Image caption Bilasi Singh's daughter Bisanti has been missing for two years

The death of a student who was gang-raped on a Delhi bus has prompted anguished soul-searching about the place of women in Indian society. The widespread killing of female foetuses and infants is well-documented, but less well-known is the trafficking of girls across the country to make up for the resulting shortages.

Rukhsana was sweeping the floor when police broke into the house.

Wide-eyed and thin, she stood in the middle of a room clutching a broom in her hand. Police officers towered above her, shouting questions: "How old are you? "How did you get here?"

"Fourteen," she replied softly. "I was kidnapped."

But just as she began to say more, an older woman broke through the circle of policemen. "She is lying," she shouted. "She is 18, almost 19. I paid her parents money for her."

As the police pushed the girl towards the exit, the woman asked them to wait. She leaped over towards the girl and reached for her earrings. "These are mine," she said, taking them out.

A year ago, Rukhsana was a 13-year-old living with her parents and two younger siblings in a village near India's border with Bangladesh.

Image caption Rukhsana (right) talks to police about her kidnapping

"I used to love going to school and I loved playing with my little sister," she remembers.

Her childhood ended when one day, on the way home from school, three men pushed her into a car.

"They showed me a knife and said they would cut me into pieces if I resisted," she said.

After a terrifying three-day journey in cars, buses and on trains, they reached a house in the northern Indian state of Haryana where Rukhsana was sold to a family of four - a mother and her three sons.

For one year she was not allowed to go outside. She says she was humiliated, beaten and routinely raped by the eldest of the three sons - who called himself her "husband".

Missing millions An estimated 25-50 million women in India are "missing", if you compare the proportion of women in the population with other countries

As well as infanticide and foeticide, some girls die of neglect - many adult women also die early

Use of ultrasound for sex determination is illegal in India, but remains widespread

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"He used to say, 'I bought you, so you do as I tell you.' He and his mother beat me. I thought I would never see my family again. I cried every day," she said.

Tens of thousands of girls disappear in India every year. They are sold into prostitution, domestic slavery and, increasingly, like Rukhsana, into marriage in the northern states of India where the sex ratio between men and women has been skewed by the illegal - but widespread - practice of aborting girl foetuses.

The UN children's agency Unicef says it's a problem of "genocide proportions" and that 50 million women are missing in India because of female foeticide and infanticide - the killing of baby girls. The Indian government disputes this estimate, but the reality of life in Haryana is hard to argue with.

"We don't have enough girls here," the woman who bought Rukhsana cried as she tried to convince the police to let her stay. "There are many girls from Bengal here. I paid money for her," she wailed.

There are no official statistics on how many girls are sold into marriage in the northern states of India, but activists believe the number is on the rise, fuelled both by demand for women in the relatively wealthy north, and poverty in other parts of India.

"Every house in northern India is feeling the pressure, in every house there are young men who cannot find women and who are frustrated," says social activist Rishi Kant, whose organization Shakti Vahini (or Power Brigade) works closely with the police to rescue victims.

In just one area, the Sunderbans in the South 24 Parganas district of West Bengal, the BBC visited five villages all of which had missing children, mostly girls.

According to the latest official data, almost 35,000 children were reported missing in India in 2011 - and over 11,000 of them were from West Bengal. Police estimate that only about 30% of cases are actually reported.

Trafficking peaked in the Sunderbans after a deadly cyclone destroyed rice paddies around the area five years ago.

Local farm worker, Bimal Singh - like thousands of people - was left without income, and so he thought it was good news when a neighbour offered his 16-year-old daughter Bisanti a job in Delhi.

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"She went on a train. She told me 'Father, don't worry about me, I will come back with enough money so that you can marry me."

They never heard from her again.

"The police have done nothing for us. They came once and knocked on the door of the trafficker but they didn't arrest him. They don't treat me well when I go to them, so I am afraid to go to the police," Singh says.

In a Calcutta slum we manage to meet a man who sells girls for a living. He doesn't want to give his name, but speaks openly about the trade.

"The demand is rising, and because of this growing demand I have made a lot of money. I now have bought three houses in Delhi.

"I traffic 150 to 200 girls a year, starting from age 10, 11 and older, up to 16, 17," he says.

Why are there fewer girls in the north? Indrani Sinha, director and founder of Sanlaap (Dialogue), an NGO that works on trafficking In Haryana, people don't want to give birth to girls, so they kill their own children. It has gone on so long it has become tradition. The main reason is dowry. Haryana is a rich state because they have a lot of land and good agriculture. But education is very, very low, and the dowry is big because of all this land. Boys work on the farm and inherit the farm. But if it's given to a girl, it is for her family too. It's a cultural thing too. In Kerala, they don't think that women are a burden. The girl child is educated and will work. But some women - particularly in Punjab - believe that if they have many girls they won't be popular in the family, so they think abortion is better. In places where there is money, they get ultrasounds done and they sometimes kill the child. When the government issues an ultrasound machine, they try to follow up to see what happens. There are many people who practise foeticide.

"I don't go to the source areas, but I have men working for me. We tell parents that we will get them jobs in Delhi, then we transport them to placement agencies. What happens to them after that is not my concern," the man says.

The man says he makes around 55,000 rupees ($1,000; £700) from each girl. Local politicians and police, he says, are crucial to his operation.

"Police are well aware of what we do. I have to tell police when I am transporting a girl and I bribe police in every state - in Calcutta, in Delhi, in Haryana.

"I have had troubles with authorities but I am not afraid - if I go to jail I now have enough money to bribe my way out."

The head of the Criminal Investigation Unit in charge of anti-trafficking in West Bengal, Shankar Chakraborty, describes police corruption as "negligible" and says his unit is "absolutely resolute" in its determination to tackle the problem of trafficking.

"We are organising training camps and awareness campaigns. We have also recovered many girls, from different areas of the country. The fight is on," he says.

The very existence of his unit, he adds, shows the government's resolve and activists agree that police are now more aware of the problem. Every police station in West Bengal now has an anti-trafficking officer. But their caseloads are overwhelming, and resources are scarce.

"Simply changing the police will not give results. When we rescue a child together with the police, then what?" says Rishi Kant from Shakti Vahini.

"What we need is fast rehabilitation. We need social welfare and judiciary systems that work."

Rishi Kant says there is a need for fast-track courts - like the court being used to try the suspects in the latest gang-rape case - to prosecute perpetrators, and make it more difficult for them to get out on bail.

Even greater, some argue, is the need for a change in attitudes.

Two weeks before the notorious Delhi rape case, a group of influential local elders, all of them men, came together in a Haryana village to discuss what they called the most pressing issues their communities face - rape, illegal abortions and marriage laws.

One speaker addressed what he called an "alarming" increase in rape cases. "Have you seen the suggestive ways that girls ride scooters?" he said. "There is no modesty in the way women dress or act any more."

Defining terms Foeticide - The act of destroying a foetus or causing abortion

- The act of destroying a foetus or causing abortion Infanticide - The crime of murdering an infant after its birth, perpetrated by or with the consent of its parents Source: Oxford English Dictionary India's unwanted girls How India treats its girls

Another man spoke about the roots of female foeticide. "These days the society has become very educated and the girls from this educated society have started eloping. When girls bring shame on their own parents and behave like that - who would want a girl?" he asked.

Rupa, a 25-year-old woman was trafficked to Haryana from Bihar. She was sold as a wife to a man who failed to find one in his own community. The family forced her to have two abortions until she was finally pregnant with a baby boy.

In India, the cycle of abuse carries on.

Natalia Antelava's Assignment airs on 10 January at 09:05, 13:05, 16:05 and 20:05 GMT on BBC World Service. You can also listen via iPlayer or download a podcast.

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