Organizations are increasingly recognizing their obligation to employee health as marked by the rise in workplace health initiatives, particularly over the last two decades [1–3]. Despite mental disorders being the leading cause of sickness absence and work incapacity in most developed countries [4, 5], mental health has remained relatively ignored in the majority of workplace health programs. With depression predicted to be the leading cause of work disability by 2020 [6], there is a growing need for evidence-based workplace mental health interventions. To date, most work-based responses to mental health problems have been reactive, with interventions only being considered once a worker is symptomatic and often on sick leave [7]. However, recent evidence suggests that many mental health problems may be prevented [8], raising the prospect that workplaces might proactively prevent the onset of mental health problems. Despite the appeal of such strategies, to date there has been very little consensus on whether such preventative programs are effective in the workplace [9].

Workplaces have been suggested as an ideal site for prevention programs for a number of reasons [9]. First, with 60% of the world’s population engaged in some form of employment and 60% of their waking hours spent at the workplace, there is potential to reach a substantial number of people in a reliable and predictable manner [10]. Second, an adverse psychosocial work environment is established as a risk factor for mental disorder [11], meaning work-based interventions can be multi-modal in simultaneously reducing known risk factors while enhancing individual coping skills and resilience. Third, if found to be effective the cost of mental health interventions based in the workplace could be shared by both the private and health sectors. A recent review suggested that interventions focused on the prevention or treatment of mental health problems were likely to produce a favorable financial return on the investment [12].

Prevention programs can be directed at an entire population (universal prevention), only those at high risk (selective prevention), or only those with emerging symptoms (indicated prevention) [8]. Although the relative effectiveness of the different types of prevention as they relate to mental health remains unclear [13], there are theoretical and practical reasons that universal interventions may be most appropriate for the workplace. From a public health perspective, universal interventions are attractive not only because they can reach more working adults, but also because they can reach selected and indicated groups without the need for screening, which has been found to be a costly exercise [14, 15]. Targeting an entire population also reaches individuals who might not want to seek treatment or disclose symptoms for fear of stigmatization and the perceived negative effects on employment [16]. Such fears may be particularly relevant in a workplace situation, where previous research has found evidence that prejudiced attitudes by employers towards individuals with depressive symptoms are common [17].

In settings outside of the workplace, preventive interventions using a variety of cognitive behavioral and psychotherapy techniques have been found to effectively reduce the incidence of mental disorders [13]. Only one review, which focused on literature published between 1997 and 2007, has specifically examined mental health interventions in the workplace. A small but positive effect on reducing symptoms of depression and anxiety was found, but the methodology of the review was limited by the inclusion of studies other than randomized controlled trials [18]. In the six years since this review, a number of new randomized controlled studies have been published. As a result, it is now timely to conduct a systematic review and meta-analysis of the evidence for work-based universal prevention of depressive illness.

This is the first published systematic review and meta-analysis examining randomized controlled trials of universal interventions to prevent the development of depression at work. Our results indicate that a range of different depression prevention programs produce small but overall positive effects in the workplace. When analyzed separately universally delivered CBT-based interventions significantly reduced levels of depressive symptoms among workers. These results demonstrate that appropriate evidence-based interventions in the workplace should be part of efforts to prevent the development of depression.

While the effect sizes demonstrated for universal symptom reduction were relatively small, this does not mean they would not have considerable impact at a population level. Universal interventions are never likely to produce large individual effect sizes, but when translated to an entire workforce, the overall impact can be substantial. Within our review, there were some individual studies which were able to demonstrate larger effect sizes. For example, Tsutsumi et al. found that when a team-based participatory intervention was used to improve workplace stress reduction, there was significant deterioration of GHQ scores in the control group while the intervention group remained the same, with an overall moderate effect size of 0.6 [48]. Interestingly, this study was also the only intervention based at the organizational level, as opposed to all other studies that were based at the individual level, suggesting the benefits of organizational level approaches deserves further attention.

The main strengths of this review are the very detailed systematic search strategy, the clear defined inclusion criteria and the objective assessment of the methodological rigor of each included study. Despite these strengths, there are a number of other limitations to this review. First, due to the limited number of studies identified, we were unable to make direct comparisons to determine which type of interventions was most effective or whether an intervention based on psychosocial education is more effective over participatory-based interventions. However, there were adequate numbers of CBT-based intervention trials to perform a separate meta-analysis in order to establish the effectiveness of this particular group of interventions. Second, given that the study populations were randomized, we conducted the meta-analysis under the assumption that pre-test depression scores were the same for the control and treatment groups. The majority of studies in our meta-analysis assessed and reported that no significant differences were present in the pre-test scores; however, there were several studies that did not perform such analyses. Thus, if the pre-test scores among the treatment arms are significantly different for these studies, some bias may be introduced. Third, as self-report measures were used in all studies, our conclusions are limited to reductions in symptoms rather than clinical diagnosis. The combination of self-report symptoms together with the fact participants were not blinded to the type of intervention they received, may have introduced some bias via the Hawthorn effect. An additional problem with the measures used in many of the studies included in this review is that they combined both depression and anxiety symptoms. Our sensitivity analysis demonstrated that the beneficial effects of universal prevention remained even when only studies with pure depressive symptoms measures were included, suggesting there is a true impact on depression. Whether there is an additional and potentially even greater impact on anxiety symptoms remains unclear. Fourth, as workplace interventions are not often reported or published in academic material, there may be some publication bias in this area of research with publications only reporting significant results. However, the regression tests we conducted to examine the possibility of publication bias indicated that this was unlikely to alter our results. Finally, as we adopted a search strategy with only English publications, there is a possibility that there might be non-English universal prevention publications that were not identified.

While no studies of true prevention were identified, the finding of effective universal symptom reduction is important as it demonstrates that universally delivered programs are effective at improving employee mental health. We defined true prevention studies as needing to select a non-depressed sample at baseline and to examine the incidence at follow-up [13, 20]. One of the key problems in attempting to undertake intervention studies of true prevention is the sample sizes required to gain sufficient statistical power. Cuijpers has demonstrated this with a series of calculations, which showed that in order to be able to demonstrate that a true preventative program could reduce the rates of new onset depression over one year by 15%, both the experimental and control groups would need to consist of over 30,000 participants [54]. While unable to definitively demonstrate true primary prevention, the studies of universally delivered interventions identified in this review have the advantage of accurately demonstrating the impact of interventions delivered to an entire sample of unselected workers, which is often more practically and ethically feasible in a work situation.

Prevention of mental health problems in a general community setting is still a relatively new area of research [8], although recent community-based research has provided promising results on the feasibility of prevention as a way of reducing the incidence and overall burden of depression [13]. The results of our review and meta-analysis suggest that the workplace is an alternative location in which preventative mental health programs can be successful. The workplace provides a unique location in which the majority of working-age adults can be engaged. The high cost of depression for employers, in terms of sickness absence and reduced work performance [55, 56], also provides an opportunity for private organizations to be encouraged to help fund prevention programs; although further economic analysis of the costs and financial benefits of work-based universal interventions will be needed to further this case. One of the main limitations of wide-spread implementation of the types of interventions included in this review is cost, both financial and time. Most of the interventions tested required substantial amounts of face to face teaching or group training time, ranging from a single four-hour session to a year-long intervention of redesigning the work environment. There is some emerging evidence that e-health technologies may be able to assist in meeting some of these practical challenges [57]. Internet-based CBT has been shown to be effective as a treatment for depression and anxiety and is able to enhance mental well-being in a community setting [58, 59]. While there are some early indications that computer-aided interventions are well received in the workplace [55], the effectiveness of universal work-based e-health prevention strategies remains unknown.

In conclusion, the current review demonstrates there is good quality evidence that universal mental health interventions can reduce the overall level of depression symptoms in a workforce. Specifically, workplace CBT-based interventions are effective at universal symptom reduction for depression. More research is required to determine the extent to which such interventions can prevent new cases of depression and to establish cost effective and practical strategies for wide scale implementation. Overall, the results of this review provide support for work-based mental health interventions and add to the imperative that depression should no longer be ignored in workplace health promotion programs.