Having more than one brother or sister may put a child at the risk of sibling bullying, with firstborn kids and older brothers more likely to be the perpetrators, according to a study.
"Sibling bullying is the most frequent form of family violence and it is often seen as a normal part of growing up by parents and health professionals, but there is increasing evidence that it can have long-term consequences, like increased loneliness, delinquency and mental health problems," said Dieter Wolke from the University of Warwick in the UK.
The study, published in the journal Developmental Psychology, analysed data from 6,838 British children born in either 1991 or 1992 and their mothers.
The kids were put into four categories: victims, bully victims -- defined as being both a perpetrator and victim of bullying -- bullies or uninvolved.
When the children were five years old, their mothers reported how often the children were victims or perpetrators of bullying in the household.
Sibling relations were analysed two years later when the mothers were asked how much time the children spent engaging with their siblings on various activities, such as crafts or drawing.
Several years later, at age 12, the children reported if they had been bullied by a sibling or if they had bullied a sibling within the previous six months.
The boys and girls were also asked their ages when they first experienced sibling bullying and when they first bullied a sibling. About 28 per cent of the children in the study were involved in sibling bullying and psychological abuse was the most common form.
The majority of those children were found to be bully victims, meaning they bullied and were bullied, according to the study.
"Bullying occurs in situations where we cannot choose our peers, like in families," said Wolke.
"Siblings live in close quarters and the familiarity allows them to know what buttons to press to upset their brothers or sisters. This can go both ways and allows a child to be both a victim and a perpetrator of bullying," Wolke said.
Family structure and gender were the strongest predictors of sibling bullying by middle childhood, according to the researchers.
"Bullying was more likely to occur in families with three or more children and the eldest child or older brothers were more often the bullies," said Slava Dantchev, also from the University of Warwick.
"Female children and younger children were more often targeted," he said.
Researchers believe bullying can happen in larger families because resources such as parental affection or attention and material goods are more limited.
"Despite our cultural differences, humans are still very biologically driven. A firstborn child will have their resources halved with the birth of a sibling, and even more so as more siblings are added to the family," said Wolke.
"This causes siblings to fight for those limited resources through dominance," he said.
Although the researchers investigated whether marital and socioeconomic status would be associated with more or less bullying, they did not find any evidence. "Sibling bullying does not discriminate. It occurs in wealthy families just as much as lower-income families and it occurs in single-parent households just as much as two-parent households," said Wolke.