The preference for boys is deeply rooted in the social and economic constraints that almost all societies place on adult women. It is surely one of the cruelest forms of sex discrimination, both because its victims are infants and children and because it operates through the institution that is supposed to be their fundamental source of nurturance - the family.
Most governments are extremely reluctant to intervene in family affairs, and so have done little effectively to combat son preference. Some, however, have begun to attack the problem.
Discrimination within the family has serious practical consequences. At best, preferential treatment for sons often makes girls feel intrinsically inferior. At worst, it produces substantially higher death rates among female infants and children than among their brothers.
Probably a majority of the world's 800 million girls under the age of 15 feel some effect as families invest less in the education of their daughters, demand more work from them at earlier ages, and place stricter limits on their freedom of movement.
The problem is not that adults just naturally dislike little girls. Both instinct and a considerable amount of evidence point to the contrary.
The desire for sons is explained in practical terms. Boys are expected to contribute more to household income and to support their parents in old age. Generally, they are better equipped than their sisters to do so because women have fewer and lower-paying opportunities for employment.
In Bangladesh, to cite an extreme example, women make up less than 5 percent of the formal, paid labor force.
Cultural patterns often dictate that family names are passed on through the male line only, that girls move away from their families and home villages when they marry, and that certain religious obligations (such as Confucian rites for the ancestors) can be carried out only by males. The net result is that families pray for sons and do everything in their power to ensure their survival and success.
Daughters are more expendable. They may even be a liability. Discrimination against girls is most virulent where custom imposes real penalties on the parents of daughters, in the form of dowries or extravagant wedding expenses. ''A daugher lets you down twice,'' declares a Korean proverb, ''once when she is born and again when she marries.''
A society's high value placed on female chastity can also impose a burden on parents - often to the detriment of their daughters. Many girls are kept from going out to school or to work for fear that their reputations may be tarnished. Often they are married off as quickly as possible to relieve their families of the responsibility of supervision.
A survey of villagers in Bangladesh found that 29 percent of the respondents thought that girls should be married before the age of 13. Nine percent voted for marriage at 10 to 11 years of age.
The combination of a preference for sons and dire poverty is particularly devastating for girls, as economic destitution forces families to make grim choices among their children.
In Bangladesh again, the mortality rate for girls under the age of five is 30 to 50 percent higher than the rate for boys in the same age group. Surveys in India suggest strongly that the ratio of girls to boys tends to fall as one descends through the economic classes.
The natural ratio between the sexes at birth is about 105 boys to 100 girls. But the sex ratio described as ''ideal'' in most surveys and studies of parents' preferences is more heavily imbalanced in favor of boys.
At the extreme, in parts of North Africa and the Indian subcontinent, the average couple would choose to have anywhere from three to six sons for every daughter.
The facts about son preference and its consequences have become better known and better documented in recent years. The problem has been brought to the attention of policymakers in many countries.
Some have adopted official policies to discourage discrimination against girls, often spurred by the realization that the desire for sons (at least two, for safety's sake) is a major obstacle to the achievement of population stability.
The Chinese government has led the way with wide-ranging policies to combat son preference. Korea, Singapore, Indonesia, the Philippines, and others have conducted campaigns of exhortation to convince parents that a daughter is as good as a son.
Sexual equality has been stressed in textbooks and in government-sponsored media. These efforts fall short of a concerted assault on the economic, social, and cultural underpinnings of son preference, however - and nothing less can be expected to uproot such deepset prejudices.
Eliminate dire poverty, and the worst manifestations of son preference will also disappear. But the basic attitude may well persist. Even in Pakistan's least-poor region (Punjab), for example, four times as many boys as girls are enrolled in school.
And in middle-income Korea, two-thirds of the parents who have only daughters refuse to limit their families to the government's preferred two-child norm.
The preference for sons is a sensitive indicator of the overall position of women in a society. If their status improves - especially in the realm of economic opportunity - son preference tends to erode.