Blame it on the hormones but I can clearly remember being moved to tears on several occasions during the late stages of my second pregnancy as I reflected on how, for my first-born, the world as he knew it was about to end.

He would no longer be the sole focus of parental attention in the house as a little usurper was about to arrive to monopolise his mother. One-on-one time together would be limited from now on and liable to interruption by the wail of a hungry baby.

How, at the age of two and a half, was he going to cope with sharing his parents, his house and, in no time at all, his toys?

For my part, there was the niggling worry about whether that all-consuming love generated by child number one would stretch to number two. Could I really be equally besotted second time around?

However, luckily, at that stage financial considerations did not come into it and I had no doubts at all that a younger sibling and companion would be a blessing, even if my eldest might not see it like that right away.

But, for an increasing number of couples, the decision to at least attempt to add to a one-child family comes with a lot more agonising, for a variety of reasons. Will their bank balance, their careers and their relationship be able to take the strain? Does it make more sense to focus limited resources of time and money on one?

Despite our much-vaunted baby boom of recent years, due to demographics more than anything else, the average number of children per family has been falling. In 1991 it was 2.0 children; by 2011 it had reduced to 1.4.

The 2011 census also showed that more than half of married couples with one child belonged to the top three social classes.

In the UK there has been a significant increase in the proportion of “only” children – in the 1970s one in five children was raised without a sibling; now it’s one in four.

Are there benefits to having at least one brother and/or sister that these children, as well as their parents and society at large, are losing out on? That is a question Sky News presenter Colin Brazier looks at in a new book, Sticking Up for Siblings, published by Civitas, a UK social-policy think tank.

"The whole purpose of the book was to acknowledge there are massive and growing contraceptive forces out there," he tells The Irish Times. He is referring to the high cost of childcare and lost income from a mother's career that is repeatedly interrupted. The publication is aimed at parents who feel that a sibling for their only child would come at irrecoverable expense, both financial and personal. They cannot imagine a calculation that tilts the ledger in favour of another pregnancy.

Eye-popping statistics
Maybe they have read too many of those eye-popping, "cost of raising a child" stories, which usually arise from the latest survey conducted by organisations with a vested interest – personal-finance companies – and never mention diminishing costs of subsequent children through hand-me-downs, pooled resources, and so on.

“Having a second child, once the ‘traditional’ choice for married parents, now presents a stark choice to parents: a life of relatively unflustered parenting with one set of university tuition fees and no need for a people-carrier, or – even with just two children – a protracted period of anarchy in the home and years of umpiring the squabbles of siblings,” he writes.

In "offering the other side of the ledger", he and Swedish researcher Therese Wallin have laid out evidence from numerous scientific studies about the potential benefits of having more than one child.

“Some of it is stunningly incontrovertible and that surprised me,” he remarks, citing examples of how siblings have been shown to be a protective influence against three of the “great epidemics of modern life”: obesity, allergies and depression.

He believes there is a strong case, all other things being equal, that siblings make children thinner, fitter, healthier and happier than children who grow up alone.

That’s all good news personally for Brazier, who is 45, and his 50-year-old wife, Jo, who have six children, ranging in age from 14 to three. When I call their home in the Hampshire village of Middle Wallop, he has to extricate himself from imminent child-ferrying duties before he can talk.

Struggling to conceive
For a few years they thought they were going to be a one-child family, as they struggled to conceive a second after the birth of their daughter Edith, when Jo was 37. They were not like couples they know who felt they had experienced parenting having had one child and that there was no need to repeat the experiment.

It was only after Jo gave up her stressful job as head of foreign news at Sky, when he was posted to Brussels, that she became pregnant again, with Agnes, who is now 10.

Brazier missed their second child’s birth, being embedded, at the time, with US troops invading Iraq, where he was rapidly acquiring a greater sense of the fragility of life. His first sight of Agnes was an emailed photo on a laptop at Baghdad airport some four weeks later.

But, as it turned out, he had plenty more births to attend as Constance (nine), Gwendolyn (seven), Katharine (five) and John-Joe (three) followed.

“My wife, Jo, really defies medical science because she is 50 . . . She has had five children [and no multiple births] in her 40s, which is meant to be impossible,” he says.

“Jo and I are not dewy-eyed about parenting,” he stresses. “There is a lot of hard yacker.” However, being journalists, he says, they are particularly interested in the different personalities.

Brazier identifies with the late historian and mother of eight, Lady Longford, who, when asked by an interviewer why she had so many children, replied: "I just wanted to see how the next one turned out." But he is "deeply conscious" that he can enjoy raising a large family because he is in a well-paid job and one that means he is not away from home all the time.

Although Sticking Up for Siblings deals primarily with the case for siblings at a personal level, he was struck in the course of writing it (at their local Starbucks in Salisbury – "I had to get out of the house!") that further study is needed on how the growth of one-child families affects society.

Birth order
"If you believe siblings are codifiably different, and I think we do – the birth-order theory is pretty nailed down – you therefore have far fewer creative middle children or rebellious younger children coming through the system. That has to change things."

“Little Emperor Syndrome” has been a concern in China for some time, since the introduction of the one-child policy. And there certainly seems to be evidence backing up the notion that distinct behaviour traits can be attributed to a generation of children raised alone.

An Australian study published in the US journal Science last January found that children born one or three years after the one-child policy was introduced in 1979 were more pessimistic, neurotic and risk averse and were less trusting, trustworthy, competitive and conscientious than children born one to three years before.

Brazier is very cautious about straying into “really dodgy pro-natal territory” and has no wish to get sucked into right-wing politics. He acknowledges that parenting politics is full of “landmines”, and undoubtedly some people who have only one child due to fertility problems or other circumstances, rather than choice, won’t want to hear him extolling the positive differences siblings can make.

"The choice to have an only child is all too explicable," he says. As well as financial pressures, there is the impact on lifestyle to consider. "People want to experience parenting but they don't necessarily want to live through the great parenting emergencies where they subjugate their ego and every wish to bring up the next generation."

Only-child disease
He has no wish for a return to the "ridiculous" idea promoted in the late 1800s by US psychologist G Stanley Hall, known as the founder of child psychology, that "being an only child is a disease in itself".

However, he feels there needs to be a counterbalance to the “resource dilution model” put forward a century later that, as parental resources are finite, and siblings are competitors for parents’ time, energy and financial resources, the fewer children the better.

Brazier describes the new publication as a labour of love. “I don’t get paid for the book,” he explains. “This is my charity work.”

While we tend to consider the effects of siblings on childhood in particular, they last from the cradle to the grave. At a time of rising concern about the welfare of the elderly, the benefits of siblings in sharing the care of parents is obvious; so too the care of each other in due course.

Brazier also believes that “siblings keep us honest” and are “custodians of conscience. In an increasingly fleet-footed, atomised world, people often don’t put down roots . . . The scope to constantly reinvent yourself is ever present – whereas a sibling will be there to remind you of your shared past and will be quick to point out, as one of his younger sisters regularly does, that, “You are saying this now, but you weren’t always saying that.”

“You might want to forget your mistakes,” he says, but siblings won’t. “You never chose them as a friend.”

Sticking Up for Siblings by Colin Brazier will be published by Civitas on August 22nd. See civitas.org.uk

swayman@irishtimes.com