Having sons is important to many Asian cultures, and now American families from those groups seem to be asserting the same preference. A new analysis of the 2000 Census shows that among U.S. born children of Chinese, Korean and Asian Indian parents the odds of having a boy increase if the family already has a girl or two.

The findings "suggest that in a sub-population with a traditional son preference, the technologies are being used to generate male births when preceding births are female," co-authors Douglas Almond and Lena Edlund said of their findings, appearing in Tuesday's edition of Proceedings of the National Academy of Sciences.

"We should emphasize that our paper does not imply that sex selection is practiced by all or even most Asian-Americans," they said in an e-mail response to questions. Most Chinese, Korean, and Asian-Indian parents do not sex select.

The discovery that some do, however, seems to be a new development in the United States, since the researchers didn't find the same variance in the 1990 census, Almond, of Columbia University and Edlund of the National Bureau of Economic Research in Cambridge, Mass., reported.

Edlund and Almond said they do not know what method is being used for sex selection, but they speculated that the most common is fetal ultrasound to determine the sex of the baby followed by disproportionate abortion of females. Ultrasound has improved in recent years and is being given earlier, they noted.

"Between 1989 and 1999, prenatal ultrasound use among non-Japanese Asian mothers rose from around 38 percent to 64 percent of pregnancies," they said, citing data from the U.S. National Center for Health Statistics.

The normal sex ratio at birth is 1.05 boys to 1 girl and that holds for first children of these families, the researchers found.

But if the first baby is a girl, the odds of a boy coming next rise to 1.17-to-1, and after two sisters the likelihood of having a son jumps to 1.51-to-1.

High sex ratios in Asia have received considerable attention and Almond and Edlund were curious whether the same could be observed in the United States.

Among the explanations in Asia is China's one-child policy, they noted. "For India, it is often claimed that dowries are necessary to marry off a daughter, while sons are money spinners who can get both a dowry and support the parents in old age," they said.

Carl Haub, a population researcher at the private Population Reference Bureau, said Indians he has studied have a high level of son preference.

"If you are a Hindu it is of great value to have a son officiate at your cremation," explained Haub, who was not part of the research team.

Haub said he was not surprised that some families from Korea, China and India brought their cultural and social values to the United States.

Phil Morgan of Duke University's Center for Social Demography and Ethnography, said he too was not surprised at the finding.

"We see this pattern strongly in places like South Korea. Why would it not show up here? I think that it is unlikely to persist in subsequent generations," added Morgan, who was not part of the research team.

The American College of Obstetricians and Gynecologists and the American Society of Reproductive Medicine say assisting patients in choosing the sex of their offspring to avoid serious sex-linked genetic disorders is ethical, but they discourage sex selection for personal and family reasons, such as family balancing.

Nevertheless, while many countries prohibit sex selection techniques without a medical purpose, the United States does not.

The research was funded by the Institute for Social and Economic Policy Research at Columbia University.