At least 10 million female foetuses have been aborted in India over the past two decades by middle-class families determined to ensure they have male heirs.
The figure is revealed by a survey of more than a million homes published today which found that sex determination in pregnancy and selective abortion accounted for 500,000 missing girls each year.
Termination of pregnancy on the basis of sex was made illegal in India in 1994, but better-off families find ways round the law. Many couples believe their family is unbalanced without a son who will continue the family name and bloodline, earn money, look after the family and take care of his parents in old age in a country which has no social security system.
Population censuses in India show that the number of girls has been falling steadily for the past 20 years relative to the number of boys. For every 1,000 boys up to the age of six the number of girls dropped from 962 in 1981 to 945 in 1991 to 927 in 2001.
Researchers say the most likely reason for the fall is the availability of ultrasound which allows parents to discover the gender of their child before birth and has been widespread in India for most of the past two decades.
Writing in The Lancet, which publishes the findings online today, Shirish Sheth of Breach Candy hospital, Mumbai, says: "To have a daughter is socially and emotionally accepted if there is a son, but a daughter's arrival is often unwelcome if the couple already have a daughter.
"Daughters are regarded as a liability. Because she will eventually belong to the family of her future husband, expenditure on her will benefit others. In some communities where the custom of dowry prevails, the cost of her dowry could be phenomenal."
Researchers from the University of Toronto in Canada and the Institute of Medical Education in Chandigarh, India, studied almost 134,000 births in 1997 among 6 million people living in 1.1 million households who are part of the ongoing Indian National Survey. They found the sex of the previous child born affected the sex ratio of the current birth, with fewer girls born to families who had yet to have a boy.
The effect was more than twice as great among educated mothers compared with those who were illiterate, but did not vary by religion.
Based on the natural sex ratio from other countries, the researchers estimated that 13.6 to 13.8 million girls should have been born in India in 1997 but the actual number was 13.1 million. The deficit amounts to between 590,000 and 740,000 female births.
Prabhat Jha and colleagues say: "We conservatively estimate that prenatal sex determination and selective abortion accounts for 500,000 missing girls yearly. If this practice has been common for most of the past two decades since access to ultrasound became widespread, then a figure of 10 million missing female births would not be unreasonable. Women who have already had one or two female children are clearly at highest risk."
The research team found that when the first birth was a girl, at the second birth there were 759 girls born to every 1,000 boys. At the third birth, the sex ratio declined further to 719 girls to every 1,000 boys when the first two births were girls. By contrast, when the first or second child was a boy, the number of girls born at second or subsequent births exceeded the number of boys.
The practice of female foeticide has taken the place of infanticide and is extensive in China as well as India, aided by the development of ultrasound. Dr Sheth says: "Female infanticide of the past is refined and honed to a fine skill in this modern guise. It is ushered in earlier, more in urban areas and by the more educated ... A careful demographic analysis of actual and expected sex ratios shows that about 100 million girls are missing from the world - they are dead."