The Chinese call them guanggun, or "broken branches": young men under 30 who, because of a cultural preference for male children and three decades of China's one-child policy, have little chance of marrying a Chinese woman. China has one of the most unbalanced gender ratios in the world: there are an estimated 40 million more Chinese men than women of marriage age.
Such an imbalance in a country's demographics can cause real problems. There is research to suggest a correlation between the number of young unmarried men and the propensity of a country to fight wars. At a minimum, there are social challenges of crime and violence.
After the arrival of the seven billionth human to share our lonely planet, thoughts have turned to the population increase in the region, in particular to what is known as the Arab "youth bulge" - that across the region, around 60 per cent of the population is under 25 years old. This, combined with high unemployment rates, have provoked fears of social unrest and, perhaps, radicalisation of youth.
But the number of people is only half the story. As the example of China shows, the problem is not the total number of people, but an imbalance in the population. Too many young men and not enough young women; too many young and old people and too few people in employment.
In fact, the Arab youth bulge could be an opportunity. Properly managed, it could begin a long period of economic growth for the region.
Here's how: A country with a high proportion of teenagers and young adults and a relatively low proportion of elderly and children is ripe for economic growth: young people are innovative, mobile and have a high demand for manufactured goods and services.
As women increasingly enter the workforce - as they will increasingly in the Arab world, even in conservative countries like Saudi Arabia - the number of workers could double. Because of careers, families are postponed and when they do arrive, they are smaller, so workers stay in the workforce longer. At the same time, the revenues the state gains from taxing young workers is not spent on pensions or health care, because there are relatively few older people.
One of the reasons for China's extraordinary growth over the past two decades has been its youthful workforce. Contrast that with Japan, which is suffering from a combination of a low birth rate, later marriage and postponed families, and long life expectancies. That same situation is now occurring in western Europe, although - in contrast to Japan - European countries are trying to solve the problem with increased immigration.
Those problems may come to the Arab world. But for now, demography can be a bonus to the region, though only if governments act decisively and swiftly.
Part of the reason why the Arab world, especially Egypt, Yemen and North Africa, were ripe for unrest was the number of young people. With so many young people in these states and the economy stagnant, it was clear to economists and demographers that big changes would soon come, although no one could say when. Revolutions based on the Arab youth bulge were predicted for a decade.
But the reason was in the detail. It was not because of the number of young people in the Arab world, but because of the failure of states to offer them opportunities, that made the region so ripe for revolution. In Egypt and the Levant, the word "hayateen" is used to refer to those young men who have nothing to do and lean against walls all day. Other Arab countries have similar nicknames.
These men are testament to the failure of the state. There are not enough jobs and the costs of education, marriage and property are too high. This is not an exclusively Arab problem: many of the young people occupying streets across America and Europe would have similar complaints. But nor is it a given in a creaking worldwide economy: there are policy solutions and the new leaders of Egypt, Libya and Tunisia will have to find them.
The youth bulge will be easier to manage in some places: small Tunisia is better placed than big Egypt; the natural resources of the Gulf will ease the transition. Moreover, it is in the interest of rich Arab countries to help their lower-income neighbours, so that fewer of those citizens seek work across borders.
But there are common strategies that Arab governments can pursue to benefit from their young people. There are three things, in particular: first, focus on education, on building skills and knowledge. Then, get institutions working: corruption, lack of competition and too much concentrated power will sap the economy. And thirdly, utilise the whole potential labour force, which means giving women the skills and opportunities to choose careers.
These three things will not guarantee high economic growth. They are a best case scenario. Education without jobs will lead to frustration and emigration. A full labour force will not tolerate underpaid jobs for which they are overqualified. And the money to buy goods and services only benefits the national economy if those things do not need to be imported.
The youthful populations of Latin American countries have not translated into strong economies, mainly due to bad policies. But the youth bulge should be seen as an opportunity for the region, not a threat. Governments of the Middle East cannot easily control populations, but they can control policies, which is more important. Even with seven billion humans, the number of young people in the Arab world matters less than what those young people do each day.
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