Sibling relationships can be difficult, and never more so than in childhood. But society often regards the scrapping and squabbling, the play fighting and not-so-playful fighting as a normal part of growing up.
"The public brushes off aggression between siblings as just rivalry," says Corinna Tucker of the Carsye Institute, University of New Hampshire.
Tucker is the lead author of a new study on the issue for the journal Pediatrics. Almost a third of the 3,600 children questioned said they had been the victim of some sort of sibling aggression in the past 12 months.
This included a range of acts from theft and psychological abuse to physical assault, either mild or severe. In comparison, research suggests that up to a quarter of children are victims of schoolyard aggression every year.
This is defined by experts as intentional acts of aggression, repeated over a period of time, where an individual or group is in a position of power over someone.
So sibling relationships would seem the perfect breeding ground for bullying, since children live together for a long period of time and there is usually an intellectual and physical power imbalance.
Although there might not be an outright malevolence, there is often reason for jealousy.
"A sibling relationship is emotionally intense - it's one of those relationships where you can love them and you can hate them at the same time," says Tucker.
"And siblings are natural competitors for family resources and parents' attention."
Laura - who grew up in a house she shared with four siblings and a foster brother - did not consider it bullying at the time, but now thinks the term captures what went on in their house in Ohio.
"My older brother - I would say he beat up on all of us," she says, her voice breaking. Although he never hurt them badly, he liked to wrestle his sisters, pinning them to the floor.
"He was bigger and stronger than us - he could put us in very powerless situations. It was really scary."
For her brother, she believes, the activity gave him a momentary sense of control at a chaotic time for the family. Her mother, who had depression, had left home.
The children were all particularly mean to the youngest sister Tracy who had been the focus of their mother's attention. In the afternoons, after school and before their father came home from work, they would tease Tracy until she phoned her mother to try to get them to stop.
"I remember us saying to her: 'Mum can't do anything about it - she isn't coming here,'" recalls Laura. "And my dad - I think he was so overwhelmed that he only ended up dealing with the most serious things."
Her father later told Laura that he had sent her foster brother back to the orphanage because he was being bullied so much by Laura's middle sisters, a pair of twins.
"If you only have sibling bullying or school bullying you are about 2.7 times more likely to have behaviour problems, but if you have both then it's 14 times more likely," says Wolke. "And the point about this is that you don't have any escape. It's 24/7."
Little is known about the long-term effects of sibling bullying but Jan Parker, a family psychotherapist and a co-author of Raising Happy Brothers and Sisters, is certain that they can be "serious and lifelong".
"We're very familiar with discussions on the impact of [school] bullying," she says. "I don't think it takes a great imaginative leap to understand the impact of it happening within your own home. You feel that you are not only let down by the person bullying you, but also by the adults in your life that are letting it go unchecked."
While sibling fights are normal, parents need to be aware of a pattern developing over time, she says. Parents are advised to talk to the children about what's happening. If they are worried, they could speak to a health professional.
They should also consider what might be causing the problems. "There is one known predictor - it's when siblings perceive there to be a wide disparity between how they are treated and how their brother and sister are treated," she says.
Robin Kowalski spoke to Health Check on the BBC World Service - listen again to the programme on iPlayer or get the Health Check podcast.
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