Researchers looked at a representative sample of about 6,500 households in five districts in states already known to have especially skewed sex ratios -- Punjab, Rajasthan, Haryana, Himachal Pradesh and Madhya Pradesh.

The sex ratio had dropped in four of the five districts, compared to 2001 census data, according to researchers working for ActionAid, an advocacy and aid organisation.

In one of the worst examples, researchers found only 300 girls for every 1,000 boys among upper-caste Hindus in urban areas of Punjab's Fatehgarh Sahib district.

"These sex ratios are disastrous," said Mary John, a researcher from the Centre for Women Development Studies in New Delhi, speaking at the release of preliminary data on Thursday.

They reflect the culmination of a trend towards ever smaller families, she said -- some families now choose to have only one child and they make sure that child is a boy, with illegal prenatal sex tests and abortions if necessary.

Indian wedding customs mean that girls are often seen as a huge cost with very little returns, partly because the practice of demanding dowries remains the norm, despite being illegal.

Sons, on the other hand, can inherit property, continue the family line, and play a vital role in important Hindu rituals.

John said they found skewed sex ratios in almost all communities, regardless of background, educational level, incomes, religion or caste.

Even in poorer, rural communities, prospective parents often prefer to pay perhaps a month or two's earnings now for an illegal ultrasound sex test rather than have to spend several years' worth of income on a dowry.

Richer families can then visit an abortion clinic; poorer, rural families often rely on herbal potions or similar traditional methods to induce a miscarriage, or, if a daughter is born, may fatally neglect her if she gets sick, researchers said.

Outright infanticide is rare, John said.

Only the very poorest communities have nearly as many girls as they do boys, partly because they have no access to sex tests, and partly because high rates of child mortality result in value being placed on any child surviving past infancy, regardless of sex.