They are left behind in grim villages all over China by parents who have joined the biggest voluntary worker migration in human history. Some are left at home with one parent, others stay back with an illiterate grandmother or exhausted grandfather. An estimated 2m are left to fend for themselves.

China has 61m so-called “left behind children”, all but orphaned by the mainland economic miracle. Interviews with social workers, non-governmental organisations, economists and parents say migrants do it for the sake of the very children they leave behind: to pay for their education, to build them a house to give them a future living among skyscrapers and not pig pens. Economists say this is the price of China’s modernisation, just as surely as the poisoned water and choking smog that is the legacy of its industrial development.

“We think of them as economic orphans,” says Jenny Bowen, head of the US charity OneSky, which has recently begun pilot programmes to help rural children being raised without parents. “Villages are unravelling because so many young parents go away to work. Generations of people who were always together are no longer together any more”.