000 back when Barack Obama’s parents wed a half-century ago.

While the increased rate of intermarriage reflects demographic changes in the American population — a more diverse pool of available spouses — as well as changing social mores, they may presage a redefinition of America’s evolving concepts of race and ethnicity.

“The lines dividing these groups are getting blurrier and blurrier,” said Jeffrey S. Passel, an author of the Pew analysis.

For instance, of the 2.7 million American children with a black parent, about 10 percent also have one nonblack parent today. Because many mixed-race African- Americans still choose to identify as being black — as Mr. Obama did when he filled out the 2010 census — the number of multiracial African-Americans could actually be higher.

How children of the expanding share of mixed marriages identify themselves — and how they are identified by the rest of society — could blur a benchmark that the nation will approach within a few decades when American Indian, Asian, black and Hispanic Americans and people of mixed race become a majority of the population.

More precise estimates of the number of people who identify themselves as mixed race will be available from the 2010 census. Other census estimates found a 32 percent increase in the mixed-race population (to 5.2 million, from 3.9 million) from 2000 to 2008.

Still, the “blending” of America could be overstated, especially given the relatively low rate of black-white intermarriage compared with other groups, and continuing racial perceptions and divisions, according to some sociologists.

“Children of white-Asian and white-Hispanic parents will have no problems calling themselves white, if that’s their choice,” said Andrew Hacker, a political scientist at Queens College of the City University of New York and the author of a book about race.

“But offspring of black and another ethnic parent won’t have that option,” Professor Hacker said. “They’ll be black because that’s the way they’re seen. Barack Obama, Tiger Woods, Halle Berry, have all known that. Will that change? Don’t hold your breath.”

Of all 3.8 million adults who married in 2008, 31 percent of Asians, 26 percent of Hispanic people, 16 percent of blacks and 9 percent of whites married a person whose race or ethnicity was different from their own. Those were all record highs.