Then what are the reasons behind mobile phone addiction among left-behind children? Their phones are usually bought by migrant parents in a bid to facilitate communication and remote supervision. Therefore, "parenting by phones" is a helpless choice for many families.

But for some primary and middle school students in those families, mobile phones are more like electronic toys than useful tools.

"The vast majority of left-behind children are cared for by their grandparents, but the elderly are generally not able to use smartphones and cannot manage their grandchildren well. Yet they can save a lot of trouble by giving their grandchildren a cellphone," said Zhang Yu, principal for a primary school in Southwest China's Guizhou province.

The addiction to mobile phones leads to a number of problems. Zong Chunyan, a consultant at Yangzhou University's mental health center, said the overuse of mobiles can result in children having less effective communication with their parents and peers, as well as an unsociable personality.

Physically, staring at a mobile phone screen for a long period is likely to cause impaired vision, cervical spondylosis and other problems.

But in Zhang Yu's eyes, hidden risks such as the decline in academic performance, inattention and exposure to inappropriate information are even worse.

"I think effective cooperation between families and schools is the most useful tool to break children's dependence on cell phones," said Zhang.