They make an odd bunch, my mentors: a protest singer, a suffragette, a sculptor, an actor turned MP, a comedian, a former US president and a first lady. But these giants of the 20th century have helped to create a historical shift in demographics.

Like me Joan Baez, Sylvia Pankhurst, Henry Moore, Glenda Jackson, Dawn French, and Bill and Hillary Clinton had only one child.

And they seem to have enjoyed close relationships with their offspring. Pankhurst, an admirer of the Ethiopian emperor Haile Selassie, went to live in Addis Ababa with her son Richard. When Jackson was elected to the House of Commons, her son Daniel worked in her office.

The percentage of women in the UK who have only one child has more than doubled in 20 years, from 10% to nearly 23%. Only-child families are the fastest growing type of families in the developed world.

We live in the era of the only child, and I think I may be one of the pioneers. But am I? My daughter Nancy is not yet three. So is she really an only child, or the first of many to come? It would be polite to ask Nancy’s mother. But I, too, have some say in the matter and, since the perils of pregnancy don’t concern me directly, I can consider the options relatively dispassionately.

Statistics for England and Wales show a birth rate of 1.77 children per woman of childbearing age. In 1964, at the height of the baby boom, the figure was 2.95. That decline can be accounted for, to some extent, by women starting families later, which leaves less time to have large families. As many as one in five women today will never have children.

It seems that both ends of the family-size spectrum are populated by successful women: professionals are the ones who can best afford to have large families (and fertility treatment); but it’s also professional women who leave childbirth until later in life.

Many women gingerly have one child, says Dr Katherine Rake of Fawcett, a women’s rights lobby group, and decide to leave it at that. Elliot McAllister was born when his mother was 42. “The tests during pregnancy ’s syndrome, among others were awful,” says Jo McAllister, a graphic designer. “Rather than go through all that again, we decided to have only one.”

Cathy Guthrie, an academic, was 33 when she had her daughter Maddy. After Maddy turned two, Cathy and her husband Andy, who works in the oil business, wondered about having a second. “We thought, ‘Shall we throw the pills away?’ ” says Cathy. “And then we thought, ‘Why?’ She is just right. We felt complete . . . to be selfish about it, we have more space to be ourselves; and that’s good for her, too.”

Then there are the environmental arguments to consider. Six billion people walk the earth, but analysis suggests the planet can support, long term, no more than three billion. World leaders know this, but few dare to broach the subject of population control. The exception is China, which introduced its birth-control directive, the “one-child policy”, in 1979. Since then, more than 100 million “little emperors” have been born every year.

The environmental impact of a new child born in Britain is seven times that of a Chinese child, according to writer Bill McKibben. He ensured there would be no second child in his family by undergoing a vasectomy, and provides an account of the grisly procedure in his book about the ethics of only children, Maybe One. But he acknowledges that readers will reject his environmental message if he doesn’t convince them that only children are capable of growing up happy and well adjusted.

The theory that a child’s position in the family affects its character became popular a century ago. The idea was that first-borns would be domineering and successful, middle children non-confrontational and confused, and last children irresponsible and power hungry. Studies done since the 1970s, however, have not confirmed such assumptions. A study by Texas University found that the personalities of only children were comparable with those of first-borns: they were no more selfish, awkward, grandiose or needy; the important factor was the quality of the parenting.

There may be wider implications though. Chinese policy has created a severe demographic imbalance, through abortion and abandonment of girl babies. One urban Chinese in three is doomed never to find a partner. The consequences are unimaginable.

In Britain, it seems, that gender bias is reversed. Researching her book, To Have an Only Child, Ros Kane asked hundreds of parents why they stopped at one. It’s mortifying for me, as a man, to report that several mothers said they had a girl and didn’t want to risk having a boy. Worse still was the mother who stopped at one child because she “already had a boy and didn’t want to risk having another”.

I grew up with siblings and so did my wife. I vaguely felt, when visiting my only-child cousin, that something was missing. But as a parent, I now see the disadvantages of siblings. We recently took Nancy to see a friend with three children. The boys were tired, but refused to go up to bed. The oldest and youngest had tantrums, so their parents took one each upstairs and left us with the third, who sullenly watched a video until his turn came. Nancy gets stroppy too, at times, but has both parents to put her to bed and plenty of time to read stories. Stories about only children, perhaps from Goldilocks to Harry Potter.

For parents, having an only child can be a luxury. You enjoy the experience of parenthood without absolutely saying goodbye to the life you led before. “We can do more,” says McAllister. “And when we go on holidays, Elliot always makes friends with adults. He joins in.”

People say that only children are spoiled. But they don’t have to be: Nancy learns social skills at nursery, and she doesn’t get her own way at home either, because she has to accommodate her parents’ wishes.

I was delighted, recently, to come across research showing that only children hold their parents in greater affection than other children do. But I’m not sure that I’m willing to follow McKibben and submit to a vasectomy for the sake of the planet. Again and again we hear from friends announcing the birth of a second child, or a third. It’s a drumbeat at once terrifying and compelling. So we may still decide, one day, to give Nancy a playmate.

The full version of this article appears in the June issue of UK Harper’s Bazaar, on sale this week