Being involved in bullying is common among adolescents. Prevalence rates of being victims of bullying vary globally from 6 to 35%, and bullying others from 6 to 32%, whereas a smaller group, from 1.6 to 13%, has experience both as a bully and victim (“bully–victim”) [1–7]. Prevalence differences are most often attributed to variations in age of participants, time range of measurement and classification of bullying. Olweus and Limber [8] defines bullying or victimization in terms of being bullied, intimidated, or victimized when a person is exposed, repeatedly and over time, to negative actions from more powerful peers. Bullying behavior may be manifested in various ways, for example, as teasing, active exclusion from a social group, or physical assaults [9]. Studies in schools have found an association between involvement in bullying—whether as victim, perpetrator or bully–victim—and elevated mental health problems [10, 11]. Surprisingly, almost no research has addressed the effects from bullying on the transition from adolescent to early adulthood when most people move on from the educational system to work-life and are expected to begin making a life apart from their parents. Accordingly, we know little about the long-term association between bullying involvement in adolescence and mental health outcomes and broader effects on development into young adulthood. Recently a few studies have indicated troubling associations between bullying involvement and later problems in adulthood [1, 5, 6]. Nonetheless, further prospective longitudinal research on bullying involvement in adolescence and later mental health outcomes is much needed.

A common way of examining mental health issues separates those reflecting internalizing and externalizing problems. Whereas, the terms internalizing and externalizing problems have traditionally mainly been used to describe symptoms occurring in childhood, they are also applied in adult psychiatric research due to the latent structure of psychiatric disorders [12, 13]. Internalizing symptoms include problems within the individual, such as depression, anxiety, fear and withdrawal from social contacts. Some research suggests that internalizing problems are more prevalent in victims of bullying [8]. However, other research has been inconsistent [14]. A recent longitudinal study has shown that both those who are bullied and bullying others in adolescence have an increased risk of developing panic-disorder or depression in young adulthood; in addition, those being bullied had an increased risk of developing anxiety disorders [1].

Externalizing symptoms reflect behaviours that are directed outwards toward others such as anger, aggression, and conduct problems including a tendency to engage in risky and impulsive behaviour, as well as criminal behaviour. Individuals who are aggressive and bully others not surprisingly concurrently display more externalizing symptoms than those being bullied and peers
who have no involvement in bullying [15]. Importantly, research suggests that bullying others in adolescence
is associated with elevation in externalizing symptoms as young adults [1, 16]. Sourander et al. [16] found that being a frequent bully at age 8 predicted antisocial personality, substance abuse, and depressive and anxiety disorders
in early adulthood. However, the sample consisted only of males during enrollment at the Finish obligatory military service. Copeland and colleagues [1] reported in a prospective study that those bullying others in adolescence have heightened risk of developing antisocial personality-disorder in young adulthood, even when controlling for preexisting psychiatric problems, family hardships, and child maltreatment.