There are around 9 million so-called "left-behind" children in rural China, according to a new survey. These are children who have either one or both parents who are migrant workers in cities.

More than 8 million have no parents to take care of them, meaning almost 90 percent are being looked after by grandparents. 360,000 of the left-behind children have no guardians at all, accounting for around 4 percent of the total.

The survey also shows the highest percentage is to be found in the 6-to-13 age group -- with more than 5.5 million. And the central rural part of China is where most left-behind children are located -- nearly 5 million, accounting for more than half of the total.