In spite of a relatively large sample, we did not find any statistically significant difference in psychiatric treatment rates between fixed-term contract workers and unemployed people in the general population of Denmark. Sixteen rate ratios and confidence intervals were estimated; two in the main effects analyses, ten in the stratified analyses and four in the sensitivity analyses. All of the estimated confidence intervals included unity. Moreover, the tests for interaction with age, gender, and education level were not statistically significant.
Results in relation to previous research
The present study looked at the effect on mental ill health of insecure employment vs. unemployment. The result of such a study needs to be interpreted in relation to results in studies on the effect of unemployment vs. employment and insecure vs. secure employment.
Studies on the effect of unemployment vs. employment
The existing evidence on the association between unemployment vs. employment and mental ill health is well summarized in an extensive review and meta-analysis by Paul and Moser , which covered 237 cross-sectional studies with a total of 458,820 participants and 87 longitudinal studies with a total of 43,899 participants. The studies were published between 1963 and 2004 and the samples were drawn from general populations in a total of 26 predominantly Western countries. One of the inclusion criteria stated that the “measurement of mental health was done via a standardized and objective quantitative procedure, usually a questionnaire or a structured interview”. The effects were measured in terms of Cohen's d, which is defined as the difference between two means divided by the pooled standard deviation . The overall effect size was estimated at d = 0.54 (95% CI: 0.50–0.57) meaning that the overall level of mental health problems was approximately half a standard deviation higher among the unemployed than it was among the employed participants. The average prevalence of psychological problems with potential clinical severity was estimated to be 34% among the unemployed and 16% among employed participants. The association was stronger among men and blue-collar workers than it was among women and white-collar workers, respectively. The association was, moreover, stronger in countries with lower GDP per capita, higher income inequalities and a weaker unemployment protection system than it was in countries with higher GDP per capita, lower income inequalities and a stronger unemployment protection system, respectively. There were no statistically significant changes in the strength of the association between unemployment and mental ill health during the four decades covered by the studies.
The longitudinal studies in the meta-analysis by Paul and Moser  showed that a change from employment to unemployment was associated with worsened mental health while a change from unemployment to employment was associated with improved mental health. It was, moreover, shown that factory closures were associated with worsened mental health. These longitudinal findings were seen as evidence of a causal link from unemployment to impaired mental health. Paul and Moser also provided evidence for selection effects, which supports the hypothesis of a causal link from mental ill health to unemployment .
A more recent meta-analysis, which included cross-sectional, case–control and cohort studies published until the end of 2020, estimated the odds ratio for depression among unemployed vs. employed people at 1.62 (95% CI: 1.40–1.87) for women and 2.27 (95% CI: 1.76–2.93) for men .
Studies on the effect of insecure vs. secure employment
The existing evidence on the association between perceived job insecurity and subsequent mental ill health is well summarized in a systematic review and meta-analysis by Rönnblad et al. , in which the odds ratio (OR) for mental ill health among employees with vs. without self-reported job insecurity was estimated at 1.52 (95% CI: 1.35–1.70).
Regarding objective job insecurity, we are aware of three studies that compare fixed-term contracts with open end contracts [39,40,41]. The Finnish Public Health Sector Study did not find an association . This study of 107 828 employees found a hazard ratio for sickness absence and disability retirement due to depression of 1.02 (95% CI 0.97–1.08). Two other studies found elevated risks. A study of up to 3.577 young people in the U.S. NLSY78 cohort found an elevated risk for depression (’Centre for Epidemiologic Studies Depression Scale (CES-D)’) in fixed-term contracts (ATT, “Average treatment effect for the treated”) 1.80; 95% CI 0.55–3.06) . A paper on 600 workers from the North Sweden Study Cohort found an elevated odds ratio for depressive symptoms (one item:’felt depressed during the past 12 months.’) of 1.79 (1.04–3.08) . The reason why the Finnish study did not find an association could be that it did not look at depressive symptoms per se but sickness absence and disability retirement due to depression, which might be a less valid measure.
Studies on the effect of insecure employment vs. unemployment
To our knowledge, the first study that explicitly aimed at comparing the effects of expectation of future job loss with the effects of actual job loss on mental ill health was conducted by Mandal et al. . Their study was based on survey data on a cohort of US citizens who were 45 – 65 years old and stably employed in 1992 (N = 6781). The cohort members were invited to be interviewed once every second year from 1992 until 2006 with questions on, inter alia, employment status, self-rated likelihood of losing one’s job within a year, actual job losses due to business closures (within the two year that had passed since the previous interview) and depressive symptoms. Multiple linear regression was used to model changes in depressive symptoms between two waves as a function of, inter alia, the presence of business closures between the two waves and the self-rated likelihood of losing one’s job according to the interview at the first of the two waves. Based on the p-values of the regression analysis of that paper, Mandal et al. concluded that “among older workers in the age range of 55–65 year, subjective expectations are as significant predictors of depression as job loss itself”. The statistical significance of their findings is, however, spurious because the final statistical model was contingent on results obtained in preliminary analyses.
The second study that explicitly aimed at comparing the effects of expectation of future job loss with the effects of actual job loss on mental ill health, that we know of, was the meta-analysis by Kim and von dem Knesebeck  that we commented on in the introduction of the present paper.
A third study along this line of research was performed by Park et al. , who estimated rate ratios of depression as a function of employment status (full-time permanent employment, precarious employment and unemployment) among ≥ 45 year old inhabitants of Korea (N = 5638). Precarious employment category covered part-time employment, temporary employment, dispatched employment, and unpaid family workers, while the unemployment category covered all who were not working, regardless of whether they were seeking work. The rate ratio for depression among unemployed vs. precariously employed people was estimated at 1.50 (95% CI: 1.17–1.92) for women and 1.39 (95% CI: 1.03–1.88) for men . The apparent disagreement between the null finding of the present study and the results obtained by Park et al.  can probably be explained, firstly by the very different definitions of the term “unemployed” and secondly by the large differences in the age compositions of the two studies. The unemployment category of the present study covered only those who were actively searching for a job and ready to start working within 14 days, and the baseline ages of the participants ranged from 20 to 59 years. The unemployment category in Park et al. covered all types of non-employed people, including disability retirees and old age pensioners, and 61.5% of the participants were 60 years or older at baseline .
In conclusion, previous studies suggest that an insecure employment may be as detrimental to a person’s mental health as unemployment itself. They, moreover, suggest that both fixed-term employment and unemployment pose a risk for depression at levels around 1.5 compared with permanent employment. Our study’s results are in line with most of the studies referred to here.
Strengths, weaknesses and limitations
The study was large enough to address our research questions, which were raised on the basis of some previous studies. The exposure categories of our study were the same as the ones used in the European labor force surveys . Another advantage was that the participants were drawn from the general working age population of Denmark.
Epidemiologic studies are often associated with substantial publication bias due to multiple testing of outcomes combined with selective reporting of results . In the present study, the hypotheses and statistical models were completely specified, peer reviewed, and published before we linked the exposure data to the outcome data . We adhered to the protocol without violations. The study is thereby free from bias due to selective hypothesis-testing. Since the endpoints of the study were ascertained through national registers, which cover all inhabitants of Denmark, we can rule out bias from missing follow-up data. For the same reason, we can also rule out recall-bias. Register data on social security cash benefits, sickness absence benefits and psychiatric treatment prior to baseline enabled us to identify and exclude potentially unhealthy workers and thereby mitigate the possibility of health selection bias. Register data on age, gender, disposable family income and education enabled us to control for and thereby mitigate the possibility of bias from demographic and socio-economic factors.
Smoking [46, 47] and overweight  have been associated with an increased risk of depression. In the present study, we did not have any person-based data on these lifestyle factors and could therefore not control for them in the analyses. Based on the prevalence of smoking and overweight in another random sample of fixed-term contract workers and unemployed people in Denmark, we have estimated that a failure to control for smoking and overweight in the present study would bias the rate ratio for mental health illnesses among fixed-term contract workers vs. unemployed downward with a factor of 0.96. Which means that a rate ratio at 0.96 without control for smoking and overweight would correspond to a rate ratio at 1.00 with control for smoking and overweight . It is, therefore, unlikely that the null finding of the present study was due to a failure to control for smoking and overweight.
Immigrants are highly overrepresented among unemployed people in Denmark . The rates of psychiatric treatment among the immigrants are, however, quite similar to the rates among native Danes. This was shown in a very large register-based Danish population study  in which the incidence rate ratio among first-generation immigrants vs. native Danes was estimated at 0.97 (95% CI: 0.93–1.01) for any psychiatric contact, 0.98 (0.71–1.32) for bipolar affective disorder, 0.81 (0.74–0.89) for affective disorders and 1.05 (0.99–1.12) for anxiety and somatoform disorders. It is therefore unlikely that the null finding of the present study was due to a failure to control for country of birth.
Some of the covariates and inclusion criteria of the study were based on records in national registers, which only were available among the DLFS-participants who had lived in Denmark throughout a one-year period prior to the interview. We therefore had to exclude those DLFS-participants who had immigrated to Denmark within the one-year period preceding the interview (cf. Fig. 2). This group constituted however less than one percent of all participants, wherefore we assess the effect of excluding them to be negligible.
It has been shown that response rates to Danish health questionnaires is affected by calendar time, age, gender, and educational level [51, 52]. By controlling these factors in the analyses, we aspired to minimize the possible effect of non-participation bias. The present project had, however, not access to data on all of the sampled individuals. We had only access to data for the responders and could therefore not calculate and compare response rates among fixed-term employees and unemployed. Unemployed are probably overrepresented among non-responders. Hence, we cannot rule out the possibility of non-participation bias.
Since the outcomes of our analyses are based on redeemed prescriptions and hospital diagnoses, we need to consider the possibility of detection, prescription, and referral bias. All citizens of Denmark are covered by a tax-funded health insurance, which, among other things, enables them to consult a general practitioner without charge. The general practitioner may in turn refer the patient to a specialist or a hospital for further examinations or treatments. If the patient is referred to a psychiatric specialist or hospital, then the treatment is free of charge. The tax-funded health insurance may be supplemented with private health insurances, which, among other things, cover the costs associated with minor surgeries and psychological therapy. The number of private health insurance holders has increased from 50,000 in 2001 to 1 million in 2008 and 1.9 million in 2017 [53, 54]. Unemployed people in Denmark do not usually hold a private health insurance; in 2015, approximately 98% of all private health insurances in Denmark were provided by the employers. As the access to psychological treatment is greater among people with than it is among people without a private health insurance, it is possible that our results have been influenced by detection, referral, and prescription bias towards lower rates among the unemployed. On the other hand, the unemployed are able to consult their general practitioner without having to take time off from their job, which may lead to an increased probability of consultation and thereby an increased probability that a mental health problem is detected. Hence, it is also possible that our results are biased towards higher rates among the unemployed.
A major limitation is the measurement of exposure as only point-prevalence self-reported data. In the primary analysis, the exposure category was defined at a single time point (the first interview). To find out if the estimated strength of the association would change if we based the exposure categories on more than one interview, we conducted a sensitivity analysis, in which we only included people who participated in two or more interview rounds and whose exposure was the same in all of their interview rounds. In this sensitivity analysis, the rate ratio for psychotropic drug use among fixed-term contract workers vs. unemployed was estimated at 0.90 (99.5% CI: 0.68—1.20) [cf. Additional file 1, Table S1], which is lower than 0.98 (the rate ratio obtained in the primary analysis). It is possible that those in fixed-term employment could have an earlier high exposure of unemployment, which could explain the lack of significance when comparing the groups. We can therefore not entirely rule out that a more rigorous control for selection processes would lead to the conclusion that fixed-term contract employments are less detrimental for mental health than unemployment.
Generalizability of the findings
Here, it should be noted that employees on fixed-term contracts in Denmark, since July 1999, are protected against discrimination through a European council directive. Which aims “to improve the quality of fixed-term work by ensuring the application of the principle of non-discrimination, and to establish a framework to prevent abuse arising from the use of successive fixed-term employment contracts or relationships” . It should, moreover, be noted that Denmark is an egalitarian country with relatively high GDP per capita and a strong unemployment protection scheme. The findings of the study might not hold good in nations with low GDP per capita, high income-inequalities or a weak unemployment protection system .
In the Organization for Economic Co-operation and Development (OECD), mental health problems constitute the most frequent single cause of disability benefits, and in Denmark, they account for almost half of all new applications for disability retirement . Unemployment is a significant and important risk factor for mental ill health. From this viewpoint, rate ratios of mental ill health between fixed-term contract workers and unemployed should be of interest in political discussions about the pros and cons of a labor market with a high vs. low proportion of temporary jobs, especially if an increased labor market flexibility is seen as a means of reducing unemployment rates. Contrary to our expectations, the present study did not find any statistically significant differences in psychiatric treatment rates between fixed-term contract workers and unemployed in the general population of Denmark, and can therefore not reject the proposition that fixed-term employment may be as detrimental to an individual’s mental health as unemployment itself [cf. 9]. Our null finding thereby suggests that an increased proportion of insecure jobs (measured as fixed-term contracts) may lead to an increased prevalence of mental ill health. The confidence intervals around the estimated rate ratios of the present study are, however, a bit too wide to allow any firm conclusions on this interesting issue.