These actions might be physical (hitting or kicking for example), verbal (name calling for example) or relational (social exclusion for example). Research suggests between 10 per cent to 50 per cent of children are exposed to bullying. Bullying has long term consequences for children, particularly impacting on their mental health: behavioural problems, depression, suicide ideation and eating disorders are some of the observed outcomes. Children who are targets of bullying are more likely to engage in delinquent behaviours in their teenage years.

Adults who are bullied (for example at work) are more likely to find parenting stressful and more likely to behave more aggressively towards their children.

Bullying creates social stress and stress is well known to change the molecular configuration around the DNA, impacting on its functionality in ways that increase risks for negative outcomes. In a recent study in the Netherlands, researchers found that being the target of bullying was associated with changes in DNA methylation. In DNA methylation a methyl-group binds to a cytosine nucleotide of the DNA (cytosine-phosphate-guanine site or CpG site).

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Children who have been exposed to bullying were found to have variance in their DNA methylation at a CpG site though the results were not altogether clear; there was some thought that the recency of the bullying might play a role in DNA methylation.

Many studies of bullying focus on the school years but there is strong evidence that young children are exposed to bullying. Younger children who bully tend to demonstrate poor social skills, and show limits in their co-operation and ability to play with others. Young children who are targets of bullying before school age tend to be more lonely as they grow up and demonstrate longer term behavioural problems. Research suggests bullying at this age is about social dominance, the use of aggressive tactics to gain control of resources and/or alliances. Aggression may be physical but it may also be relational (promising friendship in exchange for co-operation for example).

The social and interactional skills children learn in their sibling relationships impact on those they form with their peers and these lay the foundation for later life relationships. Children with warm and caring sibling relationships tend to have similar peer relationships.

Children who are bullied by their sibling(s) are also more likely to be bullied by their peers. Older adults who were targeted by their siblings are more likely to be targeted in elder abuse. Mothers who were targeted by their siblings are more likely to experience anxiety and depression, which impacts on their ability to parent their own children.

Research suggests that sibling bullying is linked to a range of family characteristics (birth order, ethnicity and number of siblings) but children's individual characteristics (emotional dysregulation and gender) also play a role. Parental factors such as parental self esteem and harsh parenting have an impact but not a particularly strong one.

However, family relationships are believed to create a model of how to manage interpersonal situations. In support of this, a recent study demonstrated that children who are exposed to hostile family relationships, family violence or maltreatment were more likely to model these behaviours outside the family.

Inconsistent or harsh parenting was found to link to sibling targeting. Sibling victimisation was found in more highly educated families where it was possible that children experienced more pressure to succeed. Children targeted by both siblings and peers were at much higher risk for poor mental health that those targeted in one context or not at all.

Parental monitoring (house rules, awareness of risk) is found to decrease bullying behaviours along with co-operation between parents and schools in jointly agreeing on a no-tolerance approach which identifies what bullying looks like and how to respond to it.