Most of those left behind say the extra money their migrant parents earn will go towards fixing up their homes -- usually low brick courtyards heated with coal stoves or traditional cave houses -- and help fund their childrens' education.

For some though, it is simply about making ends meet.

"There is no special goal," Yangyang said, when asked why his father went away. "It's just so our family can get by."

Some village officials encourage such migration in order to boost incomes, but they typically give less thought to the consequences for those left behind.

Village head Han Qunyan, who himself worked for a decade in Xi'an while his parents cared for his three children, said the biggest challenge facing those left behind is their education.

"Their grandparents can't really help with their education. They can only make some food for the children after they come home and then they have to rest," he explained over steaming bowls of peppery noodles.

Powerless

Others say the psychological impact of being separated from their parents could have longer lasting effects.

"Conditions for left-behind children can be extremely arduous and, moreover, all the households in a rural community face pressures just to live," said Ye.

"So when they need help and their mother and father aren't there, they feel powerless."

On one occasion, Xiao Bo, Xiao Yun and their sister were on their own when torrential summer rains that sweep the usually arid region flooded their cave home, its mud walls carved from the loose soil of the plateau.

Their sister went into a panic as she struggled to stop the rising waters.

It's just that kind of situation -- needing support and having no one to turn to -- that Professor Ye finds worrying.

"When they grow up, their perception is that there is no mutual help between people," he said.

"Twenty-three million people growing up and having this kind of perception of society is a very dangerous situation."

Most left-behind children have some telephone contact with their parents, but studies show their conversations are brief and rarely go beyond their parents' admonitions to behave well and study hard.

For the moment, Xiao Bo and Xiao Yun's father is back in their village to spend a few weeks with his children over the Lunar New Year holiday.

"He brings cake and oranges for us," Xiao Yun said, smiling for the first time at the thought of holiday treats.

But his older brother realises their father will soon leave again.

"My sister gets very tired," he said. "She has a lot of responsibilities when our dad's gone."

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