Conversations about when to start having children are not, broadly speaking, a male specialty. Conversations initiated by men about the effect of age on male fertility may be even more of a rarity. “The age thing?” says Mark, a middle-aged father who spent his 30s and 40s vaguely wanting children but working and travelling and developing complicated interests instead. He pauses, slightly puzzled at the question. “I’ve never really thought about it.” Simon, a recent first-time father of similar age, worried a little more about reproductive biology in his 30s. But not about his own: “Always the consideration was my girlfriend’s biological clock,” he says. “You just think you can be Charlie Chaplin if necessary, and have a baby you’re too old to pick up.”

This comfortable state of affairs for men may now be at an end. This week, writing in the journal Fertility and Sterility, a French scientist, Elise de La Rochebrochard, who led a recent mass study of French pregnancies, concluded that a father aged over 40 “is a key risk factor for reproduction.” For women under 30, a male partner aged 40 or over reduced their chances of conceiving by a quarter; for women between 35 and 37, a partner over 40 reduced conception to a one-in-three possibility.

The French findings feel boldly counter-intuitive, in the light of decades of newspaper articles warning women, often in the most finger-pointing and sensationalist terms, about the perils of delaying having children. Few women in their 30s can be unaware of the notions that if they are too busy building a career, or too materialistic or selfish or preoccupied to get pregnant before the biological deadline of their early 40s, then they’ll have no family. Midlife infertility, essentially, is seen as a female problem.

In fact, over the past five years similar investigations in Britain and the United States have anticipated the French findings, and have also found late fatherhood to be riskier than traditionally assumed. One study found would-be fathers over 40 half as likely to make their partners pregnant as men under 25; another found fathers over 50 quadrupling the likelihood of having a child with Down’s Syndrome.

If anyone notices, there are very large implications for the rhythm of male lives and the balance of power between men and women. “This research will be a relief to a lot of women who are used to a culture where they carry the responsibility for fertility,” says the author and journalist Melissa Benn, who has written extensively about the politics of motherhood. “If men were to take this information seriously, it could help to synchronize male and female desires and bodily needs.”

But men could take some persuading about their reproductive frailties. It has long been known to medical professionals, says Richard Kennedy, spokesman for the British Fertility Society, that across all relevant age groups, “the man is the leading cause of fertility problems. Yet still there is an attitude from men that, ‘It can’t be me that’s the problem.'” It is women who buy ovulation predictor kits, take their temperature in the mornings and have tests to discover how many eggs they have left. By contrast, says Kennedy, public and media attitudes to male ageing and fertility remain unconcerned.

Meanwhile, British culture celebrates fathers who leave it late. Currently in Britain, according to the Office for National Statistics, the average age of a first-time father is 32 and rising. One in 10 babies is born to a father over 40, and one in 100 to a father over 50. Unlike middle-aged mothers, these late fathers tend to be regarded as physically heroic rather than freakish or selfish: “The flag goes up the pole and everybody says, ‘What a terrific guy,'” says the novelist and writer on masculinity Dave Hill.

Besides admiring their virility, there are reasons to see older fathers as a good thing. Jack O’Sullivan of the pressure group Fathers Direct says: “Research suggests that older fathers are more confident with their children. They are more likely to be fathers by choice. They’ve got over ‘provider fever’ – horror about being able to manage financially. They are less bothered about their careers. They tend to share the care with the mother more.”

But O’Sullivan and Hill, who are from the more enlightened end of the fathers’ rights spectrum, agree with Benn that a more realistic and less gender-obsessed attitude to fertility would be welcome. O’Sullivan points out that all the emphasis on the female biological clock has acted to exclude men as well as restrict women: “Historically men have become fathers when women wanted to become mothers. In Britain now, about 20% of men become expectant fathers at a time they wouldn’t have chosen. The smaller that figure is, the better it is for children.”

Simon says his approach to becoming a father would have been different if he had known his fertility was a wasting asset. “We would have tried much earlier. We thought about it when I was in my early 30s, but it didn’t seem urgent. But it would have been better … Now I sometimes think, ‘I’ll be dead when my daughter’s 30.'” Hill, who is 48, has similar thoughts about his own mortality. “My youngest son is eight years old. I’m pretty fit, but I have to work pretty hard when we’re running round a playing field. And my three-year-old daughter – I could be dead before she has children of her own. That’s a pretty depressing thought.”

That men have a lower life expectancy than women has always been a flaw in the argument that the former need worry less than the latter about when to have a family. The new discoveries about male fertility may challenge the male confidence – you could say complacency – that papered over this flaw. “Intimations of mortality are good for all of us, not just for reproducing but for living,” says Benn. “The biological clock has always been that early nudge for women. Now men are getting one too.” O’Sullivan suggests one possible social implication for middle-aged men: “A woman who is 35 and thinking about her biological clock may look at a man of 50 and think, ‘I have to look for someone younger.'”

Then again, as the British Fertility Society point out, the decline of fertility with age is still much shallower for men than women. And less terminal: “For men we can sometimes overcome the problem fairly easily,” says Kennedy. “For women, we can’t. I don’t think men should get overly anxious.”

It would be too simple, anyway, to portray most modern would-be fathers as blindly confident about reproduction like Victorian patriarchs. Men are already more involved – attending antenatal classes, being present at scans and births, making plans to take time off work – than their fathers most likely were. And many men now are more self-conscious about their health through fitness magazines and heavy-handed lifestyle advice from government.

But the test of the new thinking about male fertility may be whether it works its way into the coded conversations men have always had about ageing, about hairlines and waistlines and whether they want to play five-a-side any more. Hill thinks the unsettling findings of French fertility scientists will take a while to become pub staples. “It’s a long slow road to raising men’s awareness in this field,” he says. “Masculine convention is a lot about custom.”

Benn thinks such a response may be healthy. “I can’t believe that information about fertility risks is going to screw men up in the same way as it has women.” It may not. Then again, the articles about bachelors selfishly trying to have it all would make interesting reading.

Defying nature: famous old fathers

John Humphrys: John Humphrys has two grown-up children, and is now the proud dad of five-year-old Owen. “My oldest son is 38 now, and, however cliched it may sound, to have a child in your life again at this stage makes you realise how wonderful the whole thing is,” he told one newspaper recently. “I was 22 when I first became a dad, and at that age there is just so much else to worry about – paying the mortgage, making a career for yourself, all of that. To have another chance to watch a small person grow is a blessing, and one I’m enormously thankful for.”

Rod Stewart: Oh, it’s nice to see a man so happily in touch with his paternal side – though given that Stewart’s baby son Alastair (with bra model Penny Lancaster) is his seventh, he has had a while to get used to fatherhood. The former mullet-haired rocker has admitted that he was “too busy touring and being a drunk rock- ‘n’roller” when some of his older children were born, but he’s thrown himself into it this time round. “I think the older you get, the more you can appreciate that you can still bring another life into this world,” he said. Such is his dedication, that he reportedly got into the birthing pool during Alastair’s birth.

Julio Iglesias, Sr.: The gynecologist father of Julio fathered Jamie when he was 89 after his 40-year-old wife underwent fertility treatment. “At my age, a child is marvelous,” he declared. “If people say I just did it for my wife, I don’t take it as an insult, but the truth is I wanted it just as much as her.” Julio Iglesias Sr passed away after suffering a cardiac arrest days after he announced he was to become a father for the fourth time at 90. “My wife wanted it and I owed it to her,” he explained. “It was an act of generosity towards her. I need her so much that I said to her, ‘Here, this is what you wanted for when I am gone.'”

Rupert Murdoch: He may run half of the world’s newspapers, TV stations and magazines, but Rupert Murdoch isn’t too busy to procreate. In a life spanning almost eight decades, Murdoch has produced six children, two of whom – with third wife Wendi Deng – arrived when the media mogul was in his 70s. Grace was born in 2001, and Chloe in 2003. Let’s hope old age has mellowed the old dog, or the girls are going to have a rough ride: it is rumored that he liked to toss his older kids in deep water to toughen them up. He allegedly did it “to test them.”

Eric Clapton: “I still regard music as my main calling. I would never regard myself as a father first.” Perhaps not the kind of thing a doting dad should say out loud, let alone to a national newspaper, but that’s what Clapton told the Times recently, when discussing life with toddlers. Though Clapton has been a dad before – to Conor, who, aged four, fell to his death in 1991 from a 53rdfloor window; and Ruth, born in 1985 – it is only in the past five years, when his wife Melia had Sophie Bell, Ella May and Julie Rose, that he has embraced full-time fatherhood. That’s full-time in the rockular sense: Melia allows him two days off a week, as long as he’s home for tea.

Charlie Chaplin: Perhaps the most famous of all elderly fathers, Chaplin was 73 when his youngest son, Christopher, was born. Legendarily fertile, Chaplin had a total of 11 children and was married four times. He is a long way off being the world’s oldest father, however: that honor is thought to belong to the now deceased Les Colley, who became a father at 92, having married his Fijian bride, whom he met through a dating agency, a year earlier. Colley died in 1998 after a brief bout of pneumonia, but his son, Oswald, lives on.

Michael Douglas: There is a 23-year age gap between Douglas’s first child, Cameron, and his second, Dylan, who was born in 2000 to his second wife Catherine Zeta-Jones. Waiting for the arrival of Carys, now three, Douglas claimed to be preparing himself mentally and physically. “I’ve got more motivation to stay in shape than I do with my movies,” he exclaimed. “I’ll be dealing with the bad backs – and everything else – on a regular basis.” But if there is any truth in the rumors of multiple facelifts, Douglas is feeling the pressure to match up to younger dads at the school gate.