Working at a workplace with a mixed gender composition was related to a higher likelihood of psychological distress compared to workplaces with more men, even after adjustment for socioeconomic position, psychological distress at age 21, psychosocial work environment factors and gender. Contextual perceptions of the psychosocial work environment based on the workplace gender composition could not explain the relations.
Our findings regarding worse health at mixed workplaces are interesting because they contradict previous research on the occupational level that suggests better health in those few occupations that are gender-integrated . A possible explanation could be that gender composition is measured on different levels – on occupational and on workplace level, and that gender integrated occupations means something different compared to mixed workplaces. Previous research also suggests that women at workplaces with more women are worse off in terms of mental health . In our study the differences in psychological distress between workplaces with more women and workplaces with more men are not significant after adjusting for gender. Our results indicate that the higher likelihood of psychological distress at workplaces with more women in the crude analysis is explained by a compositional effect, i.e. that there are more women at these workplaces, and women as a group have a higher prevalence of psychological distress. However, these results must be interpreted with caution as there are few men at these workplaces and there might not be enough statistical power to detect an association. The descriptive analyses indicate that the prevalence of psychological distress for men at workplaces with more women is at the same level as for women.
Concerning the second part of our aim exploring the work environment hypothesis, our results show that the differences in psychological distress between the three gender compositions could not be explained by differences in their perception of the psychosocial work environment, and consequently the work environment hypothesis is not supported. Previous research has suggested an influence of gender composition on the association between high strain jobs and sickness absence for both women and men . Our results indicate that when the outcome is self-reported psychological distress rather than sickness absence, gender composition does not influence the association between the psychosocial work environment and the outcome in a corresponding way. As the explanation to differences in prevalence of psychological distress between gender compositions does not seem to be found in the psychosocial work environment, more research is needed to explore other possible explanations. For the psychosocial work environment factors included in this study, only one factor (not important work) differed significantly in prevalence between workplace gender compositions. Previous research also indicates that workplaces with different gender composition do not differ significantly in the reported character of their psychosocial work environment . These findings indicate that the explanation to differences in health connected to workplace gender composition might be found elsewhere. One possible way forward could be to explore how workplace cultures regarding health behaviour fits into this picture. However, although the psychosocial work environment cannot explain the differences in psychological distress between the gender compositions, our study confirms that demand, control and support are all related to mental distress at all types of workplaces. Our study also adds the status variables such as important work and being looked down upon as factors that can be of interest to explore in future research.
The longitudinal design allows us to reflect on the development of psychological distress prevalence over time. The lowest prevalence of psychological distress at age 21 was found among men who at age 42 worked at mixed workplaces; but at age 42 the differences compared to men at workplaces with other gender compositions were no longer significant. These results suggest that there might be a positive health selection of men into the mixed workplaces, i.e. that the group of men who at age 42 worked at mixed workplaces started their working life with lower prevalence of psychological distress compared to men who ended up at workplaces with other gender compositions. We can also see that there might be a negative health selection of men into workplaces with more women. The lack of significant differences in psychological distress between gender compositions for men at age 42 is consistent with several previous studies on the occupational  and workplace level [7, 14]. However, the new finding of a potential positive health selection of men into the mixed workplaces and negative health selection of men into workplaces with more women requires more attention in future studies. The longitudinal design also indicate that the health differences between the gender compositions might evolve during working life (between ages 21 and 42). At workplaces with a mixed gender composition the prevalence of psychological distress increased considerably during working life for both women and men, but women start and end up at a higher prevalence compared to the men. This suggests that the conditions in working life might have a negative influence on mental health for both women and men at mixed workplaces, and that the differences that were present at an earlier age have been scaled up. It is possible that the work environment is similar for women and men at these workplaces and that initial differences in mental ill-health explain the differences at age 42. It is also possible that the workplaces are not really gender integrated, but that women and men have very different work situations with diverse associations with mental ill-health. The latter interpretation is to some extent supported in the descriptive analysis where more women than men at these workplaces reported low control. Also, previous analyses on the same population showed that a mixed gender composition often co-existed with gender inequalities in salaries, educational levels and parental leave at the workplace . Regardless of the explanation to the differences in prevalence between women and men, the similar development of psychological distress over time for women and men in the same gender composition gives some support for the assumption that the gendered organizations are of importance for mental ill-health, and perhaps more so than gender roles attached to the individual, which was suggested previously , . However, more research on larger samples is needed to clarify the importance of gender at different levels.
On the method
A major strength of the present study is the high quality of data from a longitudinal cohort with high response rate. The longitudinal cohort design makes it possible to adjust for previous psychological distress, which has not been possible in previous cross-sectional studies [8, 9]. The use of register data for determining gender composition of the workplace also ensures high validity compared to previous studies that have mainly relied on self-reported gender composition. A limitation of the study is that the relatively small sample size makes separate analyses for women and men impossible in the logistic regression. In future research, gender stratified analyses should be used to further explore if men and women at workplaces with various gender compositions are similarly affected . Future research is also needed to analyse the importance of factors in unpaid work, such as family responsibility, in relation to gender composition of workplaces and psychological distress. Another limitation is that both exposures (work environment factors) and the outcome (psychological distress) are self-reported. However, although it is possible that respondents with poor mental health also report worse psychosocial work environment, there is little reason to believe that such “over reporting” would differ between the gender compositions which is the main focus of this study. Both severe and less severe complaints of psychological distress have been shown to be related to later ill-health and all-cause mortality .
In our study we have chosen to include demand, control and support as separate dimensions of the psychosocial work environment rather than combining them into job strain and iso-strain. It has been debated whether to use the variables in combination or as separate dimensions, and especially in longitudinal studies the support for the combinations in relation to psychological wellbeing has been scarce [19, 20]. In empirical testing of the demand-control-support model, many studies have also found differences between women and men with more support for using the high strain combination on populations of men . It has also been suggested that men, to a higher degree, work in organisations and occupations where they have more possibilities to influence their work situation, although this is not always caught by the control measure. This suggestion was supported by an externally assessed validation of the demand-control model in which women who described their work as active did not have more creative tasks than women in high–strain or passive jobs .
The Northern Swedish Cohort is a homogenous group in terms of age and geographic location, although 41 per cent lived elsewhere than Luleå at the latest follow up. However, the labour market structure of Luleå is representative of Sweden as a whole in regards to the distribution between branches of business in 2007 at the latest follow up , and the cohort has also proven to be comparable to the country as a whole with regard to socio-demographic factors as well as health status and health behaviour . The sample in the present study is comparable to Sweden regarding socioeconomic position in the age group between 25 and 44 years . The workplace sample (all employees at the participants’ workplaces that were used to calculate the gender composition exposure) has a similar age structure as the general population in Sweden, but a higher educational level . The generalization of the results may be limited to similar age groups and workplaces as well as to other countries with similar gender segregated labour market and high labour market participation for both women and men.