NEW DELHI — India’s increasing wealth and improving literacy are apparently contributing to a national crisis of “missing girls,” with the number of sex-selective abortions up sharply among more affluent, educated families during the past two decades, according to a new study.

The study found the problem of sex-selective abortions of girls has spread steadily across India after once being confined largely to a handful of conservative northern states. Researchers also found that women from higher-income, better-educated families were far more likely than poorer women to abort a girl, especially during a second pregnancy if the firstborn was a girl.

“This has deep implications,” Shailaja Chandra, one of the study’s authors and the former director of the National Population Stabilization Fund, said Tuesday during a panel discussion after the release of the findings. “The scale is very large and requires intervention beyond what has been done so far.”

The study, being published in the British medical journal The Lancet, is the latest evidence of India’s worsening imbalance in the ratio of boys to girls. The 2011 Indian census found 914 girls for every 1,000 boys among children 6 six or younger, the lowest ratio of girls since the country gained independence in 1947. The new study estimated that 4 million to 12 million selective abortions of girls have occurred in India in the past three decades.