AKIPRESS.COM - 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," CNN reported.

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.

Some of the ingrained preference is due to the norms governing inheritance, the continued practice of paying a dowry for female children to be married and the tradition of "patrilocality" -- women joining their husband's households -- and rituals which need to be performed by male children.

According to the report, 55% of couples who have a girl will try for another child and will keep trying until they have a boy. It's referred to in the report as fertility "stopping rules."

Subramanian told India's NDTV the decision to have more babies until there was a boy had offset the number of girls lost through infanticide, sex-selection and differential survival -- the difference in mortality rates between male and female children. More females die than males in early childhood.

"What this says is that even if you didn't have all those things, you have fertility stopping rules, where people say, 'if I have a (male) child we stop... and if we don't we continue'," Subramanian said.