More than 65 per cent of babies born in Merthyr Tydfil are to unmarried mums - the highest proportion in England and Wales.
A record-breaking 50.3 per cent of Welsh babies are now born out of wedlock, and nearly 18 per cent of those are to teenage mums. The comparable figures in England are 41 and 15 per cent.
However, most such babies are jointly registered by both parents (83 per cent in Wales), of whom three-quarters are living together, according to statistical analysis commissioned for TES Cymru.
Research suggests that children who suffer divorce or parental break-up do less well educationally and are more likely to truant or leave school at 16, although the differences compared to families that stay together can be small. There has been less research on mothers who were never married, but more than half of lone parents are poor - and poorer children tend to do less well at school.
Dr Marion Kloep, a reader in psychology at Glamorgan university, said the decline of the traditional married family and the rise in cohabitation was a worldwide trend, reflecting women's increasing economic independence.
But she suggested the explanation for the particularly high figures in the Welsh valleys relates to poverty and teenage pregnancy.
"Some girls, particularly in the Valleys where there isn't a future for them, might choose to get pregnant to get some status. They don't have much education and job opportunities are poor."
Schools, she added, can help break the cycle by supporting pregnant pupils and ensuring they complete their education. Rhondda Cynon Taf runs a Books and Babies scheme for teenage mums but admits few return to school.
But sociologist Patricia Morgan, a research fellow for Civitas, the Institute for the Study of Civil Society, said children were better off being poor than having unmarried mums.
Quoting large-scale studies in the United States, she said: "Once you are past the subsistence line, with basic housing, food and medical care, income tends to make little difference to outcomes.
"That's because the things parents can do to help are cheap or free, such as trips to the library, reading books together, monitoring children's behaviour and having aspirations and expectations.
"In the UK, studies suggest in the long run the child will do better being in poverty than being in an unmarried family."
That view was challenged by the charity One Parent Families. "Fifty-two per cent of children living in one-parent families are poor and we know that poverty affects outcomes for all children," said a spokesperson.
"Children from poor families are likely to experience not only a loss of resources while growing up but also (and in part as a consequence of such loss) lower opportunities for success in later life."
Vernon Morgan, Merthyr's outgoing education director, said family change had no apparent effect on schools.
A programme to help schoolgirl mums was suspended two years ago for lack of numbers. In 2003, 94 unwed Merthyr teenagers had babies.
"There's a wide mix of family types, and our schools deal with children as they come to us. The key element is that they come from a stable background," he said.
Three other South Wales Valleys authorities are in the top 10 for births outside marriage: Blaenau Gwent, Torfaen and Rhondda Cynon Taf.
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