A total of 165 students consented to participate in the study in Sweden. One boy was excluded because of an incomplete questionnaire. In total, 46 students had no information on class. Two individuals had missing information on sex. In total, nine classes from three different schools with, on average, 13 students in each class participated from Sweden. A total of 472 students consented to participate in Bulgaria. Seven individuals had no information on sex. In total, 26 classes from six different schools with on average 18 students in each class participated from Bulgaria (Table 1).

The girls in both Sweden and Bulgaria reported higher scores on the PSS-scale than the boys within the same country (Table 1, Fig. 1 and Table 2).

PSS total score for boys and girls in Sweden and Bulgaria (nS = 147, nB = 462)

A higher proportion of the Swedish adolescents, both boys and girls, reported PSS-values in the two upper quartiles (PSS total score divided into quartiles, 0–13, 14–27, 28–41, 42–56), compared with the Bulgarian adolescents (Swedish boys 65%, Bulgarian boys 24%) (Swedish girls 67%, Bulgarian girls 41%) (Table 2).

Girls in both Sweden (p = 0.01) and Bulgaria (p = 0.002) reported that they were more stressed by their schoolwork than the boys in the same country. Girls also reported feeling low (Sweden p < 0.0001, Bulgaria p = 0.003), being irritated (Sweden p = 0.03, Bulgaria p < 0.0001), being anxious (Sweden p = 0.01, Bulgaria p < 0.0001), feeling dizzy (Sweden p = 0.05, Bulgaria p = 0.001), having stomach ache (Sweden p = 0.03, Bulgaria p < 0.0001) and headache (Sweden p < 0.0001, Bulgaria p < 0.0001) to a higher extent than boys in both countries (Table 2). Most of these results remained significant also after the Bonferroni correction (i.e. a p-value below 0.002).

In both countries, girls reported feeling worse than the boys on the question “How are you, in general?”, which reached significance among the Swedish sample, (p = 0.04), and was a trend in the Bulgarian sample (p = 0.10), albeit without the Bonferroni correction (Table 2).

In Bulgaria, the girls reported higher frequency of restless sleep than the boys (p-value < 0.0001, Table 2). In Sweden, there was no difference between the sexes regarding restless sleep (p-value 0.54).

No significant difference between the sexes (Sweden p = 0.13, Bulgaria p = 0.92) was observed in either country on the questions “How often have you felt happy in the last 6 months?” or “How easy is it for you to talk to adults?” (Sweden p = 0.60, Bulgaria p = 0.72) (Table 2).

There was an association (assessed with Spearman’s rank correlation and expressed in ρ (rho)), between perceived stress and feeling stressed by schoolwork for both boys and girls, in both countries and in Bulgaria also with a Bonferroni correction calculated as 0.05/72 = 0.00069 for Table 3 (Swedish boys borderline significant, ρ = 0.22, p = 0.05, Bulgarian boys ρ = 0.30, p < 0.0001, Swedish girls ρ = 0.30, p = 0.02, Bulgarian girls ρ = 0.32, p = 0.0002). In all groups, the association between perceived stress and feeling low (Swedish boys ρ = 0.3, p = 0.008, Bulgarian boys ρ = 0.38, p < 0.0001, Swedish girls ρ = 0.43, p = 0.0005, Bulgarian girls ρ = 0.51 p < 0.0001), being irritated (Swedish boys ρ = 0.49, p < 0.0001, Bulgarian boys ρ = 0.37, p < 0.0001, Swedish girls ρ = 0.46, p = 0.0002, Bulgarian girls ρ = 0.54, p < 0.0001), significant also with a Bonferroni correction. The association with feeling dizzy (Swedish boys ρ = 0.26 p = 0.02, Bulgarian boys ρ = 0.35, p < 0.0001, Swedish girls ρ = 0.28, p = 0.03, Bulgarian girls ρ = 0.56 p < 0.0001) was significant in all groups, but after the Bonferroni correction only in the Bulgarian sample. There was also an association between perceived stress and being anxious (Swedish boys ρ = 0.22, p = 0.05, Bulgarian boys ρ = 0.37, p < 0.0001, Swedish girls ρ = 0.42, p = 0.0008, Bulgarian girls ρ = 0.54, p < 0.0001) and remained significant also with a Bonferroni correction in all groups except among the Swedish boys where it was only borderline significant. The associations were similar between boys and girls, which was confirmed by the non-significant interaction tests where all p-values were higher than 0.08. (Table 3).

There was no significant association between perceived stress and the question “-How easy is it for you to talk to adults?” for boys and girls in neither of the countries (Swedish boys p = 0.11, Bulgarian boys p = 0.79, Swedish girls p = 0.23, Bulgarian girls p = 0.14) (Table 3).

In Sweden, there was a significant difference (also after the Bonferroni correction) between girls and boys in the association between perceived stress and the question “-How often have you felt happy in the last 6 months?” (p-value from interaction test < 0.0001) where Swedish boys reported no such association (ρ = 0.009, p = 0.94) in contrast to the girls (ρ = 0.48, p = 0.0001).

No gender differences in the associations between perceived stress and any of the health outcomes were seen in Bulgaria (all p-values from interaction tests >  = 0.08) although nearly all associations were stronger for girls compared to boys (Table 3).

The Swedish boys reported their strongest significant association between perceived stress and being irritated/in a bad mood (ρ = 0.49, p < 0.0001), followed by (in descending order of association (but no longer significant after the Bonferroni correction) feeling low (ρ = 0.3, p = 0.008) and feeling dizzy (ρ = 0.26, p = 0.02) and with a borderline significant association between perceived stress and feeling stressed by schoolwork (ρ = 0.22, p = 0.05) and feeling anxious or worried (ρ = 0.22, p = 0.05). The Swedish boys reported no significant association between perceived stress and feeling content with oneself (p = 0.45), having stomach ache (p = 0.25), having headache (p = 0.90), experiencing restless sleep (p = 0.65), feeling happy (p = 0.94), general wellbeing (p = 0.25) or ability to talk to adults (p = 0.11) (Table 3).

The Swedish girls had the strongest significant association between perceived stress and feeling happy (ρ = 0.48, p = 0.0001), followed by (in descending order of association), being irritated/in a bad mood (ρ = 0.46, p = 0.0002), and feeling low (ρ = 0.43, p = 0.0005). The following associations were not significant after the Bonferroni correction; feeling anxious or worried (ρ = 0.42, p = 0.0008), general wellbeing (ρ = 0.38, p = 0.003), feeling content with oneself (ρ = 0.33, p = 0.01), feeling stressed by schoolwork (ρ = 0.30, p = 0.02), feeling dizzy (ρ = 0.28, p = 0.03), and experiencing restless sleep (ρ = 0.28, p = 0.03). They reported no significant association between perceived stress and having stomach ache (p = 0.48), having headache (p = 0.10) or ability to talk to adults (p = 0.23) (Table 3).

The Bulgarian boys had the strongest significant association between perceived stress and general wellbeing (ρ = 0.44, p < 0.0001), followed by (in descending order of association), feeling low (ρ = 0.38, p < 0.0001), being irritated/in a bad mood (ρ = 0.37, p < 0.0001), feeling content with oneself (ρ = 0.37, p < 0.0001), feeling anxious or worried (ρ = 0.37, p < 0.0001), feeling dizzy (ρ = 0.35, p < 0.0001), feeling stressed by schoolwork (ρ = 0.30, p < 0.0001), feeling happy (ρ = 0.29, p < 0.0001), having stomach ache (ρ = 0.23, p < 0.0001), and having headache (ρ = 0.22, p = 0.0001). The associations between PSS and experiencing restless sleep (ρ = 0.18, p = 0.001) was not significant after the Bonferroni correction. They reported no significant association between perceived stress and ability to talk to adults (p = 0.79). (Table 3).

The Bulgarian girls reported their strongest significant association between perceived stress and general wellbeing (ρ = 0.60, p < 0.0001), followed by (in descending order of association), feeling dizzy (ρ = 0.56, p < 0.0001), being irritated/in a bad mood (ρ = 0.54, p < 0.0001), feeling anxious or worried (ρ = 0.54, p < 0.0001), feeling low (ρ = 0.51, p < 0.0001), feeling content with oneself (ρ = 0.49, p < 0.0001), having stomach ache (ρ = 0.34, p = 0.0001), experiencing restless sleep (ρ = 0.33, p = 0.0001), feeling stressed by schoolwork (ρ = 0.32, p = 0.0002), feeling happy (ρ = 0.30, p = 0.0006). The association with having headache (ρ = 0.22, p = 0.01) was no longer significant after the Bonferroni correction. They reported no significant association between perceived stress and ability to talk to adults (p = 0.14) (Table 3).

When comparing the countries (Additional file1: Table S1), the Bulgarian adolescents had a tendency towards stronger associations (with higher rho-values and lower p-values) than Swedish adolescents for almost all health outcomes in both boys and girls, but the differences did not reach statistical significance. The only exceptions was that compared to the Swedish boys (ρ = 0.08, p = 0.45), the Bulgarian boys (ρ = 0.37, p < 0.0001) had a significantly stronger association between perceived stress and feeling happy (p < 0.0001), which remained significant also after the Bonferroni correction (calculated to 0.05/72 = 0.00069), whereas the association between perceived stress and general wellbeing (Swedish boys-Bulgarian boys p = 0.001, Swedish girls-Bulgarian girls p = 0.02), as well as between perceived stress and feeling content with oneself (Swedish boys-Bulgarian boys p = 0.003, Swedish girls-Bulgarian girls p = 0.02) was no longer significant after the Bonferroni correction.