The current study examined trends in three indicators of adolescent mental well-being using nationally representative cross-sectional data from Czechia (2002, 2006, 2010, 2014 and 2018). Importantly, the study also investigated whether the trends in adolescent mental well-being were moderated by gender, age and family affluence. Our first main finding indicates that in Czechia, the adolescent mental well-being has changed between 2002 and 2018 across all three mental well-being indicators, but each indicator had a different pattern of change over time. During this period, a consistent increase in the prevalence of psychological symptoms emerged from 2002 onwards, whereas for life satisfaction a decline was observed up to 2014. From 2014 to 2018, an increase in life satisfaction was observed. For somatic complaints, an increase was observed up to 2010, followed by a subsequent improvement over time. These findings despite small in size are at odds with other studies that reported rather a stable state of emotional and behavioural symptoms between 2003 and 2013 in the Netherlands  or Norway .
Nonetheless, the fact that we observed no further deterioration in life satisfaction and somatic complaints is in line with other recent studies that support either a stabilization or further decline of self-reported mental well-being [16, 47]. In an international comparison, adolescents from Czechia and the United States were the only ones whose self-rated health worsened between 2002 and 2006, and then showed an increase from 2006 to 2010 . Our findings are comparable to this pattern and indirectly suggest the presence of potential buffering factors such increase in family support and communication. Future studies should explore this in more depth.
The diverging trends in psychological complaints, somatic complaints and life satisfaction reinforce the idea that adolescent mental well-being is not a unidimensional construct and that its different components of mental well-being can show different trajectories and may have differential susceptibilities. Life satisfaction, which refers to global cognitive evaluations about one’s life, can be considered a global construct of subjective well-being, and may therefore be influenced by broader life experiences and relationships . In contrast, psychosomatic complaints may represent symptoms of more immediate stress which, at the more severe end, may impair everyday functioning and could be associated with problems from the internalizing spectrum. Furthermore, emotional components of well-being (i.e. psychological complaints) tend to be more prone to fluctuations compared to life satisfaction, which is usually described as a more stable component . Nonetheless, these findings emphasize the need to view mental well-being as a multi-dimensional construct  and suggest a need for greater understanding of the associations between risk factors and different aspects of mental well-being.
Adolescent girls reported lower mental well-being compared to boys, but this gender gap has not systematically increased over time. This result confirms that girls are more likely to report poorer mental well-being outcomes , and also supports a consistent body of research (e.g. ) which found increasing trends in girls only for emotional problems [8, 16] or psychological and somatic symptoms [17, 43]. This increase in gender gap over time could be explained, among others, by the exposure to gender role expectations and the socially defined roles for women and men in society together with exposure to gender-specific stressors . Furthermore, there is considerable evidence that girls are expected to be more emotionally sensitive , experience more restricted gender roles and body dissatisfaction , are more likely to experience and communicate health symptoms , or experience more school performance pressure , which may all contribute to the gender disparities in mental well-being we observed in adolescents from Czechia.
Similarly to the consistent gender gap, our results indicate that older adolescents were more likely to report low mental well-being and this age gap has increased over time but not across all outcomes. The interaction analyses revealed, though, that these age differences remained stable across the survey years except psychological symptoms where the age gap increased in 2010 and 2018 as compared to 2002. Nonetheless, including a three-way interaction parameter in the regression model revealed that the increase in psychological complaints had been the strongest among older adolescent girls. This is in line with the results reported by Bor et al..
Furthermore, previous research argued that interaction of mental health outcomes and socio-demographic characteristics as gender, age and socio-economic status showed a large cross-national variability (e.g., ), which may explain why the changes in associations over time were less emphasized in Czechia. These results do not follow previous findings which indicated that the decline in mental well-being is slightly stronger for older adolescents compared to younger ones [5, 11]. In Norway, an increasing trend in health complaints among adolescents from 1994 to 2014 was found, especially among older adolescent girls . In Sweden the increase over time in psychological complaints (1985 to 2005) was seen in older adolescents (boys and girls), whereas no significant change was seen in the youngest groups (11-year olds) . Given these mixed results, there is a need for a more comprehensive study which includes more countries over a relatively longer time frame and employs a uniform set of mental health and well-being outcomes for boys, girls, adolescents of different age groups, and socio-economic backgrounds.
Interestingly, the gap between Czech adolescents coming from different family affluence families remained relatively stable in the investigated time frame. This is in line previous studies that showed that the inequalities in adolescent health complaints in Czechia has been stable from 1994 to 2010 , and confirms that this trend has remained stable. This could be partly explained by the demographic characteristics of the Czech population. According to the Gini index , Czechia is one of the countries with the lowest income inequality worldwide. In addition, its population is also very homogeneous as regards nationalities of its inhabitants, because only 5% of them are of non-Czech origin and this has been quite consistent over time .
A key strength of the present study is investigating nationally representative samples of adolescents using identical study protocols across a 16-year period. Nonetheless, this inherently fosters the limitation that data collected across time is cross-sectional and self-reported. Secondly, the measures used were restricted to those available in the HBSC study since 2002, therefore providing a relatively limited perspective on adolescent mental health. Further research should include a broader range of mental health outcome measures and other potential drivers of mental health trends, such as changes in the school or family environment, or social media use, which are required to better understand this complex issue. Nonetheless, the present study provides essential and up-to-date information about changing mental health trends in early adolescence from the Central European region.