Sibling squabbling is a rite of passage. Brothers and sisters argue; they roughhouse; they steal each other's clothes and toys.
But a provocative new study claims that even mild acts of sibling aggression are just as detrimental as the bullying that many children face at school.
"Historically [sibling bullying] has been accepted as something that's normal, as something that's benign. Oftentimes it's just dismissed," study author Corrina Jenkins Tucker, an associate professor of family studies at the University of New Hampshire, told The Huffington Post. "Some people actually view it as a good thing, thinking it teaches kids how to fight and develop conflict resolution skills."
The new study, published online Monday in the journal Pediatrics, suggests otherwise.
In telephone interviews, researchers asked more than 3,500 children 17 and younger, or their caregivers, whether they'd experienced at least one act of sibling aggression in the past year. It could include psychological bullying, property damage, physical fighting (with no weapons that resulted in no injuries), and more serious physical assault.
Researchers questioned participants about feelings of depression, anxiety and anger. Children who experienced even just one, relatively mild act of sibling aggression in the past year reported greater mental health distress than those who had not. Kids aged 9 and under were more distressed after experiencing physical aggression than their teenage counterparts, but all age groups were equally affected by other forms of bullying.
The new study is the first major investigation of the consequences of sibling aggression, which is generally considered less serious or harmful than the peer bullying that children may face online or in school.
"This is an extremely important study for parents, psychologists, and pediatricians," said Susan Swearer, who did not work on the study, but is a professor of school psychology at the University of Nebraska-Lincoln and co-director of the Bullying Research Network. "The findings clearly connect sibling aggression with youth mental health difficulties."
The difficulty for parents and health care providers is distinguishing between "normal" fighting among children who are growing up together under one roof, and something more serious.
"This study examined sibling aggression [but] not sibling bullying, per se," said Amanda Nickerson, director of the Alberti Center for Bullying Abuse Prevention at the University at Buffalo, who did not work on the new research. "To assess bullying, it is important to assess the intent of the aggression -- was it done to cause harm? -- the repetition of it and the power imbalance between the perpetrator and the victim."
For example, Nickerson said, it is common for brothers and sisters to argue over who gets what items and privileges, such as picking TV shows, as well as for them to annoy each other. However, if a child's aggression is aimed at intentionally hurting a sibling physically or psychologically, then it is different. Repeated behaviors are also a red flag, Nickerson added.
The Centers for Disease Control and Prevention has identified bullying as a major public health concern, citing estimates that suggest nearly 30 percent of adolescents in the U.S. have either been a bully, been the victim of bullying, or both. Children who are bullied are at greater risk for depression and anxiety that may last into adulthood, as well as lower academic scores and broader health complaints. Being a bully also carries health risks: Children who are the aggressors are more likely to abuse alcohol or drugs as they get older, engage in early sexual activity and abuse their partners or children.
Jenkins Tucker and her colleagues hope to next explore how long the mental health effects last and whether patterns vary according to birth order or gender.
But outside experts hail the new study as an important first step.
"This study is the first to unequivocally show that sibling aggression is connected to mental health problems among youth," Swearer said. "In order to effectively treat mental health problems in youth, parents and mental health providers must recognize and understand the role that sibling aggression plays."