Children who are bullied by siblings several times a week in early adolescence are twice as likely to become clinically depressed as young adults, according to a new study published in the journal Pediatrics. The bullied kids are also twice as likely to report self-harm compared to kids who were not bullied by siblings.

The study, conducted by researchers at the Universities of Oxford, Warwick and Bristol, and University College London, is the first to investigate the connection between sibling bullying and clinical depression and self-harm in young adults.

“Forms of bullying where victims are shoved around the playground or targeted at work have been well documented, however, this study uncovers a largely hidden form of bullying. Victims of sibling bullying are offered little escape as sibling relationships endure throughout development,” said lead author Dr. Lucy Bowes, from the Department of Social Policy and Intervention at the University of Oxford.

“We are not talking about the sort of teasing that often goes on within families, but incidents that occur several times a week, in which victims are ignored by their brothers or sisters, or are subjected to verbal or physical violence.”

Participants were the children of women enrolled in the Avon Longitudinal Study of Parents and Children (ALSPAC) in the 1990s.

At the age of 12, nearly 7,000 children completed questionnaires about whether they had experienced any form of sibling bullying and if so, how often it occurred. The same children were followed up at the age of 18 years.

Of the 3,452 children who provided data on both sibling bullying and mental health, 1,810 said they had not been bullied by a brother or sister. Of these, 6.4 percent had depression scores in the clinically significant range, 9.3 percent experienced anxiety and 7.6 percent had self-harmed in the previous year. Of the 786 children who said they had been bullied by a sibling several times a week, clinical depression was reported by 12.3 percent, 14 percent had self-harmed in the previous year and 16 percent of them reported anxiety.

Victims were more likely to be girls, and bullying was more common in families with three or more children. Older brothers were often the perpetrators.

On average, victims reported that sibling bullying had started at the age of eight. The link between being bullied by their siblings as a child and later mental health disorders was found to be similar for both boys and girls.

“Social learning and how to behave with peers starts at home, and when siblings are bullied it can have serious long-term consequences, as we found in our study. It is important that parents set clear rules about what is allowed in conflicts and they should intervene consistently when their children maltreat each other repeatedly,” said co-author Dr. Dieter Wolke, from the Department of Psychology and Division of Mental Health and Wellbeing at the University of Warwick.

Children who said they had been frequently bullied by siblings were more likely to report increased feelings of anxiety. However, anxiety was not found to be a significant effect after individual and family characteristics had been taken into account.

“Even though we cannot be certain that this relationship is causal, we think it likely that interventions to reduce sibling bullying would improve the mental health in the longer term,” said co-author Glyn Lewis, Ph.D., from the Division of Psychiatry at UCL.

This article first appeared ‘Psych Central’ on 20 September 2014.