Kaess et al. BMC Public Health 2011, 11:597 http://www.biomedcentral.com/1471-2458/11/597 to 33.8) of these adolescents indicated having thought about suicide at some point in their life [8]. One of the strongest predictors of completed suicide or further suicide attempt has been found to be a pre- vious suicide attempt [9,10]. Therefore, suicide attempts and also suicidal thoughts in adolescents must always be taken seriously, in spite of the fact that for every death of a young person from suicide, many suicide attempts have already been undertaken, especially by girls [9,11]. In most western countries, females are more likely to engage in suicidal behaviour, but are less likely to die as a result of a suicidal act than males [12-14]. This “gen- der paradox” is known to be extremely distinctive in adolescents; during this period of life suicide attempts are 3-9 times more common in girls while completed suicides rates are 2-4 times higher in adolescent males [15]. Many epidemiological studies so far have reported higher rates of non-fatal suicidal behaviour in females which could indicate a gender-specific predisposition for the experience of suicidal thoughts and suicide attempts during this life-period [6,15,16]. As regards mental illness as one of the strongest risk factors for suicidal behaviour, depression and anxiety in particular seem to function as mediators of adolescent suicidal behaviour [17]. But drug abuse, risk behaviours [6] and other types of externalizing psychopathological behaviours [18] are also associated with an increased risk of suicide among adolescents. There is only little empirical research which investi- gated the reasons underlying the correlation of being female and showing higher rates of suicidal thoughts and suicide attempts during adolescence. One theory is that gender differences in psychopathology could play an important role in this issue [12,19,20], suggesting that different types and characteristics of emotional and behavioural problems may primarily lead to unequal rates of suicidal behaviour in female and male indivi- duals. Therefore, our hypothesis was that gender differ- ences in non-fatal suicidal behaviour among adolescents could mainly be explained by the gender differences in emotional and behavioural problems. Methods Study population and design The Heidelberg School Study, a cross-sectional, nation- ally representative, school-based sample was devised to measure the prevalence and common psychological cor- relates of non-fatal suicidal behaviour, self-destructive behaviour and other forms of risk behaviour in adoles- cents. The investigation took place in cooperation with the Heidelberg Public Health Service and the University of Education between October 2004 and January 2005. All schools in the Rhein-Neckar District were invited to participate. The Rhein-Neckar District is typical for Page 2 of 7 geographically mixed populations in Germany and therefore shows a representative distribution of types of schools and parental socioeconomic status [21]. Of 121 schools contacted (N = 6,842), 116 agreed to participate. Five schools declined participation without giving rea- sons. All ninth-grade students of the 116 participating schools (N = 6,534) were requested to take part in the study; 349 students were absent on the day of the assessment, 100 students did not return their question- naire (N = 6,085). Additionally, 573 students had miss- ing data (N = 5,512). Hence, we could include data for 89.1% of the students who were approached and for 80.6% of the entire population of ninth-grade students in this school district [22,23]. Study procedures The Heidelberg School Study was performed in compli- ance with the Helsinki Declaration. It was appraised and approved by the ethics committee of the faculty of med- icine at the University of Heidelberg. While parents or legal guardians were informed about the study by letter about 4 weeks before it commenced, informed and writ- ten consent was obtained from all participating students. The student’s anonymity and voluntary status were ensured before starting the investigation. Data collectors, who had been trained in test administration by the research team, administered the questionnaire to all par- ticipating subjects during a regular class period. Although teachers were in attendance during this class period for legal reasons, they did not interact with the students at any time during the assessment. For adoles- cents in special education classes, the survey personnel assisted subjects who had problems filling out the ques- tionnaire and offered them additional time to complete it. However, this kind of support was only necessary in a few cases. Assessment Students were given a self-report booklet, including diverse questions regarding socio-demographic charac- teristics as well as items comprising suicidal and self- destructive behaviour, different forms of risk behaviour, and emotional and behavioural problems. Suicidal beha- viour was assessed by pertinent parts of the German version [24] of the Schedule for Affective Disorders and Schizophrenia for School-Age Children [25] that had been inserted into the self-report booklet. Suicidal idea- tion was assessed by the question “Did you ever ser- iously think about taking your own life?” Suicide attempts were assessed by the question “Have you ever tried to take your own life?” A separate question on self-harm (e.g. cutting, burning, etc.) was also asked dur- ing the survey and has been published elsewhere [22], therefore the students were probably able to distinguish