In China in 2005, 118 boys were born for every 100 girls born. This ratio is far above the normal biological ratio of about 106 boys to 100 girls. The sexual disparity in China has resulted from a combination of low birth rates, a preference in China for boys when parents only have one or two children, and the spread of ultrasound techniques in that country that allow the sex of fetuses to be identified and then aborted if parents do not like the sex. Similar trends have emerged in India and South Korea as well. More sophisticated and expensive methods permit parents to raise their chances of a male baby even before a woman becomes pregnant. Considered most reliable is a method that involves in vitro fertilization, drugs to stimulate the mother‚Äôs ovaries, surgery, and other steps. The total cost can exceed $20,000, so this method clearly is only available to richer persons. Are there good reasons to object to sex selection, either by abortion or more sophisticated methods? On Feb. 1 the Committee on Ethics of the American College of Obstetricians and Gynecologists (the ACOG) did issue an opinion objecting on the grounds that it is unethical for physicians to participate in sex selection by parents that was based not on potential for sex-linked genetic disorders, but solely on family balancing of personal preferences. This opinion about the ethics of sexual selection applied "regardless of the timing of the selection (i.e., preconception or post conception) or the stage of development of the embryo or fetus". Such an opinion seems strange in light of the general support by physicians and the Supreme Court of abortions by parents "solely" to satisfy their personal preferences about timing or number of children. What is so different about sex-selected abortions that would lead the ACOG with its over 51,000 members who provide health care to women to oppose abortions to satisfy parental desires for additional boys or girls while supporting the general right to abortion? The ACOG tries to provide an answer by claiming that sex selection through any method may "ultimately support sexist practices." It is not clear what the ACOG means by sexist practices, but all the evidence on sexual preferences in the United States and other richer countries indicates an overwhelming desire for variety-boys and girls- rather than a strong preference for either sex. So sex-selected abortions in these countries is unlikely to have much of an effect on the overall sex ratio, although it would affect the distribution of boys and girls in different families. I concentrate my remaining discussion on the implications of sex-selected abortions in countries where it raises the number of boys relative to girls. China, South Korea, and other countries have tried to implement control over sex selection by making it illegal to use ultrasound techniques to select the sex of children. However, these regulations are notoriously difficult to implement since doctors may say "congratulations" when an ultrasound test reveals a boy, and remain silent when the fetus is a girl. Abortions of girl fetuses would reduce average family size if parents who prefer boys would end up with larger families than they would like because they cannot control the sex of their offspring. The effect on family size could go the other way, however, if the fear of having girls discourages parents from having additional children. These effects on family size could be important, but I ignore them in the following discussion and concentrate on the effects of a lower number of girl babies relative to boys compared to the biological natural girl-boy ratio of a little below 50-50. One might expect parents who abort fetuses of sexes they do not want to treat their children better than they would otherwise since they now are satisfied with the sexes of their children. In such cases, sex-selected abortions against girls would improve rather than worsen the average treatment of girls since parents would be happier with the girls they have than if they had girls who were not really wanted. It is no surprise, for example, that orphanages in China predominantly have girls (and some handicapped boys), given the preference for boys in the traditional Chinese culture. What about the overall effects in a society of skewing the sex ratio of births toward boys? The fewer girls who are born presumably would be better off since they would be better educated, and in other ways better treated by parents who want them. This would be reinforced if the effect of sex selected abortions is to lower the overall birth rate since it is well established that families with fewer children invest more in each one, girls as well as boys. As children become adults in cohorts with a high ratio of boys, the advantage of girls and women increases since they are scarcer. It is claimed that young women in China are already at a premium as potential mates because strong sex-selection has been going on ever since the one child policy was introduced in the early 1980's. Prior to the spread of ultrasound techniques, sex selection occurred through sending girls to orphanages, neglect, and in some case even engaging in female infanticide. To be sure, if the value of girls as wives and girlfriends, and in other ways, rises because they are scarcer, then the value of boys as husbands and boyfriends tends to fall. However, it is not apparent why that should call for policies that prevent sex-selected techniques, unless the interests of men were motivating these policies. To use an analogy, a shift of demand in an economy toward services and away from manufacturing because of a shift in "preferences" toward services- as has occurred in the United States and other rich countries- benefits women relative to men since women are more likely to work in services than are men. Yet no one would claim that society should prevent such preferences because they help (indirectly) one sex over another. The great statistician and biologist, R. A. Fisher, used a celebrated biological analysis to explain why the sex ratio remains close to 50-50 in non-human species. An economic analysis based on incentives gives results that are related to Fisher's result. An improvement in the position of women due to a decline in the number of girls relative to boys leads to some correction in the sex ratio as parental choices respond in the long run to the more favorable position of girls. If women are in greater demand as wives and in the economy when they are in scarcer supply, some parents will decide that having girls has advantages, possibly through receiving generous bride prices when daughters marry. This would shift "preferences" toward having girls. The long run outcome would not necessarily be the biological natural ratio of a little more boy births than girl births, but it should be closer to that ratio than the current ratios in some Asian countries.