This longitudinal study showed that in Switzerland during the years 1991–2013 suicide in adolescents was associated with socioeconomic and demographic factors: being male, living with only one parent and living in rural areas were risk factors for suicide among youth. There was some evidence that the effect of the household composition on probability of youth suicide depended on the age group, with stronger associations in younger adolescents. Hanging was the most common method of suicide overall and in boys, whereas in girls, railway suicides were most common. Only about 20% of all adolescents dying by suicide from 1991 to 2013 had a psychiatric diagnosis recorded on the death certificate. There was no evidence for an increase or decrease over calendar time.

The rate of suicide was lower than the rate observed in Austria, where 4.57 per 100,000 adolescents from 10 to 19 years died by suicide from 2001 to 2014 [20]. Eurostat, the statistical office of the European Union, reports suicide rates only for age group 15–19 years: the rate of the 28 EU countries for the year 2013 is 4.5 per 100,000 and therefore lower than the rate in Swiss youth aged 15–18 years observed in this study [29]. The rates across countries ranged widely, from 1.29 in Greece to 21.4 in Lithuania [29]. There are only few data on trends over calendar years for suicides among adolescents. In analyses based on WHO data, the suicide rates in European adolescents decreased in boys 10–14 years old and in both genders 15–19 years old [21, 22]. In Austria the rate in 10–19 year old males decreased from 2001 to 2014, from around 10.0 to 8.0 per 100,000 person-years [20]. A slight decrease from around 2.5 to 1.5 per 100,000 person-years was also seen in girls, but it failed to reach statistical significance [20]. In England and Wales, the rates of suicides in adolescents 15–19 years old decreased between 2001 and 2010, from around 8.0 to 4.75 per 100,000 person-years, while there was no trend over time in the age group 10–14 years [19]. We found no evidence for a trend over time in suicide rates in adolescents living in Switzerland, in contrast to adults in whom the rate of suicide decreased over the last decades [30]. The methods of suicide observed confirm the results of an earlier study from Switzerland [31]. While hanging is also a common method of youth suicide in other countries [7, 13, 17, 32], railway suicides in adolescents have increased in the past years in Switzerland [31]. International comparisons emphasize the importance of the availability of suicide methods and the associated potential in prevention [32, 33].

Over 70% of the adolescents dying by suicide were boys, in line with the literature [3, 6, 8]. Mirroring the situation in adults, female adolescents have a higher rate of self-harm and suicide attempts, but a lower rate of suicide [3]. Our data confirm that the male-to-female ratio of suicide is increasing from prepubescent children to adolescents [18]. Another risk factor known from suicide in adults is living in rural areas [34,35,36].

Other factors are more youth-specific and often similar to risk factors for self-harm in adolescents [3]: the higher risk associated with living with one parent only is consistent with results from studies showing a higher risk for children not living in with their biological parents [6, 13]. It is unclear from our data to what extent the family constellations identified in our study, or the factors leading to these situations are causally related to the risk of suicide, or if issues such as unemployment or mental illness in parents pose a greater risk if the child has no other caregiver in the household. An earlier study found an association between parental separation and suicide, but the effect decreased when the mental health of the parents was taken into account [6].

Earlier studies found differences in the possible factors triggering suicide between younger and older adolescents: suicides in adolescents younger than 15 have often been associated with family conflicts, maltreatment or problems at school, while romantic relationships and psychiatric illness were more common risk factors in older adolescents [7, 16,17,18]. Brothers and sisters may be an important source of support in parent–child conflicts, which might explain the association with only children found in this study among younger adolescents.

We found a higher suicide rate in middle-born adolescents. Studies of birth-order effects on personality and psychological development have produced contradictory results [11, 37, 38]. In the context of suicide in young adults some studies showed an increasing risk with increasing number of siblings [11, 15]. In our study, less than 10% of adolescents came from families with more than three children at census.

In only few suicides, a psychiatric disorder was noted on the death certificate. The proportion of about 20% is comparable to the results of a study from Norway, although the authors restricted analyses to youths under 16 years of age [39]. In their study, the parents of those who died by suicide more often reported mental health problems in their children than parents of accident victims, but there were no significant differences in the Kiddie-SADS criteria for any psychiatric diagnoses [39]. Conversely, studies from the US found that 80–90% of all suicides in adolescents were associated with psychiatric morbidity [4, 6]. In Australia, 50% of children aged 10–14 years and 57% of adolescents aged 15–17 years who died by suicide had a mental disorder [40]. Whereas it is likely that the information on the death certificate underestimates the prevalence of mental illness among youth dying by suicide, the post hoc psychological autopsy approach will overestimate the prevalence of psychiatric morbidity [41]. In the USA and Canada studies based on coroner data also found a considerably higher proportion of adolescent suicides with mental disorders [42, 43]. It is unclear whether Swiss adolescents dying by suicide are suffering less often from psychiatric disorders or if they are underdiagnosed. Of note, a recent report on the care of mentally ill persons in Switzerland identified a shortage of resources in the psychiatric care for children and adolescents [44].

An important strength of our study is its national coverage, and the detailed socioeconomic and demographic data available both at the individual and household level. This study was, however, based on routine data collected at the census and on the death certificate, and we could not investigate the importance of mental illness in the adolescents or their parents in any detail. We also had no information on the relation of the adolescents with their parents, teachers and schoolmates. All our data on sociodemographic factors are based on the census 1990 and 2000. Therefore some factors as type of household, highest education in household or birth order might have changed during the study period. In addition, we could only identify the parents of the adolescents if they were living in the same household with the child at least at the time of one census.

Misclassification of suicide as accidental or other unnatural death [45, 46] is another limitation of studies based on death certificate data. For example, a survey among medical examiners in the USA revealed potential for misclassification particularly in suicides of youth [47]. Of note, suicide notes and data on past suicidal behaviour or diagnosed mental disorders are important in decision-making processes of medical examiners in the determination of child suicide [47]. Unfortunately, we did not have any information on suicide notes and suicidal behaviour, and only limited information on diagnosed mental disorders from the death certificates. However, systematic misclassification is unlikely in Switzerland because the family and the communal authorities do not receive a copy of the death certificate, and death registration is anonymous.

In conclusion, further research is needed to better understand the complex effects of distal and proximal risk factors and protective factors on suicide in adolescents. Mental health problems of children or their parents, parent–child conflicts and mobbing have been shown to be risk factors for suicide [7, 14], with a dose–response association [9]. Familial and sociodemographic factors such as the composition of the household may reflect the level of social support available to the adolescent, which in turn may mediate the resilience of adolescents to deal with conflicts and stressors. These factors should therefore be considered when designing preventive interventions in adolescents at risk.