What's it like to grow up in a world where no-one has brothers or sisters? Are siblings really that important? Researchers have been asking those questions for years - and China, with its famous one-child policy, has been a good place to look for an answer.
Chinese families used to have an average of four children each, but life changed radically in 1979, when a law was introduced dictating that most parents could only have one child. Last week, we learned that the policy will now be relaxed, after being enforced across the world's most populous country for more than a generation.
"On the township roads, there are slogans written on flamboyant red banners, telling people to have fewer children and raise more pigs," says art photographer Fan Shi San, recalling a recent trip to the impoverished province of Gansu. Fan, himself an only child, takes photographs of single children alongside their "phantom" brothers or sisters - the siblings they never had.
"Most of my audiences don't realise they have a special identity," he explains, noting that many parents even stopped questioning why they couldn't have more than one child and forgot that things had ever been different.
In 1979, when the policy was first unveiled, the new rules were a major adjustment for those accustomed to large families. But children growing up under the policy were unaware of this. And in the early years, the parents of most new single children came from large families - so instead of siblings the children were able to forge close relationships with cousins.
Since 1997, sociologist Vanessa Fong of Amherst College in Massachusetts has followed a group of 2,273 Chinese "singletons" as she calls them. Every year, she interviews and surveys between 600 and 1,300 of the original group so she can track how their lives have been affected by growing up without siblings.
To begin with, the very notion of "sibling" was a hard one for the children to grasp - a task made more difficult by the Chinese tendency to use the term for "brother" or "sister" when talking about cousins.
Even when the children were in their teens, she would have to explain the difference to this group of people that had never encountered genetic siblings.
"They'd say, 'Well, yes, I have many brothers and sisters.' And I'd say, 'How did that happen? Most people have no siblings.' And they'd say, 'Oh, I'm talking about my aunts' children'."
The first singletons born under the one-child policy experienced other changes, too, apart from the absence of brothers and sisters.
"Every family suddenly had a huge amount of discretionary income to invest in education and also in consumption," Fong explains. The resources that had been spread among several children in past generations were now focused on one child.
The result - China's new singletons were more educated than generations before them. And Chinese education costs soared overnight. In the past, parents would usually choose just one of their children to progress in school. But after the one-child policy came into practice, each single child shouldered this focused pressure from two parents.
Ge Yang, a 32-year-old woman who grew up in Beijing, says the unflinching, relentless attention she received from her father, a driver, and her mother, an accountant, altered the course of her life.
"If my parents had had other children, they would have paid less attention to me, in which case I might have spent more time and energy doing things that interest me. Chinese parents of my parents' generation like to plan life for their children," she explains.
Ge now works as a pharmaceutical sales representative, a solid middle-class job. But things might have been different, she says, if she had had siblings to share the burden of her parents' expectations. She might have chosen a different career, or moved away from Beijing.
"I think if I had another chance, I might choose to work in the tourism industry, or live in another city," she muses.
"But as a single child, I have the responsibility to look after my parents. I couldn't leave my city. I need to be with them. This is something I cannot change."
Nonetheless, she sees her singleton status in a positive light.
"As an only child, I have my parents' love all to myself," she says firmly. "I don't want to share my parents with others."
But what about Little Emperor Syndrome - the notion that China's children would grow up spoiled and self-centred.
It was widely feared, but a number of studies - including many conducted by Chinese researchers - have failed to turn up any nasty personality traits among those who grew up in China's one-child families. There's no real evidence that China's singletons are any different than other children, they argue.
But other studies contend that China's singletons are different. A study of only children in Beijing released by a group of Australian researchers this year used a series of games and surveys to test behavioural traits. The study attempted to unveil the subjects' real personalities by using games tied to real financial rewards, explains University of Melbourne economist Nisvan Erkal.
"What we found was that people born after the policy, and who are single children because of the policy are significantly less trusting, less trustworthy, more risk averse and less competitive," he says. "From the surveys, we find they are also more pessimistic and less conscientious."
Those born in the later stages of the one-child policy will provide researchers with even richer material. While children born in the 1970s and 1980s were usually surrounded by large extended families, those born more recently will typically have been born to parents who were single children themselves. With fewer cousins, aunts and uncles in the mix, children grow up in much smaller families than before.
An increasing tendency for people to move home for the sake of a job also makes it more likely single children will grow up without close ties to their grandparents, or even childhood friends, notes the sociologist, Vanessa Fong.
"China has changed a lot, so relationships are not as intimate as before," she explains. "In previous generations, people were not able to move to other cities or other countries. Now, there is a lot more migration, both within China and between China and other countries."
Ge understands that concern. Her three-year-old daughter is experiencing a much different childhood than the generations before her.
"My child will have very few stable friends. She will have many new friends as she grows up, but she will have very few long-term friends who grow up together," she ays.
Under the new relaxation of the one-child policy, Ge and her husband qualify for a second child. However, she knocks down that idea with a quick wave of her hand. A second child would be too expensive, she explains, if she wants to be able to afford a good lifestyle.
"It is not that we don't want to raise more children, it is that we cannot create that many opportunities for them. If I cannot create that much opportunity for my children, I think that my children will feel lost in competition against other children," she says.
Although the one-child policy is still in place for many in China, it is possible that one day in the not-too-distant future, China's one-child generation will become a chapter in the country's history books.
But even when that happens, the ultimate verdict as to whether China's singletons were hurt by the policy, or benefited from it, may still be the subject of debate.