After Colin Brazier and his wife had their first child, they thought they would never have any more children, but ended up with six. He tells Joanna Moorhead why siblings are the greatest gift a parent can give a child

People are usually surprised when Colin Brazier tells them how many children he's got. "There's a definite double-take – it's not the norm," he says. "I'm regarded as quite eccentric, certainly swimming against the tide."

Brazier has six children – Edith, Agnes, Constance, Gwendolyn, Katherine and John-Jo, aged from five to 14. But the funny thing is, says the Sky television news presenter, that he and his wife Jo, 50, had expected to have only one. "We had secondary infertility after Edith was born, and had given up hope of having more. Then to our surprise Jo discovered she was pregnant again," he says.

After that, the Brazier babies arrived one after another and as he watched the relationships between his children blossom, Colin, 45, started to think more deeply about what it meant for his children. "It seemed such a wonderful thing, that as well as the relationship between them and us – their parents – they have these very rich and interesting relationships with one another," he says.

Siblings, he concluded, were one of the greatest gifts a parent could bestow on a child. And yet, he says, the predominant trend is clearly towards families with just one child. "A generation ago," says Colin, "only children accounted for about one in five. Today it's one in four, and the prediction is that in another generation it will be one in three."

That, he is sure, has implications. "Siblings are life-changing, and they are world-changing. Having a sister or brother changes an individual life; it changes your experience of childhood, it changes your relationship with your parents and it changes how you relate to your sibling and to the world around you," he says.

"Siblings help to round your personality. And it changes the world because birth order affects the types of people there are in the population. Where will diplomats come from, if there aren't youngest children who can hone their negotiating skills? Where will we be without the creativity that's a typical trait of middle children?"

Somewhere between his working day and helping his wife Jo – who gave up her job on the Sky newsdesk after having Edith and is now a full-time mother – to look after their brood at their home in Hampshire, Brazier found time to start reading up on the research around siblings: and the more he read, the more convinced he became. Now he has written a book about it published by the think-tank Civitas: because, he says, the evidence is too compelling to ignore. "There's increasing pressure, and some of it is very subtle, on people to limit the size of their families to as few as one child – my contention is that there's another side of the coin to explore," he says.

Take finances, which is the main factor militating against family growth. Each year, Brazier points out, the media trots out figures on how much it costs to raise a child – the latest report, for 2013, puts it at £222,458 per child. "What that fails to take into account is economies of scale – having a second or subsequent child – where you've already got the buggy, the car seat and the clothes. It doesn't cost anything like as much as having the first," he says.

What's more, the report is compiled by an insurance company with a vested interest in spooking us into worrying us about the cost, in the hope of encouraging us to buy its products.

The truth is, says Brazier, the argument about siblings isn't solely or even mostly about cost. "When you look beyond the finances, at what siblings give one another and give the world, my question is: can you really afford not to have another child?" he says.

Not, he points out hurriedly, that he's encouraging the rest of us to emulate him and Jo: they are thrilled to have six, but he would never say that it's for everyone. "What I am saying is: if you've got one child and the cost of having another is putting you off, I'd gently encourage you to think again. Especially in today's world, because there's evidence that having siblings is the best support when it comes to two trends that are growing in our society: divorce and caring for ageing parents."

On divorce, he says the evidence is clear. "When a family goes through the trauma of a split, siblings can be saviours. They provide support – children who go through this sort of experience without brothers or sisters have more serious adjustment issues than only children." There's also "eldercare", another growing issue in family life. "Research has found that grown-up children with elderly parents suffer less hardship if they have siblings, because there are more people to share the burden," he says.

Interestingly, though, even where an elderly parent has several children, one of them tends to be the primary caregiver and (surprise, surprise) that tends to be the daughter who lives closest to the parental home. This hints at other sibling issues that Brazier is far from starry-eyed about. "Of course siblings complain about one another and they also argue and fight, and not only while they're children. But that's part of the experience too. It's another aspect of why having brothers and sisters is a good thing," he says.

In the long term, he argues, it's the fact that your siblings are your most knowing critics that could make them your best compass. "Siblings hold us to account. In some ways they know more about us, and across a longer period, than anyone else does. No one is more likely to highlight our contradictions, to identify our hypocrisies, than our siblings. They're the custodians of our consciences … they keep us grounded."

But in the midst of these more nebulous advantages, says Brazier, evidence has emerged – and his fear is that it hasn't been properly noticed – that children who have brothers and sisters tend to be healthier. "Research is showing that children with brothers and sisters are fitter, less obese and less allergy prone," he says. "We all know these are genuine worries. It's likely that the advantages work in different ways – it could be that being exposed to a bigger pool of micro-organisms, for example, makes them less allergy-prone. And it could be that having siblings encourages children to run around more.

Brazier adds: "We're in danger of moving towards the one-child template as the gold standard of parenting, and I think we should be wary of that. We're already seeing, for example, a limit on the number of children you can take to the swimming pool … if you've got a big family, as I have, you can easily find yourself breaking regulations when you take your own kids out."

Surely, though, the real backlash against big families is about resource-guzzling; on a planet where there isn't enough of everything we need, the argument is that it's fairer for families to limit themselves to replacement, rather than growth?

"My counterblast to those who are worried about the future is this: what better way of generating the future do we have than by having children to send into it?" he says. "And when it comes to housing, and the perceived lack of homes, the truth is that the crisis isn't about the people who want to live in big families, it's people who want to live alone."

A century ago, says Brazier, it was commonly believed that to be an only child was to be disadvantaged but that has changed completely. "These days when you talk to people about the 'problem child', you're more likely to find yourself talking about middle children – they're the ones who are seen as having the biggest hurdles to get across."

Brazier knows he's treading a fine line. "I don't want to demonise people who've only got one child," he says. "I'm very aware of how sensitive what I'm saying is, particularly given how many children I've got. But my big fear is that we're moving towards a time when the multi-child family isn't going to be tolerated, and I think if we let that happen we'll be ignoring the huge value that siblings bring."

Why siblings are good for you

• According to a 2004 study in Ohio, nursery-age children with siblings get into fewer fights, make friends more quickly and keep them for longer.

• Research into GCSE results in 2001 found that the elder sibling in a two-child family tended to do better than an only child by 10% in the English exam, but by 25% in maths.

• In a US survey of 7,000 adults in 1992, two-thirds said "sibling" when asked who they would call in an emergency, needed to borrow money, or were depressed or confused and needed advice.

• A study in 2006 that looked at asthma, eczema and hayfever concluded there was "a robust inverse association between sibship size and allergic disease".

• One US study found the odds for obesity dropped by 14% for each additional sibling in the household.

Source: Sticking up for Siblings