Bullying is quite prevalent in the school setting and has important adverse effects on many areas of the adolescents' life. It is a specific type of aggression in which an intension to harm or disturb can be identified, occurs repeatedly over time and there is an imbalance of power, with a more powerful person or group attacking a less powerful one . According to a widely used research definition of bullying a student is being bullied or victimized when he or she is exposed, repeatedly and over time, to negative actions on the part of one or more other students . Negative actions are further defined as when someone (the "perpetrator") intentionally inflicts, or attempts to inflict, injury or discomfort upon another (the "victim"). Negative actions can be verbal, including threatening, taunting, teasing, or name-calling, or physical, such as hitting, kicking, pushing, shoving or pinching. Being a victim of bullying has been associated with lower self-esteem , depressive symptomatology [4–7], anxiety , physical and psychosomatic symptoms [8–10], suicidal ideation [11, 12] and suicide . On the other hand, being a perpetrator has been associated with aggression , antisocial personality, criminality and substance abuse . As regards the direction of causality, studies have shown that bullying appears to be a potential risk factor for mental health problems, since it usually precedes the onset of emotional difficulties .
A large number of studies conducted in different countries indicate that bullying at school occurs all over the world and is not confined to any geographical region, socioeconomic or cultural group. The prevalence, however, of bullying varies considerably between countries. Studies show prevalence rates of the overall phenomenon between 8% in Germany  and 29.9% in the United States , 30% in Italy  and 40% in Korea . Prevalence rates of perpetrators vary between 4% and 50% , while rates of victims of bullying vary between 4.1% for girls in Sweden and 36.3% for boys in Lithuania . A recent international study which investigated the prevalence of bullying victimization in 66 countries and territories reports that on average 32,1% of the children were bullied at school at least once within the past 2 months, while 37,4% of children were bullied at least one day within the past 30 days . Boys are more often perpetrators than girls , while rates of victimization may not differ between the two genders [17, 24, 25]. Both behaviors appear to be more common in younger ages [17, 26, 27].
According to Craig et al. the prevalence of bullying combined (i.e., bullying others, being bullied and being both a bully and a victim) in Greece was 41.3% and Greece occupied the third place among 40 countries in the number of adolescent students being involved in bullying-related behaviours . Possible explanations of this relative high prevalence are the lack of national policies against bullying in Greece, as well as a number of cultural variations, for example the way bullying is conceptualized and understood .
The cross-national variations in the prevalence of bullying may reflect the different distribution of culture-specific risk factors among countries and the different methods used in research. Regarding socioeconomic status, lower parental education  and poorer academic achievement of the student in school have been associated with bullying . Recently, an international study showed that being a victim was more common among adolescents from families of lower socioeconomic position and this association appeared to be relatively strong across several countries . Another study, which investigated socioeconomic associations of bullying using a sample of preschool children, has also shown that children from families with lower educational level present an increased risk of victimization  A study conducted in Germany and England has also reported associations between social class and both victims and perpetrators . Moreover, not only the presence but also the persistence of bullying over time has been associated with lower socioeconomic status of the family . A recent review suggests that bullying is not only a socially patterned life experience, but it tracks over time and there are indications of a socially differential vulnerability to its effects. Exposure to bullying may be an element of a pathway through which socioeconomic position in adolescence contributes to adult health inequalities . At the school level, Whitney and Smith (1993) reported that junior and middle schools with higher proportions of families from lower social classes had a higher prevalence of bullying . Finally, at the country level countries with higher income inequality had a higher prevalence of bullying among preadolescents than countries with lower income inequality .
As described in the previous paragraphs psychiatric morbidity has been associated with bullying-related behaviours. In addition previous studies have shown a strong association between several socioeconomic variables and psychiatric morbidity [34–36]. Therefore, an important confounding variable in the association between socioeconomic status and bullying is the presence of psychiatric morbidity. Not all previous studies have adjusted for the full spectrum of psychiatric morbidity including sub-threshold forms of illness. It is likely that a more detailed assessment of psychiatric morbidity including also sub-threshold forms of illness could explain part of the confounding and could reduce possible associations between bullying and socioeconomic status. It is noted that confounding is an important issue irrespective of the study design and could influence the results of both cross-sectional and longitudinal studies. Clarifying whether low socioeconomic status is associated with bullying after adjustment for all potential confounding factors may contribute to the discussion about possible causal pathways of bullying-related behaviours.
The aim of the present study was to investigate the association between bullying-related behaviours (being either a victim or a perpetrator or both) and socioeconomic status in a sample of adolescents attending senior high schools in Greece. Since the burden of psychiatric morbidity is significant in adolescence , we explored possible effects of the full spectrum of psychiatric morbidity on the association between bullying and socioeconomic indicators. We made the hypothesis that socioeconomic indicators would be independently associated with bullying-related behaviours after adjusting for psychiatric morbidity. We have also investigated the association of obesity problems with bullying-related behaviours, since there is evidence that obesity or overweight may be associated with both bullying and socioeconomic status [38–40].