By James Reynolds
BBC News, Beijing
Boys outnumber girls in this classroom
China's future problems are easy to spot. You can see them in the front row of the Hui Kang kindergarten class in the city of Huizhou.
Four boys sit next to a girl. Further back, a girl in pigtails and a pink jumper stands out amid a group of boys.
In recent years, the kindergarten has been getting more and more boys.
Since the late 1970s Chinese couples have been allowed just one child - and most parents here prefer to have a son.
"It's having a real effect," said Ms Zheng, the school's head teacher.
"One of our classes has 39 boys and just eight girls. It's a serious problem. When there are more boys than girls, the girls copy the boys and they become more aggressive," she said.
At break time, the girls need to be pretty good at defending themselves. One girl beats off four boys who try to have a look at what she was drawing.
China is worried about what may happen in 20 years or so when these kids grow up and start trying to settle down.
An official estimate says that, by 2020, there may be 30 million men of marriageable age who will not be able to find a wife.
The gender imbalance has various causes. Abortions on female foetuses are believed to be widespread as couples, particularly in rural areas, hope for a son who will look after them in their old age. There is also suspected under-reporting of female births.
'No-one to impress'
On China's tropical island of Hainan, you get an idea of the problems the country might face in a bachelor-heavy future.
In a small village, made up of shacks and pig pens, there are already too many men, and not enough women.
Villages in Hainan are full of single men of marriageable age
Liu Yaxiao introduces his three brothers - all in their 30s, all unmarried.
They sit along the edge of a bed, looking a bit sheepish and forlorn.
They were born before the one child policy began - but they still cannot find anyone to marry.
All the single women left their village long ago in search of work.
At home they have to cook for themselves. That is embarrassing in a country where women are expected to make the food.
Liu Yading is one of the brothers. He has a motorbike and leather jacket and does his best to look fairly cool. But there is no one for him to impress.
"There are more than a hundred men aged between 18 to 40 who are unmarried in our village," he says.
"Nearby villages are all like us. How can we get married? I don't know what to do or where to start finding a wife. I'm stuck - unless God can help me."
The brothers eat lunch by themselves.
Outside the village shop, men and teenagers hang about.
They keep an eye on the main road - in case any eligible women wander by. But the road is empty.
In years to come, for the single men of China, things will only get worse.