I’ve been thinking a lot about siblings this week because of an important anniversary in our family. ‘What do you like about having siblings?’ I asked Molly the other day. ‘Having someone to tease,’ she said. ‘And when I was little there was always someone to poke on car journeys.’

Over the years there’s been a lot of research into parent-child relationships but sibling relationships have only attracted serious interest more recently. And yet they’re clearly very important. Psychologist Daniel Shaw put it well:
“Parents serve the same big-picture role as doctors on grand rounds. Siblings are like the nurses on the ward. They’re there every day.”

There are benefits to being stuck with siblings. Constant arguments make for choppy family life so you have to negotiate. A Canadian study found that on average, siblings aged two to four years, have some kind of conflict at least once every ten minutes. As they get older the conflicts usually get less frequent—but things don’t always work out well.  Noel and Liam Gallagher are famously distanced brothers. When asked about an Oasis reunion, Noel said that “It won’t ever happen unless they do it without me,” adding modesty, “but without me it would be rubbish.”

My four children squabbled a great deal when they were little but I’m relieved to find that they get on well as adults. Past offences are mostly forgiven, though they’re not all forgotten. Molly is the youngest and once said wistfully, ‘All the accidents I had when I was little were Henry’s fault.’ She then reeled off a list of episodes that I’d pushed conveniently to the back of my mind. Suddenly they all came rushing back. The ‘being up a tree incident’ didn’t end well nor did the time he pulled her out of a bunk bed on a barge. And there was the occasion when he told her to ride ‘no hands’ on her bike as she whizzed down a hill for the very first time.

In amongst all the falling out and making up, siblings are in a unique position to provide support when there’s family trauma. They can often appreciate what their brothers and sisters are going through in a way that no-one else is able to do. But not everyone has a sibling. Average family size is gradually shrinking and in the UK it’s increasingly common for parents to have just one child. As with all human activity people are quick to try to argue about which situation is best. But this seems pointless. Not everyone has a choice about the number of children they have and those parents who actively choose to have one child do so for their own particular pragmatic or economic reasons. And there are advantages to being an only child like getting undivided parental attention and more exposure to adults. You also learn from an early age how to be happy in your own company.

I feel as though I have a foot in both camps as my upbringing was similar to that of an only child but with some sibling benefits. My sister is twelve and a half years older than me so I didn’t get the experience of squabbling, sharing secrets, or swapping make-up. Instead she was like a second mother and had more formative influences on me than anyone else.

There’s been a lot of speculation about the effects of birth order ever since Adler linked this to personality in 1928. Oldest children are popularly believed to be assertive and conformist while youngest ones are rebellious and adventurous. But this is not substantiated by research findings. It’s possible that what are interpreted by parents as personality traits, are in fact an effect of age rather than of birth order. At any point in time, the older child has simply had more experience of life and so will be seen as different from their younger sibling. It’s hard to view them equally. It seems logical that birth order affects each individual within their particular family situation, but at the moment there’s no evidence to suggest that you can extrapolate this more broadly and call it a personality trait rather than a behaviour pattern.

These sibling behaviour patterns are deeply rooted. I know a family where the brother never married and the three sisters all outlived their husbands. In their seventies when they were all single once more, the four siblings went on holiday together. I was amused to hear one of their daughters recount with exasperation how they reverted to the same kinds of squabbles that they’d had as children.

Throughout life it’s easy to slip into old familiar roles within your family group even though you might behave quite differently in the outside world. Emma’s flatmate went to visit her younger sister who had just spent her first term at Cambridge. This younger sister was a successful student and managed her life perfectly well. But as they walked and walked and walked, the visiting older sibling eventually asked where they were going as she’d never been to Cambridge before. Her younger sister looked astonished and said, ‘I’ve no idea, I was following you.’ I can relate to that. For as long as I can remember, a defining role for me has been as a younger sister and I slot into that easily. I’ve turned to Bonnie when life has been difficult and feel fortunate that she’s always been there.

I started this post by mentioning an important family anniversary. Happy Special Birthday to Bonnie. I may be quite grown-up most of the time but just for you, I’m happy to be a little sister.