People bullied by their siblings during childhood are up to three times more likely to develop a psychotic illness.
A study suggests children picked on by their brothers and sisters end up traumatised as adults.
Eldest children are the most likely bullies within families, with girls most likely to be victims. Parents often believe the behaviour is normal and that their children will outgrow it.
But researchers led by Warwick University say being bullied at home means there is no ‘safe space’ to escape the torment.
Children who fall victim to sibling bullying several times a week or month are two to three times more likely to develop a psychotic disorder such as schizophrenia compared to other children.
For those bullied both at home and in school, their odds are four times as high.
The findings are based on analysis of almost 3,600 British children questioned about bullying aged 12 and about psychotic symptoms aged 18.
Senior author Professor Dieter Wolke, from the University of Warwick’s Department of Psychology, said: ‘Bullying by siblings has been until recently widely ignored as a trauma that may lead to serious mental health problems such as psychotic disorder.
‘Children spend substantial time with their siblings in the confinement of their family home and if bullied and excluded, this can lead to social defeat and self-blame and serious mental health disorder – as shown here for the first time.’
Lead author Slava Dantchev, also from the University of Warwick, added: ‘If the bullying occurs at home and at school the risk for psychotic disorder is even higher. These adolescents have no safe place.’
Previous evidence shows children bullied several times a week by their siblings in middle childhood are twice as likely to suffer depression and self-harm.
But the latest study is the first to look at psychotic symptoms, which include hallucinations and delusional thinking.
WHAT IS SCHIZOPHRENIA?
Figures suggest around 1 per cent of the world population suffer with schizophrenia, with 220,000 diagnosed in England and Wales and around two million in the US.
Seven in ten hear voices at some point, making auditory hallucinations of the most common symptoms.
These voices, may be ‘heard’ as having a variety of different characteristics, for example as friendly or threatening.
Hearing voices – known as ‘verbal hallucinations’ – is highly distressing and a third of patients do not respond to medications.
Children were asked how often brothers and sisters bullied them, by saying ‘nasty and hurtful things’, kicking, pushing or shoving them and telling lies about them.
Of the teenagers looked at by researchers, 664 were victims of sibling bullying, 486 bullied their siblings, and 771 children were both a bully and a victim of bullying within their family. By the age of 18, 55 had developed a psychotic disorder.
The more youngsters were involved in sibling bullying, as victims or bullies, the more likely they were to develop a psychotic disorder. But the victims, and those who were both victims and bullies, had the highest risk.
The study, published in the journal Psychological Medicine, states that childhood trauma creates a lasting ‘cognitive vulnerability’ in the brain, with bullying found to make people more sensitive to stress.
Children who are bullied at home are more often bullied by their friends, and are more likely to repress or act out their feelings.
The researchers state that ‘sibling aggression is the most common form of family violence’, adding: ‘Nevertheless, parents and health professionals continue to perceive aggression between siblings as benign and normative behaviour that children will outgrow.’
They conclude that parents should be made aware of the long-term mental health consequences that sibling bullying may have – and that interventions must be developed in order to reduce and even prevent this form of aggression within families.