Viewpoint: Is it better for children to have siblings?
There are frequent warnings about how expensive it is to have a child in the UK, but father-of-six and Sticking Up For Siblings author Colin Brazier says scare stories shouldn't detract from the benefits of children having siblings.
Financial service providers - and their press offices - like to treat us to bulletins about the cost of raising a child.
There are several studies each year and they peddle catchy acronyms like Cots (the Cost Of The Sibling).
But it would be naive to assume the authors of these reports are entirely disinterested. Most PR folk in the City are there to plug a product.
In recent years Cots estimates have been running well ahead of inflation. Recently the figure has been put around the £250,000 mark.
So I was relieved this week to see that the Child Poverty Action Group, in tandem with the Joseph Rowntree Foundation, had arrived at an estimate of £150,000. While still eye-wateringly high, it lacks the contraceptive impact of those quarter-of-a-million-pounds scare stories.
I say scare stories advisedly. Because all these studies usually ignore the way costs fall as family size rises.
There are economies of scale to offspring. It will set you back the same to run a bath for one child as it does for three. Children share heat and light, bedrooms and toys. Clothes are handed down.
Sibling discounts can be modest (hotel rooms), or worth thousands (private school fees). An older brother or sister can actually make childhood cheaper, by babysitting in loco parentis or reducing the need for a play-date or expensive excursion.
I've set out to recalibrate the cost/benefit analysis of family expansion, taking not as its starting point the fact that large families are a good thing, but the argument that simply giving an only child a sibling is demonstrably "worth it".
That assertion is the product of five years of work, much of it with Swedish researcher Therese Wallin. We found strong and, at times, compelling evidence, that belonging to a multi-child family has benefits.
Having a sibling while growing up can help a child resist allergies, obesity, and depression.
On obesity, there is a mountain of literature showing that siblings help increase energy expenditure (they run around a lot) and robust circumstantial proof that "sibship" - having a brother or sister or multiples thereof - reduces calorific intake (an only child may eat more simply by dint of sitting down to eat with adults who dole out bigger portions).
Obviously the world is full of skinny only children and my new book deals, I am not ashamed to say, in massive generalisations. There are many exceptions, but "sibling science" does a good job of making all other things equal by controlling for things like socio-economic background.
On mental health, the findings are not the stuff of rocket science, but they do demolish the relativistic idea that there is no difference in outcomes for a child who does or does not have a sibling. There are, sometimes, advantages to a sibling-free childhood, but dealing with family crises - such as parental divorce or death - without a brother or sister, is not among them.
And on allergies the research is more emphatic still. If you grow up with a sibling you stand a better chance of avoiding eczema-urticaria, hay fever and, quite possibly, asthma.
Auto-immune disorders are strongly inversely-related to sibship. Studies have shown that siblinghood has a little-understood deterrent effect against conditions as varied as multiple sclerosis and Crohn's Disease. The most likely explanation is the Hygiene Hypothesis, which posits that our body's defences are primed by early exposure to germs, of which siblings provide highly efficient carriers.
A sibling, of course, is for life not just for childhood. The policy debate about eldercare, and its colossal mounting cost, does not recognise the importance of siblings. Older sisters, in particular, save the country millions. Looking after an elderly parent is easier if the role is shared between siblings.
Less obviously, declining sibship may have a huge societal impact far beyond the usual demographic extrapolations. This is because of the pecking order - the codification of character according to where a child arrives in the family.
Birth order theory shows that certain traits apply to eldest, middle and youngest children. A middle child has a better chance of staying married, for instance, or securing a job in a "creative" industry. That is because children with senior siblings often cannot coerce older offspring into submission. So they learn to be natural diplomats, communicators or comics. Without the iron fist, they rely on the silver tongue.
As the number of only children in the UK rises from a fifth of all children born to a quarter - in the space of a generation - we are entitled to ask what impact this will have, from the boardroom to the bedroom. Will a workforce with fewer middle children become structurally weaker?
Might a state with more only children, whose parents may be more reluctant to see them risk life and limb, find it harder to wage war?
And then there is impact of falling sibship on parents. On current trends my twilight years are likely to be spent in a country - the UK - where a third of all children have no siblings. What will it be like being one of those parents of an only child?
Sibship can be the antidote to helicopter parenting. Children who teach each other how to take risks, also help with language skills and social capital. Much else besides too. A snip at £150,000 or less.