Story highlights Deeply-ingrained preference for boys has led to a massive gender gap in India

Study finds that many couples who give birth to a girl will try again for a boy

(CNN) India's preference for sons over daughters has led to the birth of millions of "unwanted" girls, according to a new report by the Indian government.

Couples' tendency to keep trying until a boy is born has led to the birth of as many as 21 million girls who are "notionally... unwanted," the Economic Survey 2017-18 states.

The preference for boys and the availability of sex-selective operations, although illegal in India, means there's a gender gap of as many as 63 million girls, classified in the report as "missing."

As a result, India has one of the most skewed sex ratios in the world. For every 107 males born in India, there are 100 females. According to the World Health Organization the natural sex ratio at birth is 105 males for every 100 females.

The report's author Chief Economic Adviser Arvind Subramanian said while progress had been made in some areas, the "deeper societal son-preference" was proving hard to shift.

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