Researchers in India and Canada for the Lancet journal said prenatal selection and selective abortion was causing the loss of 500,000 girls a year.
Their research was based on a national survey of 1.1m households in 1998.
The researchers said the "girl deficit" was more common among educated women but did not vary according to religion.
The unusual gender balance in India has been known about for some time.
In most countries, women slightly outnumber men, but separate research for the year 2001 showed . . . . . . .