WASHINGTON - 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.

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.