Led by Professor Dieter Wolke (senior author) at Warwick's Department of Psychology, this is the first study to explore the relationship between sibling bullying and the development of psychotic disorders.

Almost 3,600 children from the Avon Longitudinal Study of Parents and Children completed a detailed questionnaire on sibling bullying at twelve years of age, and then subsequently filled out a standardized clinical examination assessing psychotic symptoms when they were eighteen years old.

Of the adolescents, 664 were victims of sibling bullying, 486 children were pure bullies to their siblings and 771 children were bully-victims (victimized by siblings and bullied their siblings), at age twelve.

Fifty-five of the total 3600 children in the study had developed a psychotic disorder by the age of eighteen.

The researchers found that the more frequently children are involved in sibling bullying -- either as bully, victim, or both -- the more likely they are to develop a psychotic disorder.

Those involved in sibling bulling (as bully or victim) several times a week or month are two to three times more likely to develop a psychotic disorder than other kids.

The children most at risk are victims of sibling bullying, and those who both become victims and bully their siblings (bully-victims).

Children who are victimised both at home and by school peers are even worse off -- being four times more likely to develop psychotic disorders than those not involved in bullying at all.

Senior author Professor Dieter Wolke from the University of Warwick's Department of Psychology, commented:

"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."

First author Slava Dantchev, from the University of Warwick, adds:

"If the bullying occurs at home and at school the risk for psychotic disorder is even higher. These adolescents have no safe place.

"Although we controlled for many pre-existing mental health and social factors, it cannot be excluded that the social relationship problems may be early signs of developing serious mental health problems rather than their cause."

The researchers conclude that parents and health professionals 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.

Psychotic disorders -- such as schizophrenia or bipolar disorder -- cause abnormal thoughts and perceptions, and often involve hallucinations or delusions. Sufferers often experience severe distress and changes in behaviour and mood and have a much increased risk of suicide and health problems.