The USA's growing diversity is cooling the melting pot in at least one way: marriage across racial and ethnic lines.

The share of Hispanics and Asians married to whites dropped between 1990 and 2000 after two decades of steady growth, new research shows.

The sheer number of immigrants who arrived last decade has made it more likely for them to marry among themselves, according to findings published this month in American Sociological Review.

Recent minority immigrants are disproportionately less educated than the native-born, earn less, do not speak English well and tend to live in segregated neighborhoods - major factors that discourage marriage outside racial or ethnic groups, says Zhenchao Qian, sociology professor at Ohio State University and co-author of the report. The researchers studied married couples between the ages of 20 to 34.

The 2000 Census is the most recent data that include enough cases to examine intermarriages at the national level, he says. "That kind of trend is likely to continue because immigrants still are coming in," Qian says. "We see immigrants as more likely to marry among themselves."

In contrast with the decline in intermarriages among whites and Asians and Hispanics, the rate of marriages between blacks and whites rose significantly from 1990 to 2000, although it continues to lag far behind those of other minorities.