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.

"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. 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.