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International Archives of Occupational and Environmental Health

https://doi.org/10.1007/s00420-022-01936-7

ORIGINAL ARTICLE

Mental illness rates among employees with fixed‑term

versus permanent employment contracts: a Danish cohort study

Harald Hannerz

· Hermann Burr

· Martin Lindhardt Nielsen

· Anne Helene Garde

1,4

· Mari‑Ann Flyvholm

Received: 28 September 2022 / Accepted: 8 November 2022

Abstract

Purpose

It has been hypothesized that employment in a fixed-term instead of permanent contract position is associated with

an increased risk of development of mental health problems. The present study aimed at estimating rate ratios between fixed-

term and permanent employees in the Danish labor force, for use of psychotropic drugs and psychiatric hospital treatment

due to mood, anxiety or stress-related disorders, respectively.

Methods

Employment data were drawn from the Danish Labor Force Survey of 2001–2013, which is a part of the European

Labor Force Survey. Full-time employed survey participants without mental illness at the baseline interview (N = 106,501)

were followed in national health registers for up to 5 years. Poisson regressions were used to estimate rate ratios for redeemed

prescriptions of psychotropic drugs and psychiatric hospital treatments due to mood, anxiety or stress-related disease. The

analyses were controlled for age, gender, industrial sector, nighttime work, level of education, calendar year, disposable

family income and social transfer payments within 1 year prior to the baseline interview.

Results

The rate ratio for hospital diagnosed mood, anxiety or stress-related disorders among employees with fixed-term

vs. permanent employment contracts was estimated at 1.39 (99.5% CI 1.04–1.86), while the corresponding rate ratio for

redeemed prescriptions of psychotropic drugs was estimated at 1.12 (99.5% CI 1.01–1.24).

Conclusion

The present study supports the hypothesis that employment in a fixed-term rather than permanent contract posi-

tion is associated with an increased risk of developing mental health problems.

International registered report identifier (IRRID)

DERR2-10.2196/24392.

Keywords

Cohort study · Fixed-term employment · Permanent employment · Psychotropic drugs · Psychiatric hospital

treatment

Introduction

It has been hypothesized that fixed-term contract workers

are at higher risk of developing mental health problems than

permanently employed workers. A reason for this hypothesis

is that the job insecurity associated with a non-permanent

Hermann Burr

[email protected]

National Research Centre for the Working Environment,

Lersø Parkallé 105, 2100 Copenhagen, Denmark

Federal Institute for Occupational Safety and Health, BAuA,

Nöldnerstr. 40–42, 10317 Berlin, Germany

Lægekonsulenten.dk, Hasselager Centerue 35, 8260 Viby J,

Denmark

Department of Public Health, University of Copenhagen,

1014 Copenhagen, Denmark

employment position may act as a stressor that may induce

fears and worries about future unemployment, which in turn

may increase a person’s vulnerability to mental ill health

(Rönnblad et al.

2019).

Another reason for believing in a

prospective association between fixed-term contract posi-

tions and the risk of developing mental ill health is that the

expiry date of a fixed-term employment contract may be

followed by a spell of involuntary unemployment, which is

a well-established risk factor of mental ill health (Paul and

Moser

2009).

The relationship between perceived job inse-

curity and subsequent mental ill health is well established.

A recent review and meta-analysis of longitudinal studies

(Rönnblad et al.

2019)

estimated the odds ratio (OR) for

adverse mental health among workers with self-reported job

insecurity compared with workers without self-reported job

insecurity at 1.52 [95% confidence interval (CI) 1.35–1.70].

The meta-analysis included 14 studies, with a total number

Vol.:(0123456789)

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of 43 568 participants. The OR was greater than one in all

of the included studies. The same review article could, how-

ever, not establish a relationship between objective indica-

tors of job insecurity, i.e., specific types of employments,

and mental ill health. In particular, it could not establish that

employment in a fixed-term instead of permanent contract

position was associated with an increased risk of mental ill

health; only a few such studies had sufficient quality, and

their results were inconsistent (Rönnblad et al.

2019).

Four longitudinal studies, one German (n = 2009),

one Swedish (n = 660) and two Finnish (N = 65,208 and

= 107,828), have estimated longitudinal associations

between fixed-term vs permanent employment and indica-

tors of mental ill health (Hammarström et al.

2011;

Virtanen

et al.

2008;

Ervasti et al.

2014;

Demiral et al

2022).

Only the

German study was on a representative employee population,

the two Finnish were on public sector employees and the

Swedish study was on a follow-up of ninth-grade gradu-

ates. So we know little regarding the possible mental health

effects of fixed-term contracts in representative employee

populations.

The effects of fixed-term contracts on health might be

dependent on welfare state type. The Danish flexicurity wel-

fare state type, characterized by low employment protection,

high compensation for unemployed—even if the level of

compensation has decreased somewhat—and a high turno-

ver (Madsen

2006, 2013).

This has led to a relatively low

fraction of long-term unemployed among the unemployed

and a low overall experience of job insecurity (Madsen

2006).

One recent international comparative analysis shows

that—in Denmark—the level of employment protection is

generally low both among fixed-term contracts and open-

end contracts—a combination which has been found to

be beneficial for general health among those in fixed-term

contracts (Voßemer et al.

2018).

This comparative study,

however, indicates that findings even from other North Euro-

pean countries, such as those mentioned above, cannot be

transferred to a Danish context.

Also, it should be noted that perceived job insecurity is

a subjective construct that may be influenced by an indi-

vidual’s personality traits. It has, for example, been shown

that neuroticism is associated with perceived job insecurity

(Blackmore and Kuntz

2011)

as well as mental ill health

(Lahey

2009).

Hence, until it has been established that the

prospective association between job insecurity and mental

ill health also holds good for objective indicators of job inse-

curity, we cannot rule out the possibility that the positive

association between self-reported job insecurity and devel-

opment of mental ill health only appears among question-

naire respondents with, e.g., varying degrees of neuroticism.

The present study aimed at estimating rate ratios between

fixed-term and permanent employees in the Danish labor

force, for use of psychotropic drugs and psychiatric hospital

treatment due to mood, anxiety or stress-related disorders,

respectively. The present study would thereby contribute

to the international literature with information on relative

rates of mental ill health between fixed-term and permanent

employees in a nation with generous unemployment benefits

and a legislation, which protects fixed-term contract workers

against discrimination at the work place (The Council of the

European Union,

1999).

Methods

The data material and statistical methods of the present

study were completely specified, peer reviewed and pub-

lished in a study protocol (Hannerz et al.

2021)

before we

looked for any relation between the exposure and the out-

come data of the study. The protocol defines two separate

studies. One of the studies would compare rates for use of

psychotropic medicine and psychiatric hospital treatment

among fixed-termed versus permanently employed people,

while the other would do the same thing for fixed-termed

employed versus unemployed people. The former of these

studies is reported in the present paper, while the latter will

be reported elsewhere.

The study protocol contains the following copyright and

license information: “©Harald Hannerz, Hermann Burr,

Helle Soll-Johanning, Martin Lindhardt Nielsen, Anne

Helene Garde, Mari-Ann Flyvholm. Originally published

in JMIR Research Protocols (http://www.researchprotoco

ls.org),

05.02.2021. This is an open-access article distrib-

uted under the terms of the Creative Commons Attribution

License (https://

creat iveco mmons. org/ licen ses/ by/4. 0/),

which permits unrestricted use, distribution, and reproduc-

tion in any medium, provided the original work, first pub-

lished in JMIR Research Protocols, is properly cited.”

Relevant methodological details from the study protocol

will be repeated or paraphrased in the method section of the

present paper.

Data material

This study was based on baseline data on employment

status from the Danish Labor Force Survey (DLFS) (Sta-

tistics Denmark

2019),

which is the Danish part of the

European Labor Force Survey (Eurostat

2021).

Data from

2001 to 2013 were used for baseline, and follow-up data

on health came from a series of registers, which cover the

entire population of Denmark. The following registers were

used: the Central Person Register (CPR) (Pedersen

2011),

the Employment Classification Module (ECM) (Petersson

et al.

2011),

the Danish Education Registers (Jensen and

Rasmussen

2011),

the Danish Family Income Register (Sta-

tistics Denmark

2020),

the Danish Register for Evaluation of

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Marginalization (DREAM) (The Danish Agency for Labour

Market and Recruitment

2019),

the Psychiatric Central

Research Register (Mors et al.

2011),

and the National Pre-

scription Register (Kildemoes et al.

2011).

Linkage on an

individual level was based on participants’ personal identi-

fication numbers (Pedersen

2011).

DLFS is based on quarterly random samples of 15- to

74-year-old residents of Denmark, with systematic oversam-

pling of unemployed people. Each participant is invited to

be interviewed four times over the course of a year-and-a-

half. The purpose of the interviews is to collect person-based

information on inter alia, labor market attachment, type of

contract, and working hours (Statistics Denmark

2019;

Euro-

stat

2021).

Among those invited for the DFLS, the response

rate decreased over time from 70% in 2002 to 53% in 2013

(Hannerz et al.

2018).

The CPR contains, inter alia, infor-

mation on gender and dates of birth, death, and migrations

for every person who is or has been a resident of Denmark

sometime between 1968 and the present time. The ECM

contains annual, person-based information on, inter alia, the

socioeconomic status, occupation, and industry of the resi-

dents of Denmark. The Danish Education Registers contain

person-based information on, inter alia, a person’s highest

educational attainment. The Danish Family Income Register

contains information on household income. DREAM con-

tains weekly, person-based information on social transfer

payments (welfare benefits payments) such as maternity and

paternity benefits, sickness absence benefits, unemployment

benefits, social security cash benefits, and state educational

grants. DREAM has existed since 1991 and covers all resi-

dents of Denmark. The weekly benefits data are recorded

if the person has been on a benefit for 1 or more days of

the week. However, as only one type of social transfer pay-

ment can be registered per week, types of benefits are pri-

oritized in the case of data overlap. The above-mentioned

social transfer payments are prioritized in the order listed,

that is, maternity and paternity benefits have higher priority

than sickness absence benefits, which in turn have higher

priority than unemployment benefits, etc. The Psychiatric

Central Research Register contains person-based informa-

tion on inpatients, outpatients, and emergency ward visits

in all psychiatric hospital departments in Denmark. The

National Prescription Register contains person-based data

on all redeemed prescriptions at pharmacies in Denmark.

•

Psychiatric hospital treatment with mood, anxiety, or

stress-related disorder (ICD-10: F30–F41 or F43) as

the principal diagnosis

The following mental disorders are included in the

above case definition:

•

F30 Manic episode

F31 Bipolar affective disorder

F32 Depressive episode

F33 Recurrent depressive disorder

F34 Persistent mood (affective) disorders

F38 Other mood (affective) disorders

F39 Unspecified mood (affective) disorder

F40 Phobic anxiety disorders

F41 Other anxiety disorders

F43 Reaction to severe stress and adjustment disorders

Exposure

The participants were categorized as “employed on a

fixed-term contract position” or “employed on a perma-

nent contract” in accordance with their responses to the

question “Do you have a temporary or permanent employ-

ment contract?”

Control variables

The analyses were controlled for gender, age (10 year

classes), calendar year of the interview (2001–2003,

2004–2006, 2007–2009, 2010–2013), disposable family

income (tertiles), educational level (low, medium, high,

unstated), industry (“agriculture, forestry, hunting, and

fishing”, “manufacturing, mining, and quarrying”, “con-

struction”, “wholesale, retail and repair of motor vehi-

cles”, “transporting and storage”, “accommodation and

food service activities”, “human health and social work

activities”, “other industries”, “unstated”), nighttime work

(regularly, occasionally, never) and reception of mater-

nity or paternity benefits (yes, no), unemployment benefits

(yes, no) and state educational grants (yes, no) sometime

during the 1-year period preceding the baseline interview.

The variables “gender”, and “age”, refer to the status at the

time of the baseline interview. The variables “disposable

family income”, “educational level” and “industry group”,

refer to the status in the calendar year preceding the inter-

view. The variable “nighttime work” refers to a 4-week

period preceding the interview. Further details about the

operationalization of the control variables are given in our

study protocol (Hannerz et al.

2021).

Clinical end points

Rate ratios were examined for the following end points:

•

Redeemed prescriptions for any type of psychotropic

medicine, that is, drugs in the ATC-code category N05

(psycholeptica) or N06 (psychoanaleptica)

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Follow‑up

The follow-up in the register data started on the date when

6 weeks had passed since the first DLFS interview and ended

on the date when any of the following events occurred: the

participant emigrated, the participant died, the participant

met the clinical end point of the analysis, 5 years had passed

since the date of the start of the follow-up, or the study

period ended. The end of the study period was set at the

end of the calendar year 2014 for redeemed prescriptions of

psychotropic drugs and 2017 for psychiatric hospital treat-

ments. Person-years at risk were calculated for each of the

included participants. Participants who died or emigrated

during the follow-up were censored at the time of the event.

The present study had access to the data on Anatomical

Therapeutic Chemical Classification System (ATC) codes

from the National Prescription Register for the time period

2000–2014 and International Statistical Classification of

Diseases and Related Health Problems, 10th Revision (ICD-

10) codes from the Psychiatric Central Research Register

for the time period 1995–2017. Thus, the follow-up periods

regarding the two outcomes differed in length.

workers is that some workers may have chosen to work part

time due to ill health. Based on survey data (cf. Feveile et al.,

2007),

we estimated that part-time workers who also had a

fixed-term contract constituted approximately 1.5% of all

employees in 2005.

Primary statistical analysis

Poisson regression was used to estimate RRs for psychiatric

hospital treatment for mood, anxiety, or stress-related dis-

orders and redeemed prescriptions for psychotropic drugs,

as a function of employment status at baseline (full-time

fixed-term contract versus full-time permanent contract).

The analyses were adjusted for age, gender, disposable fam-

ily income, educational level, calendar year of the interview,

baseline industry group, nighttime work and reception of

maternity or paternity benefits, unemployment benefits and

state educational grants sometime during a 1-year period

preceding baseline. The logarithm of person-years at risk

was used as an offset. Likelihood ratio tests were used to

test first for main effects and then for effects of interaction

with gender, age, and education level. The main effects were

tested both for psychiatric hospital treatments and redeemed

prescriptions for psychotropic drugs. Due to power concerns,

the interaction effects were only tested for redeemed pre-

scriptions for psychotropic drugs. Each of the tests were

conducted at the significance level 0.005.

We controlled for industry, as a previous study has found

significant industry-related inequalities in the rate of mood

disorders among employees in the general working popula-

tion of Denmark (Hannerz et al.

2009).

We controlled for

unemployment benefits and state educational grants in the

1-year period preceding the interview, as we believe that

people’s attitudes toward fixed-term and permanent contracts

may depend on their previous labor market attachment. We

controlled for nighttime work because it has been shown

that the prevalence of psychotropic drug usage in Denmark

is greater among shift workers than among workers without

shift work (Albertsen et al.

2020).

We controlled for recep-

tion of maternity or paternity benefits, since the birth of a

child may result in maternal (O'Hara and McCabe

2013)

and paternal (Scarff

2019)

postpartum depression. The

remaining control variables were included, since the litera-

ture suggests that the risk of mental ill health depends on

gender (Parker and Brotchie

2010;

McLean et al.

2011),

age

(Wittchen and Hoyer

2001;

Tjepkema

2005;

Kessler et al.

2010),

calendar year (Steinhausen and Bisgaard

2014),

edu-

cation level (Andrade et al.

2000),

and income (Orpana et al.

2009;

Schlax et al.

2019;

Kosidou et al.

2011;

Patel et al.

2018).

We tested for interactions, as it has been suggested that

the strength of adverse health effects of fixed-term contracts

Inclusion criteria

The primary analyses were based on data from the partici-

pants’ first interview in the time period 2001–2013. Par-

ticipants were eligible for inclusion if the following criteria

were fulfilled:

1. The participants were aged between 20 and 59 years at

the time of the interview.

2. They were employed, according to the interview.

3. They usually worked ≥ 32 h a week, according to the

interview.

4. They did not receive any social transfer payments (other

than holiday allowance, unemployment benefits, mater-

nity/paternity benefits, or state educational grants) dur-

ing the 1-year period preceding the interview.

5. They did not receive any psychiatric hospital treatment

with mental disorders (ICD-10: F00–F99) as the prin-

cipal diagnosis during the 1-year period preceding the

start of follow-up.

6. They did not redeem any prescription for psychotropic

drugs (ATC: N05–N06) during the 1-year period preced-

ing the start of follow-up.

Since the fulfillment of inclusion criteria 4–6 only

could be ascertained for participants who lived in Den-

mark throughout the 1-year period preceding baseline, we

excluded all participants who migrated within this period.

We also excluded participants with missing values on the

covariates of the analysis. The reason for excluding part-time

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depends on gender (Pirani and Salvini

2015),

age (Wanberg

et al.

2016),

and education level (Virtanen et al.

2008).

Sensitivity analyses

A series of pre-specified sensitivity analyses were conducted

to: (i) estimate rate ratios in a subset of the study population

where exposure is more stable over time, (ii) estimate rate

ratios without control for industrial sector, nighttime work,

calendar year, disposable family income and welfare benefits

within the 1-year prior to baseline, (iii) estimate rate ratios

by industrial sector, (iv) compare rate ratios obtained with

and without exclusion of former cases of psychiatric treat-

ment, (v) examine rates as a function of reason for having a

fixed-term employment contract, and (vi) estimate relapse

rate ratios.

The motivations, methods and results of the sensitivity

analyses are presented in the appendix.

Results

Between 1 January 2001 and 31 December 2013, 325 553

persons participated in the DLFS, whereof 106 501 were

eligible for inclusion in the primary analysis of the present

study. A flowchart for inclusions and exclusions of the pri-

mary analysis is given in Fig. 1.

Among the included participants, we detected 11 616

cases of redeemed prescriptions for psychotropic drugs and

948 cases of mood, anxiety or stress-related psychiatric hos-

pital treatment, in 430 733 and 519 162 person-years at risk,

respectively.

Among the cases of psychiatric hospital treatment, 0.6%

were manic episodes (ICD-10: F30); 2.0% were bipolar

affective disorders (F31); 27.5% were depressive episodes

(F32); 8.9% were recurrent depressive episodes (F33); 0.7%

consisted of persistent (F34) other (F38) or unspecified

affective mood disorders (F39); 2.5% were phobic anxiety

disorders (F40); 9.0% were other anxiety diagnoses (F41);

and 48.7% consisted of adjustment disorders and reactions

to severe stress and (F43).

Among the cases of psychotropic drug use, 2.6% of the

prescribed drugs were antipsychotics (ATC-code: N05A);

21.9% were anxiolytics (N05B); 35.8% consisted of hyp-

notics and sedatives (N05C); 39.1% were antidepressants

(N06A); 0.57% were psychostimulants (N06B); 0.00% were

antidepressants in combination with psycholeptics (N06C);

and 0.05% were antidementia drugs (N06D).

The rate ratio for hospital diagnosed mood, anxiety or

stress-related disorders among employees with fixed-term

vs. permanent employment contracts was estimated at 1.39

(99.5% CI 1.04–1.86), while the corresponding rate ratio for

redeemed prescriptions of psychotropic drugs was estimated

at 1.12 (99.5% CI 1.01–1.24). The rate ratios for redeemed

prescriptions of psychotropic drugs seemed to be statisti-

cally independent of age, gender and education level; the

values of the tests for interaction were estimated at 0.62,

0.23 and 0.22, respectively. The statistical power was too

low to allow testing for interaction effects for psychiatric

hospital treatment.

The rate ratios, number of persons and person-years at

risk in the analyses of redeemed prescriptions of psycho-

tropic drugs are given in Table 1, with and without stratifi-

cation by gender, age and education level. The results of the

hospital treatment analysis are given in Table 2.

None of the results obtained in the sensitivity analyses

were drastic enough to invalidate the findings of the pri-

mary analyses [cf. Appendix: Tables S1–S6]. The sensitivity

analysis, which stratified rate ratios for use of psychotropic

drugs by industrial sector, suggested, however, that the effect

of having a fixed-term versus permanent employment con-

tract may be especially high in the transport and storage

industry, where the rate ratio was estimated at 1.87 (99.5%

CI 1.14–3.07) [cf. Appendix: Table S6].

Apart from the pre-specified sensitivity analyses, we con-

ducted two post hoc sensitivity analyses. In one of the anal-

yses, we excluded phobic anxiety disorders from the case

definition. All other details of the analysis were the same

as in the primary analysis of the psychiatric hospital treat-

ments. In this post hoc sensitivity analysis, the concerned

rate ratio was estimated at 1.42 (99.5% CI 1.06–1.91). In the

other post hoc analyses, we extended the required period of

“no psychiatric hospital treatment” from 1 to 5 years prior

to the baseline interview. All other details of the analysis

were the same as in the primary analysis of the psychiat-

ric hospital treatments. In this post hoc sensitivity analysis,

the concerned rate ratio was estimated at 1.34 (99.5% CI

0.98–1.82).

Discussion

Main findings

We found that the rate ratios for use of psychotropic drugs

and psychiatric hospital treatment due to mood, anxiety or

stress-related disease, in the Danish labor force, were statis-

tically significantly higher among employees with fixed-term

vs. permanent employment contracts. The tests for interac-

tions with age, gender and education level were not statisti-

cally significant.

Results in relation to previous research

We found four relevant studies that estimated longitudinal

associations between fixed-term vs permanent employment

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Fig. 1

Flowchart for inclusions

and exclusions of the primary

analysis

and indicators of mental ill health, one from Germany

(Demiral et al.

2022),

one from Sweden (Hammarström et al.

2011)

and two from Finland (Virtanen et al.

2008;

Ervasti

et al.

2014).

The German study dealt with employees in employ-

ments subject to social security payments (Demiral et al.

2022)

aged 31–60 years—representing 80% of all peo-

ple working in that age range (n = 2009). Odds ratios for

depressive symptoms as a function of fixed-term employ-

ment contract (yes vs. no) were 2.20 (95% CI 0.80–6.06)

among men and 1.42 (0.61–3.32) among women. The

analyses were adjusted for baseline (2012) age, partnership

status and socioeconomic position. The study population

of the Swedish study (Hammarström et al.

2011)

consisted

of all ninth-grade graduates of the calendar year 1981, in

Luleå, who held temporary and/or permanent employment

contracts between the age of 30 and 42 years (n = 660).

Questionnaire data were collected at the age of 30 and

42 years. Odds ratios at the age of 42, for the contrast

“temporary employment for a total time of more than

10 months” versus “permanent employment during the

whole 12-year period” were estimated at 1.90 (95% CI

1.33–2.71) for psychological distress and 1.79 (95% CI

1.04–3.08) for depressive symptoms. The analyses were

controlled for gender, self-rated health, psychological dis-

tress and depressive symptoms at age 30.

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Table 1

Rate ratio (RR) with

99.5% confidence interval

(CI) for use of psychotropic

drugs, as a function of type of

employment contract among

full-time employees in Denmark

2001–2013

Type of population

Type of

employment

contract

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Fixed-term

Permanent

Persons Person-years Cases

99.5% CI

All employees

Gender strata

Men

Women

Age strata

20–29 years

30–39 years

40–49 years

50–59 years

Educational level strata

High

Medium

Low

Unstated

7460

99,041

3400

54,590

4060

44,451

3522

13,222

1814

26,526

1056

30,547

1068

28,746

2279

30,425

3208

51,442

1908

16,393

781

29,242

401,491

13,162

223,737

16,079

177,754

13,490

54,039

7171

109,248

4277

122,505

4304

115,698

8710

120,019

12,855

210,732

7498

67,850

179

2890

903

10,713

323

4843

580

5870

284

1043

246

2615

171

3469

202

3586

259

3090

404

5374

228

2154

1.12

1.18

1.08

1.05

1.18

1.10

1.16

1.10

1.15

1.06

2.05

1.01–1.24

–

1.00–1.40

–

0.95–1.23

–

0.87–1.27

–

0.97—1.43

–

0.88–1.38

–

0.94–1.44

–

0.92–1.33

–

0.99–1.34

–

0.86–1.29

–

0.87–4.86

–

Adjusted for age, gender, industrial sector, nighttime work, education, calendar year, disposable family

income and state educational grants, unemployment benefits and maternity/paternity benefits within 1 year

prior to baseline

Adjusted for all of the above control variables except gender

Adjusted for all control variables except age

Adjusted for all control variables except education level

Table 2

Rate ratio (RR) with 99.5% confidence interval (CI) for psy-

chiatric hospital treatment due to mood, anxiety or stress-related dis-

orders, as a function of type of employment contract among full-time

employees in Denmark 2001–2013

Type of

employment

contract

Fixed-term

Permanent

Persons

Person-years

Cases

99.5% CI

7460

99,041

35,635

483,527

132

816

1.39

1.04–1.86

–

Adjusted for age, gender, industrial sector, nighttime work, educa-

tion, calendar year, disposable family income and state educational

grants, unemployment benefits and maternity/paternity benefits

within 1 year prior to baseline

One of the Finnish studies (Virtanen et al.

2008)

examined associations between temporary employment

and redeemed prescriptions for antidepressant medica-

tion (1998–2002) among 17,071 men and 48,137 women

employed municipalities in Finland. After adjustment for

age, socioeconomic status (SES), and calendar year, the odds

ratio for the contrast “fixed-term > 6 months vs. permanent

employment” was estimated at 1.18 (95% CI 1.03–1.37)

for antidepressant use in men and 0.99 (95% CI 0.93–1.06)

in women. The corresponding odds ratios for the contrast

“fixed-term < = 6 months vs. permanent employment” were

estimated at 1.43 (95% CI 1.19–1.73) in men and 1.18 (95%

CI 1.09–1.28) in women, and for the contrast “subsidized

temporary work vs. permanent employment” they were esti-

mated at 1.57 (95% CI 1.23–2.02) in men and 1.38 (95%

CI 1.20–1.59) in women. The association between type of

employment contract and use of antidepressants was statisti-

cally significantly weaker among women than it was among

men (p = 0.007). The association was, moreover, weaker

among men with high SES than it was among men with low

SES (p = 0.033).

The other Finnish study (Ervasti et al.

2014)

examined

associations (2005–2011) between temporary vs. permanent

employment and sickness absence due to medically certi-

fied depressive disorders (ICD-10 codes F32–F34) among

107,828 Finnish public sector employees. The concerned

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rate ratio was estimated at 1.02 (95% CI 0.97–1.08). The

analysis was adjusted for age, gender, level of education,

chronic somatic disease and history of work disability due

to mental or behavioral disorder (ICD-10 codes F00–F99).

No significant interaction with gender, age, or education was

observed (p > 0.25).

The associations between fixed-term contracts and men-

tal ill health that were observed in the present study aligns

well with the findings of the Swedish study (Hammarström

et al.

2011)

and the first of Finnish studies (Virtanen et al.

2008).

The German study’s relatively small population size

combined with low prevalence of fixed-term contracts might

explain its insignificant findings (Demiral et al.

2022).

possible explanation for the null-finding observed in the

second of the Finnish studies (Ervasti et al.

2014)

is that it

did not estimate rate ratios for depressive disorders but for

sickness absence due to depressive disorders. Some work-

ers with depressive disorders may call in sick while others

may continue to work, and it has been shown that temporary

employees, due to job insecurity, tend to have higher rates of

sickness presence than permanent employees do (Virtanen

et al.

2003;

Reuter et al.

2019).

It might also be that differ-

ent levels of employment protection in fixed-term contract

and in permanent contracts across countries could lead to

country dependent associations between contract type and

health (Voßemer et al.

2018).

Methodological considerations

Our study has several strengths. The study was quite large

and the statistical power was high enough to investigate main

effects of having a fixed-term versus permanent employment

contract. Bias from missing follow-up data was substantially

reduced, since the endpoints of the study were ascertained

through national registers that cover all inhabitants of the

target population. Within-study selection bias was elimi-

nated, since all hypotheses, significance criteria, endpoints

and statistical methods were completely defined and pub-

lished before we looked at any relation between the exposure

and outcome data of the study (Hannerz et al.

2021).

The major drawback of the study is that it is observa-

tional and thereby has a weaker design than a randomized

controlled trial, which is the golden standard in determining

causality. Another weakness is the low response rate, which

means that we cannot rule out the possibility of non-response

bias. We believe, however, that any such bias has been miti-

gated by the many control variables that were included in

the analyses. Individual participant data were available on

a large variety of socioeconomic and occupational factors,

which enabled us to control the analyses for a series of pos-

sible confounders and health selection effects such as age,

gender, education, industry, nighttime work, unemployment

benefits and income. Control for unemployment is relevant

in order to take selection into part-time work into account.

Control for income is important, as it has been found that

effects of insecurity in employment can be alleviated by

increased wage levels (Böckerman et al.

2011).

It has been shown that the risk of developing depres-

sion is associated with smoking habits (Pasco et al.

2008;

Korhonen et al.

2007)

and body mass index (Luppino et al.

2010).

In the present study, we did not have any individual

participant data on smoking habits and body mass index,

and could therefore not include these factors as control vari-

ables in the analyses. We had, however, access to collateral

data, which we have used to estimate age, gender and educa-

tion standardized prevalence of smoking, overweight, and

obesity among 20–59 year-old employees in Denmark, by

type of employment contract (Hannerz et al.

2021).

The esti-

mated prevalence among people with fixed-term contracts

were very similar to those among people with permanent

contracts. It is therefore unlikely that the results of the pre-

sent study have been influenced by differential prevalence

of smoking, overweight and obesity.

We have not conducted any validation study of self-

reported information on employment contract. We believe,

however, that most (if not all) employees know if they have

a permanent or temporary employment contract and that the

question that was used to obtain the information in the pre-

sent study is very easy to understand and difficult to misin-

terpret. Moreover, the question is not sensitive and it is not

subject to recall bias. It should, however, be noted that our

analysis do not account for time-variant unobservable char-

acteristics that may have an impact on the results. It is, for

example, possible that a person with fixed-term employment

at baseline will become permanently employed or unem-

ployed during the 5 year follow-up period. It is also possible

that a person with permanent employment at baseline will

become unemployed or shift to fixed-term contract position.

Such transitions are probably associated with a bias toward

unity.

In the present study, we used rate ratios of hospital treat-

ment and redeemed prescriptions of drugs as proxy measures

for underlying morbidity ratios. Hence, we need to consider

the possibility of detection, prescription, and referral bias.

In Denmark, all citizens are covered by a tax-funded health

insurance, which enable them to consult a general practi-

tioner and to receive psychiatric treatment free of charge,

whenever it is needed. Since fixed-term and permanent

employees have equal access to general practitioners as well

as psychiatric hospitals and specialists, we do not think that

the present study is subject any detection, prescription or

referral bias of practical importance.

Psychiatric treatment is a rare event; hence, insufficient

statistical power restricted the study of that outcome to

a main effect only model. Psychotropic drugs include a

few types that are used for disorders not expected to be

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associated with stressors like fixed-time contracts, e.g.,

psychostimulants and antidementia drugs. However, only

a few promille of the cases were due to such drugs.

The underlying research hypothesis of the present pro-

ject was that objective job insecurity may act as a stressor

that increases a person’s vulnerability to mental ill health,

without further specification. From this viewpoint, it may

seem natural to include all types of mental disorders in

the case definition of psychiatric hospital treatment. We

chose, however, to exclude the vast majority (87%) of the

diagnoses listed in the chapter on “mental and behavioral

disorders” of the ICD-10 classification, and to focus solely

on diagnoses that are labeled as mood, anxiety or stress-

related disorders. We excluded F00–F09 “Organic mental

disorders” because of their etiology in cerebral disease

or brain injury, which make them quite irrelevant to the

context of the present study; F60–F69 “Personality disor-

ders”, F70–F79 “Mental retardation”, F80–F89 “Disorders

of psychological development” and F90–F98 “Behavioral

and emotional disorders with onset usually occurring in

childhood and adolescence” because such disorders typi-

cally develop well before the entering of the labor mar-

ket; somatoform disorders, firstly, because of an extraor-

dinarily long expected duration between the onset of the

complaints and the diagnosis (Herzog et al.

2018)

and,

secondly, because the labeling of such disorders as mental

illnesses is controversial (Rief and Isaac

2007;

Kroenke

2007);

F10–F19 “Mental and behavioral disorders due to

psychoactive substance use”, F42 “Obsessive–compulsive

disorder” and F50–F59 “Behavioral syndromes associated

with physiological disturbances and physical factors”

because we wanted to keep our case definition simple

and easy to communicate, which would not have been the

case if we had included these diverse sets of behavioral

disorders; and F20–F29 “Schizophrenia, schizotypal and

delusional disorders” because they are associated with an

extraordinarily high heritability (Hilker et al.

2018)

and a

low labor market attachment (Marwaha and Johnson

2004;

Rinaldi et al.

2010),

which make them quite irrelevant to

the context of the present study. Here, it should be noted

that the last mentioned category contains F25 “Schizoaf-

fective disorders” and that manic, bipolar and depressive

schizoaffective disorders thereby were excluded from our

case definition.

We chose to base our case definition on diagnostic stand-

ard groupings at the two- or three-character level rather than

on an ad hoc collection of four-digit level sub-categories,

for several reasons. Firstly, because we wanted to decrease

the probability that relevant cases were missed. Secondly,

because the probability of misclassifications, i.e., false posi-

tive and false negative diagnoses, are likely to be higher at

the four-character level than they are at the two and three-

character level (Jensen et al.

2010).

Thirdly, because a wider

diagnostic category is less sensitive to random variation than

a narrower diagnostic category.

Since (i) participants who received social security cash

benefits, sickness absence benefits, psychotropic medicines

or psychiatric hospital treatment within a 1-year period

prior to baseline were excluded from the analysis and (ii)

most of the mental disorders that are likely to depend on

factors occurring before adulthood were excluded from the

case definition, we do not believe that the study is subject to

reverse causality bias of practical importance. The case defi-

nition included, however, phobic anxiety disorders (ICD-10:

F40), which often manifest themselves already in childhood

or adolescence (Kessler et al.

2007;

Solmi et al.

2022).

It is

possible that some of the cases of phobic anxiety that were

observed in the present study existed already at the start of

the follow-up. It is also possible that that some people may

be unable to obtain or hold a permanent employment posi-

tion due to phobic anxiety disorders. Hence a possibility of

reversed causation. To explore this possibility, we conducted

a post hoc sensitivity analysis in which we excluded phobic

anxiety disorders from the case definition. All other details

of the analysis were the same as in the primary analysis of

the psychiatric hospital treatments. In this post hoc sensitiv-

ity analysis, the concerned rate ratio was estimated at 1.42

(99.5% CI 1.06–1.91).

To further explore the possibility of reversed causation

in the analysis of the hospital treatment data, we conducted

a post hoc sensitivity analysis in which we extended the

required period of “no psychiatric hospital treatment” from

one to five-year prior to the baseline interview. All other

details of the analysis were the same as in the primary analy-

sis of the psychiatric hospital treatments. In this post hoc

sensitivity analysis, the concerned rate ratio was estimated

at 1.34 (99.5% CI 0.98–1.82).

In the analysis of psychotropic drugs, we aimed at esti-

mating the association between our exposure variable and

redeemed prescriptions for psychotropic medicine, and with

such an aim, it made sense to include all types of psycho-

tropic medicine in the case definition.

Two types of health selection bias need to be considered

in the interpretation of the results. The first one concerns

the possibility of bias due to health selection into a fixed-

term or permanent employment position. It is, for example,

possible that some people are unable to obtain or hold a per-

manent employment position due to lingering mental health

problems. The second type of bias concerns health selection

into the analysis. In our primary analysis, we included only

DLFS participants with no social security cash benefits, no

sickness absence benefits, no redeemed prescriptions for

psychotropic medicines and no psychiatric hospital treat-

ment during a whole year prior to the baseline interview. It

was, moreover, required that they were full-time employees

at the time of the interview. The purpose of the rigorous

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inclusion criteria was to counter potential bias from health

selection into a fixed-term employment position. The con-

sequence of the rigorous inclusion criteria is that the sub-

set of fixed-term contract employees that was included in

the primary analysis was far from representative of the full

set of DLFS participants with a fixed-term contract posi-

tion at baseline. It goes without saying that those who were

permanently employed at baseline are more likely to have

been permanently employed also prior to baseline and vice

versa. Hence, if there are any health risks associated with

not having a permanent employment contract then the fixed-

term employees who were included in the primary analysis

are likely to be more privilege and less vulnerable to the

consequences of not having a permanent employment than

the ones who were excluded. Seen from this perspective,

selecting away cases 1 year—and especially 5 years—prior

to baseline can be regarded as a very conservative approach

underestimating possible effects of fixed-term contracts on

depressive symptoms, as effects can have occurred before

the follow-up period.

To shed some light on these health selection effects, we

conducted two sensitivity analyses. In one of the analyses,

we (i) removed the requirement of not receiving sickness

benefits or social security cash benefits during a 1-year

period prior to the baseline interview and (ii) removed all

control variables except for gender, age, and education. We

kept, however, the requirement of full-time employment at

baseline and no redeemed prescriptions for psychotropic

medicines and no psychiatric hospital treatment during a

whole year prior to the baseline interview. The purpose of

this analysis was to obtain an unbiased estimate of the rate

ratio of psychotropic drug use between “a representative set

of the DLFS participants with a full-time fixed-term contract

position” and “a representative set of the DLFS participants

with full-time permanent employment” after standardization

for gender, age and education. The rate ratio in this analy-

sis was estimated at 1.31 (99.5% CI 1.21–1.42). In another

sensitivity analysis, we extended the required period of “no

redeemed prescriptions for psychotropic medicines and no

psychiatric hospital treatment” from 1 to 5 years prior to the

baseline interview (on top of the rigorous inclusion criteria

and potentially over-adjusted confounder control of the pri-

mary analysis). In this sensitivity analysis, the rate ratio of

psychotropic drug use between employees with fixed-term

vs. permanent employment contracts was estimated at 1.05

(99.5% CI 0.90—1.23). Further details about our pre-speci-

fied sensitivity analyses are given in the appendix.

employment contract protection and generous compensa-

tion levels regarding unemployment benefits (Bredgaard and

Madsen

2018;

Madsen

2006, 2013).

Effects of fixed-term

contracts on health might be dependent on the welfare states’

employment protection regarding fixed-term and permanent

contracts (Voßemer et al.

2018).

This means that experi-

enced job insecurity in fixed-term and permanent contracts

could vary considerably between welfare state regimes mak-

ing inference of study results across countries difficult.

Conclusions

We know very little about the possible effects of contract

type and mental health across countries. Increased coop-

eration between labor market and health researchers could

contribute to shed more light into this question. The present

study supports the hypothesis that employment in a fixed-

term rather than permanent contract position is associated

with an increased risk of developing mental health problems

in Denmark. The results in themselves do not warrant spe-

cific interventions regarding fixed-term contracts as they can

range from (a) restrictions in establishing fixed-term con-

tracts over (b) improvements in working conditions for this

group of workers to (c) specific health-related interventions.

We can, however, conclude that the results of the study lend

support to the necessity of the EU council directive 1999/70/

EC of 28 June 1999 concerning the framework agreement on

fixed-term work (The Council of the European Union

1999).

The notably higher RR within transport and storage and to a

lesser extent construction industries might warrant a particu-

lar focus on possible preventive efforts in these industries.

Supplementary Information

The online version contains supplemen-

tary material available at

https://doi.org/10.1007/s00420-022-01936-7.

Acknowledgements

The project was funded by the Danish Working

Environment Research Fund, grant number 20195100796. The data

were supplied by the Danish Health Data Authority and Statistics

Denmark.

Author contributions

The authors (HH, HB, MLN, AHG and MAF)

contributed equally to the design of the study and to the writing of the

manuscript. The statistical analyses were conducted by HH.

Funding

Open Access funding enabled and organized by Projekt

DEAL.

Declarations

Conflict of interest

No potential conflict of interest was reported by

the authors.

Ethical approval

The present study complies with The Act on Process-

ing of Personal Data, Denmark (Act No. 429 of May 31, 2000), which

implements the European Union Directive 95/46/EC on the protection

of individuals. The data usage was approved by the Danish Data Protec-

tion Agency (file number 2001-54-0180). The ethical and legal aspects

Generalizability

The results of the present paper should be seen in the light

of specific conditions at the Danish labor market, which

have been labeled flexicurity, a certain combination of low

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International Archives of Occupational and Environmental Health

of the project were approved by Statistics Denmark, account number

704291. In Denmark, register studies, which do not include medical

procedures, are not part of the ethical committee system.

Open Access

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