Sydney - It's not surprising that Lebanon-born Keysar Trad has nine children.

His homeland abounds with big families and he simply brought that tradition to Australia with him.

By the same measure, immigrants from China, where the one-child family is the norm, often stick with that paradigm in their new country - even though there is no state apparatus enforcing it.

"It seems that they genuinely prefer it," said Macquarie University demographer Nick Parr.

His study of census data found that in 2001 over 11 per cent of couples had only one child, compared with 8 per cent 10 years earlier. If the trend persists, 15 per cent of couples would have a single child in 2021.

Identifying the little-emperor trend is easier than explaining it. This is because demographers like Parr normally look at the birth rate in aggregate rather than teasing out who is having babies and who isn't.

"We don't look at the reasons," he said. "But we looked at which groups were more likely to have the one child. It's probably about costs and the investment of time."

What he found was that the single-child pattern travels.

"There are particular migrant groups that have a high prevalence of one-child families," he said. "In many parts of Europe we are now seeing birthrates at very low levels and one of the very main reasons for that is one-child families."

He also found that the single-child pattern travels down through generations. "Generally speaking, children who grow up in smaller families are more likely to go on to university and to earn more and they tend to have smaller families themselves," he said.

But why this is so, there is no easy answer to be extracted from the data.

Melbourne economist Mark Wooden speculated that many professional women simply run out of time to have bigger families. "Women are often prepared to risk waiting until they are settled in their careers despite their fertility continuing to decline and falling away quickly in their mid to late 30s," Wooden said. (dpa)