A new study has found that children who revealed they had been bullied by their brothers or sisters several times a week or more during early adolescence were twice as likely to report being clinically depressed as young adults.
They were also twice as likely to say they had self-harmed within the previous year compared with those who had not been bullied.
The findings, published in the journal, Pediatrics, are the results of the first longitudinal study to investigate possible links between sibling bullying and clinical depression and self-harm in young adults.
The research, conducted by the Universities of Oxford, Warwick, Bristol and UCL, suggests interventions are needed to specifically target a form of bullying which it says, to date, has been largely ignored by academics, policy makers and clinicians.
Lead author Dr Lucy Bowes, from the Department of Social Policy and Intervention at the University of Oxford, said: '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.
'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.'
Children of women who enrolled in the Avon Longitudinal Study of Parents and Children (ALSPAC) in the 1990s took part in the research. Nearly 7,000 children, aged 12, completed postal questionnaires in 2003-4 about whether they had experienced any form of sibling bullying and if so, how often it happened.
The same children were followed up at the age of 18 years, so their mental health could be assessed using a validated online questionnaire, known as the Clinical Interview Schedule. The teenagers attended a clinic to fill in the questionnaire that asked them about their feelings and any recent self-harming behaviour.
Of the 3,452 children with information on both sibling bullying and psychiatric outcomes, 1,810 said they had not been bullied by a brother or sister. Of these, 6.4% had depression scores in the clinically significant range, 9.3% experienced anxiety and 7.6% 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%, 14% had self-harmed in the previous year and 16% of them reported anxiety.
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.
Victims were more likely to be girls than boys, with this form of bullying more common in families where there were three or more children. Older brothers were often the perpetrators. On average, victims reported that sibling bullying had started at the age of eight.
Co-author Professor Dieter Wolke, from the Department of Psychology and Division of Mental Health and Wellbeing at the University of Warwick, said: '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.'
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.
Co-author Professor Glyn Lewis from the Division of Psychiatry, Faculty of Brain Sciences, UCL, said: '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.'
The study highlights existing programmes in the United States that specifically deal with relationship problems between brothers and sisters more broadly. It suggests these programmes should be assessed systematically to see whether they can reduce sibling bullying thereby reducing the potential psychological damage.
BeatBullying, a bullying prevention charity, welcomed the findings. Its CEO and founder, Emma Jane Cross, said: 'Being bullied as a child can have a devastating effect which lasts a lifetime. Parents who are concerned about this issue should speak to their children as early as possible before the problem escalates. It's important to tackle the underlying issues behind more frequent bullying behaviour rather than dismissing it as normal sibling rivalry.'