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As the daughter-in-law rolled open the rusted doors to her garage, light spilled onto a small figure on a straw mattress. A curious face peered out.

It was that of Kuang Shiying's 94-year-old mother-in-law, Zhang Zefang - better known as the little old lady who sued her children for not taking care of her. With her thin frame and soft smile, Zhang hardly looked the vindictive matriarch many assumed she must be.

In the village of Fusheng, Sichuan, where Kuang and Zhang live, the pace is slow and the atmosphere placid. But inside their home, there is war. Resentment hung in the air, acrid and sharp like the stench from the urine-filled bucket next to Zhang's bed.

"I never thought about whether my kids would take care of me when I was old," Zhang said. "I just focused on taking care of them."

The drama playing out inside this house reflects a wider and increasingly urgent situation. The world's population is aging fast, because of longer life spans and lower birth rates, and there will soon be more old people than young for the first time in history. This has left families and governments struggling to decide: who is responsible for the care of the elderly?

Family loyalty is a cornerstone of Chinese society, and on the mainland, more than 1,000 parents have sued their children for financial support over the last 15 years.

But in December, the government went further, amending its elderly-care law to require that children also support their parents emotionally. Children who do not visit their parents can be sued by mum and dad.

The law pits societal change against the complexities of family, and raises the question: how do you legislate love?

Zhang was born in 1919 and married at 14, Her husband died of dysentery. Her second husband was too poor to support her, so they moved in with his parents.

Her father-in-law, she said, was a gambling addict with a violent temper. Yet Zhang never considered leaving - that would have made her a social outcast. Three decades later, her husband died, leaving her at the mercy of her offspring. But the world had changed.

When Kuang had stepped outside the room, Zhang said that her family locked her in this room all day. She dared not scream for help for fear she would be beaten.

She pinched her cheek hard, slapped a visitor's arm. That's what they do to me, she said.

All she wanted, she said, was to go to a nursing home. But the nursing homes in the country supply only 22 beds for every 1,000 senior citizens, and most families cannot afford them. Zhang has no money. She said her children had taken it all.

By 2050, the mainland is projected to have 34 per cent of its population made up of people aged 60 or above, an increase from 12 per cent in 2010, according to UN figures.

The mainland's one-child policy, which has been in force for more than three decades, has led to fewer working young adults. And a lack of jobs means rural youth must leave their parents to find work in distant cities. The result is an emotional and generational tug-of-war.

Frustration is etched into every line of Kuang's face. Her mother-in-law's accusations, she said, were lies.

It is Kuang who looks after her mother-in-law, part of the common practice in which women in China shoulder most of the responsibility of caring for the old.

Where the government falls short, the kids are left to solve the problems - but often they cannot, and sometimes will not.

No one knew what to do in Zhang's situation. So the family took their case to the village court. Officials told Zhang she could sue her children. Then the court could force them all to care for her equally. Suddenly, everyone in the village knew her story.

The court ordered the two sons and their sister to take turns caring for their mother, and Zhou Yinxi, the second son, to pay her 60 yuan per month. So far, Yinxi has paid nothing.

And so Zhang returned to Kuang's garage.

"I won't get any appreciation for taking care of her," an exhausted Kuang said. "I also can't abandon her."