They were known as China’s “little emperors” – the offspring of one-child families born after the country’s draconian family planning policy was introduced in 1979. They became the “spoilt generation” of teenagers who didn’t experience the joys and heartache of sibling rivalry or share and share alike – at least this was the simplistic stereotype of the singleton children born in modern China, based on little more than anecdote and hearsay.

But now, scientists have produced the first convincing evidence to suggest that the one-child generation of China has indeed become a rather maladjusted lot.

The one-child policy came about after a rapid growth in the Chinese population in the 1950s and 1960s. It was strictly enforced in urban areas, with reports of forced abortions and sterilisations, as well as heavy financial and social penalties for those who dared to transgress the one-child law.

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Chinese authorities claimed the policy was a great success, preventing more than 250 million births between 1980 and 2000 and helping to set the country on the right demographic track for its spectacular economic growth.

However, more than 30 years after it began, an unintended consequence has emerged; it has fundamentally changed the psychology of young Chinese men and women, scientists said.

Chinese children born after the policy have grown up to become less altruistic and trusting, more timid, less competitive, more pessimistic and less conscientious than the Chinese who were born just before the policy, they claimed.

The study has broken new ground by analysing the attitudes of young Chinese adults using games designed by economists to test behaviour and intentions, such as whether the subjects are likely to share something with a stranger or are ready to trust someone they do not know.

When the scientists compared two age groups born a few years before the policy was introduced, with two age groups born just after, they were surprised to find such marked differences in the kind of personality traits which influence social relationships, that could have important ramifications for China’s future.

“We find quite large impacts. Those who are the only children as a result of the policy are considerably less trusting, less trustworthy, more risk averse, less competitive, more pessimistic, less conscientious and possibly also more neurotic,” said Lisa Cameron, of Monash University in Melbourne, Australia. “These behavioural impacts could have economic consequences in addition to the more obvious social implications.”

“For instance, we find that those born under the policy are less likely to be employed in risky occupations, such as self-employment, freelancing or the financial sector. So it may be that the one-child policy generation will be less entrepreneurial,” she said.

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The study analysed 421 individuals born just a few years before and a few years after the 1979 policy, which resulted in a dramatic decline of large families. In 1975, just 27 per cent of Chinese families consisted of only one child, whereas it was nearly 91 per cent in 1983.

The scientists asked the volunteers, now in their 20s and 30s, to carry out a series of economics games designed to test features of their personality, using real money as an incentive. The “dictator game” measured altruism, the “risk game” tested boldness and the “competition game” looked at rivalry.

“Economic experiments have the advantage of allowing the researcher to observe particular, well-defined types of behaviour,” Dr Cameron said.

“Experimental participants are also incentivised with money, the amount of which depends on the decision made in the experiment, which, experimental economists argue, provides a greater motivation for participants to reveal their true preferences,” she explained.

What became clear, the scientists say in their study published in the journal Science, is that one-child offspring suffer from what they term “sibling deprivation”, meaning that a lack of brothers or sisters appeared to make them more self-centred, less co-operative and less likely to get along with their peer group.

Previous work on one-child families in the West, where parents have largely chosen to have just one baby, have found little differences in behaviour between them and the offspring of larger families. What was different here was the ability to look at a whole society where parents were coerced into having no more than one child.

This was like “a huge natural experiment” that allowed the scientists to distinguish between growing up as an only child in a one-child society, and being an only child in societies where parents had the choice, Dr Cameron said.

On paper, singleton children should have an advantage in that they have the full attention of their parents and do not have to compete with siblings. But, in practice, the findings suggest that singletons may have missed out on the rough and tumble and give and take of growing up with other children of about the same age.

As one commentator noted: “Perhaps the biggest surprise of the study is how thoroughly the only-child subjects lived up to their bad reputation.”