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Children with multiple siblings are more likely to be bullied than those with just one. But there's a reason why firstborns and older brothers tend to pick on their younger siblings. The problem is, there are long-term consequences, researches say.

SALT LAKE CITY — In families with more than two kids, children are more likely to be bullied by a sibling than in other families — and they're also more likely to be bullies themselves. According to new research from the University of Warwick, just published in Developmental Psychology, it's all about supply and demand — and status.

The "supply" is parental affection and attention, as well as material resources that can thin as they are spread out among more children. The "demand" for that attention arises most often from older siblings, particularly older brothers, who don't want to lose what they have or to share it with younger siblings. Nor do they want to lose the dominant status they have for being older and not having many siblings.

And while some parents tend to view siblings picking on each other as a natural part of childhood, evidence is growing that being bullied has long-term negative impacts, such as loneliness and behavioral and mental health problems later in life, according to lead author Dieter Wolke, professor of developmental psychology and individual differences at the University of Warwick in Coventry, United Kingdom.

Wolke was among researchers who reported in an earlier study in Pediatrics that "being bullied by a sibling is a potential risk factor for depression and self-harm in early adulthood. Our results suggest that interventions designed to target sibling bullying should be devised and evaluated."

The study said being bullied by a sibling is associated with double "the odds of both future depression and self-harm at 18 years."

When researchers previously "looked at the effects on later mental health and risk behavior," Wolke said, "we found a dose response: the more often victimized, the worse the adverse effects."

" Siblings fight about the resources that are available, whether attention or food in poorer households or the new smartphone in rich families. "
Dieter Wolke, professor at the University of Warwick

Other studies also point out the dangers of sibling bullying.

"Being targeted and bullied by your siblings may lead to more severe psychological harm than being bullied by peers alone, and that this type of bullying uniquely predicts these outcomes even when traditional peer bullying is taken into account," wrote Christine Malecki, director of the School Psychology Program at Northern Illinois University, and Michelle K. Demaray, a professor there, in a 2014 Psychology Today post.

"It appears that sibling bullying is cause for concern and is not just 'kids being kids,'” they wrote.

Family roles

Still, some parents see conflict between their children as a natural part of negotiating childhood or an example of sibling rivalry. They may consider it benign or inevitable when it is neither, experts say, warning that sibling bullying is a problem that needs to be addressed.

"If normal sibling squabbles become truly mean-spirited, one-sided, unprovoked, and incessant, especially if from the stronger, bigger or older child, then it may be time to step in. Oftentimes, the bully is feeling deprived of love, attention, acceptance and support. If this isn’t the case, perhaps it may be worthwhile to get some outside help," suggests Susan G. Groner, author of "Parenting: 101 Ways to Rock Your World" and founder of The Parenting Mentor.

Few studies examine this issue, said Wolke, who hopes the new study will help educate families about preventing bullying and raise awareness "that firstborns are fighting resource loss and how to counter this."

A solo child gets all the parent-offspring attention at first, but typically finds that attention at least halved with the birth of a subsequent child. If parents have more children, that means an even smaller share of the attention and resources craved. It can kick into action what Wolke and others refer to as the "selfish gene."

" It is important to understand that bullying occurs in situations where we cannot choose our peers and have to get on somehow. "
Dieter Wolke, professor at the University of Warwick

Other important factors include the presence of early conflict in the sibling relationship, tendencies to low self-esteem or aggression and parenting differences, but the effects were smaller than that created simply by virtue of having more than one sibling, Wolke said.

What didn't make a difference was poverty or wealth or whether kids were in a two-parent or single-parent household. Kids in families of all types bully and are bullied.

"Siblings fight about the resources that are available, whether attention or food in poorer households or the new smartphone in rich families," Wolke said. "It is about the relative dominance within the family."

Hard to escape

While children who are bullied by peers rather than siblings may be able to get away from the torment, it's trickier when the bullying occurs within families.

"It is important to understand that bullying occurs in situations where we cannot choose our peers and have to get on somehow," Wolke said. "The prime example is the family. Siblings are caged; they cannot choose or ask their brother or sister to move out. They also live in close quarters and the familiarity allows (the bully) to know which buttons to press to upset the sibling and how to manipulate the parents."

" Sibling bullying can be in extremes highly abusive, and our research raises the awareness that it is not a normal rite of passage to be tormented by a sibling. "
Dieter Wolke, professor at the University of Warwick
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The research showed that bullying increased as the siblings spent more time together, he noted. And unlike peer bullying, when it comes to sibling bullying the largest group of children were those who were both bully and victim. So while the older sibling may bully his little brother or sister, that younger child also knows "how to upset the older by interrupting" when the big brother's pals are over, or by hiding clothes and doing other things, he said.

Wolke said the new findings could be helpful on two fronts: While most research into mental health problems looks at parenting, genetics or social issues like poverty, sibling influence isn't typically considered.

"Sibling bullying can be in extremes highly abusive, and our research raises the awareness that it is not a normal rite of passage to be tormented by a sibling," he said.