(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.
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.
Poor treatment of women points to India's widening gender gap
The report, which aims to present solutions addressing inequalities across society, states that the "intrinsic values of gender equality are (incontestable)."
It points to "growing evidence that there can also be significant gains in economic growth if women acquire greater personal agency, assume political power and attain public status, and participate equally in the labor force."
In terms of countering gender imbalance it points to a 2015 initiative which roughly translates as "Save the Daughter, Educate the Daughter" as a positive step that the government has taken to redress the imbalance.
But the report acknowledges that it has a long way to go to India's gender bias "is long-standing, probably going back millennia," and despite initiatives like the 2015 program to educate girls, India is lagging in its quest to address the gender gap.
In terms of economic participation and opportunity for females, India is one the worst countries -- 139th -- and similarly poorly in women's health and mortality rates. It is also in the bottom third -- 118th -- for education of women and girls, according to a 2017 World Economic Forum report.
A report by Kanya.Life, an anti-infanticide organization which uses data analysis to provide insight into the problem, reports that the worst city for gender imbalance is Mahesana in the state of Gujarat, with a ratio of only 762 women for every 1,000 men.
"The problem of female infanticide does not seem confined to smaller villages, contrary to common perception," its report states. "In fact, relatively large urban areas also have this problem."