"No wonder sibling rivalry exists. By having another child, you knock your eldest one straight off their perch. Meanwhile, the younger one always has the example of someone bigger, faster and ahead of them. Why would they not hate each other?" she says.
It's not all bad, of course. But we all know the scenario – one day, they get on brilliantly, the next they're at each other's throats. So should you be worried? Can you prevent sibling rivalry? And, strange as it sounds, could it ever be a good thing?
There's no doubt that sibling rivalry can be horrible to watch as a parent. But it's important to remember that it's natural and it's therefore unhelpful to have unrealistic expectations, says chartered psychologist Mandy Bryon.
"Children naturally act to meet their own needs. So the best thing parents can do is to role model thoughtful behaviour and sharing. Other simple things you can do include things like establishing a rota of turn taking for things like TV programme choice."
Make time for each of your children separately and focus on what makes them unique as children, she says. And think particularly carefully about how you, as parents, communicate with each other and how you act when you're cross or frustrated.
"I think parents underestimate how much children copy their behaviour. On the positive side, if children see their parents supporting one another in all sorts of ways, they get the idea this is normal behaviour."
Watch out for common triggers to bickering. Children who are hungry, bored or tired are more likely to become frustrated and start fights. And children who don't know the positive ways to get attention from their parents or haven't learned how to start playful activities with a sibling may also be more likely to resort to picking a fight.
These are things parents can do something about. Don't forget to praise your children when they play nicely too, letting them know you appreciate the effort they're making to get along.
When sibling rivalry does happen, your instinct will most likely be to step in and break it up. Stop yourself.
Bryon says: "If it's a question of one child over another, then as long as no one is being hurt, walk away and say, 'Sort it out yourselves.' They will learn good social negotiation skills from each other – research confirms this."
This can prepare them for important relationships when they are older, says Hayman. "Sibling relationships teach children a huge amount about sharing, co-operating, empathy and communication."
Of course there are times you will need to intervene. Hayman advises parents to consider saying, "I'll give you 10 minutes to sort it out, otherwise I'm stepping in." "You'll often find that children come up with fair solutions under those circumstances," she explains.
If you do have to step in, don't always assume the older one is the bully and don't focus entirely on telling off the child who appears to be more in the wrong, she says.
"The one who finished it isn't always the one who started it. Even if it is clear who is in the wrong, it's wise not to focus all your energy in telling them off because it means they get all the attention and may wind up fighting with their sister or brother just to gain more of it. It's much better to gather up the child who is wronged, then go back to the other one and say, 'I don't like that behaviour' and deal with it from there."
If one of your children has strong feelings about one of their siblings, let them express these feelings, advises Hayman. "It's so easy, as a parent, to tell them, 'Don't say that, he's your brother, so please just try to get on'. Your child's words might give you some clues as to how you can minimise sibling rivalry in the future."
You might discover their perception is that their sister really is mean and spiteful much of the time; perhaps they feel their brother gets all of the attention. "If you don't hear what your children tell you about their problems with their siblings, your attempts at leaving them to sort out their battles may be interpreted by them as you not caring," she says.
Parents can inadvertently encourage sibling rivalry in other ways, says Dorothy Rowe, psychologist and author of My Dearest Enemy, My Dearest Friend: Making and Breaking Sibling Bonds (Routledge). "It's classic for parents to say things like, 'Why can't you be more like your sister? You don't see her getting down from the table in the middle of lunch. The problem is that it's a sure way of creating lasting sibling conflict."
The main thing is not to think you can eradicate sibling rivalry altogether, says Judy Dunn, professor of development psychology at the Institute of Psychiatry. "Siblings wind up knowing each other so well that they understand exactly what will annoy the other. Sometimes that can be just teasing, but it can turn more aggressive.
"But all parents need to realise is how common this is. Particularly parents who were themselves only children or who remember only the positives of their sibling relationship in childhood, it can feel like the end of the world to see their own children setting out to get at each other, but it really isn't.
"Siblings can be extremely argumentative in the early years, and things usually smooth out later on when they get friends outside the home and their lives are no longer dominated by this sibling."
Indeed, the chances are your children's relationship will develop into a close one. "In middle adulthood, siblings can be a real source of comfort, especially when breakdowns occur in adult relationships," says Dunn – and studies show that 80VIRTUAL-Gallery-169521VIRTUAL-SkimlinksPromo%
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