MilCohort Baseline Mental Health Feb07 J Clin Epidemiol

Journal of Clinical Epidemiology 60 (2007) 192e201

Millennium Cohort: The 2001e2003 baseline prevalence
of mental disorders in the U.S. military
James R. Riddlea, Tyler C. Smithb,*, Besa Smithb, Thomas E. Corbeilb, Charles C. Engelc,
Timothy S. Wellsb, Charles W. Hoged, Joyce Adkinse, Mark Zamorskif,
Dan Blazerg, for the Millennium Cohort Study Team
a
Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, USA
Department of Defense Center for Deployment Health Research, 5000 North Harbor Drive, San Diego, CA 92106, USA
c
Deployment Health Clinical Center, Walter Reed Army Medical Center, Silver Spring, MD, USA
d
Department of Psychiatry and Behavioral Sciences, Walter Reed Army Institute of Research, Silver Spring, MD, USA
e
Assistant Secretary of Defense for Health Affairs, Force Health Protection, the Pentagon, Washington, DC, USA
f
Deployment Health Section, Directorate of Medical Policy, Canadian Forces Health Services Group Headquarters, Ottawa, Ontario, Canada
g
Duke University Medical Center, Durham, NC, USA
b

Accepted 17 April 2006

Abstract
Objectives: The 12-month prevalence of common mental illnesses in the United States is estimated to be 26%, accounting for an increasing fraction of all disability in the general population. The U.S. military is a unique group involved in response and defense during
times of conflicts and disasters. The mental health of service members affects organizational productivity and effectiveness and is of great
importance to the health of U.S. military members and public health in general.
Study Design and Setting: In the present report, the authors describe the baseline prevalence of mental disorders in a large U.S. military cohort, the Millennium Cohort, established for a 22-year longitudinal study of the health effects of military service. Using crude and
weighted prevalence and multivariable logistic regression, the mental health morbidity of the Millennium Cohort is reported for various
demographics.
Results: These analyses suggest that although the cohort compares favorably to other populations, there are military subpopulations,
including women, younger, less educated, single, white, short-term service, enlisted, and Army members, who are at greater odds for some
mental disorders.
Conclusion: With ongoing U.S. involvement in combat operations around the world, these baseline data are essential to assessing longterm mental health morbidity in U.S. military service members. Ó 2007 Elsevier Inc. All rights reserved.
Keywords: Mental health; Morbidity; Military medicine; Military personnel; Cohort studies; Veterans

1. Introduction
In addition to the authors, the Millennium Cohort Study Team is composed of Margaret A.K. Ryan,1 Tomoko I. Hooper,2 Gregory C. Gray,3
Gary D. Gackstetter,2 Edward J. Boyko,4 and Paul Amoroso5 from the
1
Department of Defense Center for Deployment Health Research at the
Naval Health Research Center, San Diego, CA, USA; 2Department of Preventive Medicine and Biometrics, Uniformed Services University of the
Health Sciences, Bethesda, MD, USA; 3Department of Epidemiology, College of Public Health, University of Iowa, Iowa City, IA, USA; 4Seattle
Epidemiologic Research and Information Center, Veterans Affairs Medical
Center, Puget Sound, Seattle, WA, USA; and 5Army Research Institute of
Environmental Medicine, Military Performance Division, Natick, MA,
USA.
* Corresponding author. Tel.: 619-553-7593; fax: 619-553-7601.
E-mail address: smith@nhrc.navy.mil (T.C. Smith).
0895-4356/07/$ e see front matter Ó 2007 Elsevier Inc. All rights reserved.
doi: 10.1016/j.jclinepi.2006.04.008

The 12-month prevalence of common mental illnesses
among adults in the United States is estimated to be 26%
[1]. This prevalence placed the United States highest out
of 14 countries chosen from the Americas, Europe, the
Middle East, and Asia. The United States also ranked highest in severe disorder prevalence (7.7%) [1]. Mental disorders accounted for nearly 11% of the disease burden
worldwide in 1990, ranking these afflictions almost as
harmful to public health as cardiovascular and respiratory
diseases [2]. In a subsequent 2001 World Health Report
focusing on mental health, the global burden of disease from

J.R. Riddle et al. / Journal of Clinical Epidemiology 60 (2007) 192e201

mental disorders was estimated to have increased to 12%
and projected to reach 15% by 2020 [3]. This report further
documented circumstances or characteristics associated
with increased mental health disorders, including poverty,
sex, age, major physical diseases, family or social environment, and conflicts or disasters [3].
The U.S. military is frequently called upon as the first line
of response or defense in conflicts and disasters, often resulting in service members’ separation from family or home life
for extended periods of time. When service-related risk factors known to be associated with increased mental health
morbidity are considered in conjunction with normal population risk factors, the mental health of this population
becomes a topic of much concern. Hoge et al. [4] reported
that 13% of all military hospitalizations and 28% of all military hospital bed days from 1990 to 1999 were due to mental disorders, and nearly half of those with a first time mental
disorder hospitalization separated from military service
within 6 months. Reports have suggested that personnel
involved in combat operations or peacekeeping missions following combat may have increased symptoms of psychological distress [5e12], with one report recently suggesting
significant risk of mental health problems after combat duty
in Iraq and Afghanistan [13]. With America’s dependence
on a fit and healthy force for the security of this nation,
maintaining sound mental health of all military personnel
is one of the primary goals of military medicine.
The mental health of military service members affects
organizational productivity and effectiveness and is of great
importance to the U.S. military for retention, readiness, and
mission capability. The military is also committed to protecting the health, both physical and mental, of all service
members. In this report, the authors describe the baseline
prevalence of mental disorders in a large U.S. military
cohort that will be longitudinally followed until 2022.

2. Materials and methods
2.1. Study population
The methodology for the Millennium Cohort Study has
been described in detail elsewhere [14]. In brief, those invited to participate in the Millennium Cohort Study came
from a sample provided by the Defense Manpower Data
Center (DMDC), Seaside, California, representing approximately 11.3% of the 2.2 million men and women in service
as of October 1, 2000. U.S. military personnel serving in
the Army, Navy, Coast Guard, Air Force, and Marine Corps
were selected and oversampled for those previously
deployed to Southwest Asia, Bosnia, or Kosovo during
the 1998e2000 time period, U.S. Reserve and National
Guard members, and female service members to ensure
adequate power for statistical inferences over the 22-year
follow-up period. Using a modified Dillman approach
[15], Web and U.S. Postal Service-based enrollment began
in July 2001. Enrollment ended on June 30, 2003, with

193

77,047 consenting participants, or 35.9% of the invited
nondeceased, eligible, and contacted target population.
Demographic data for the Millennium Cohort Study participants were provided by DMDC and reflected status as of
October 1, 2000. These data included date of birth, marital
status, sex, race/ethnicity, Department of Defense primary
and duty occupations, service branch (Army, Navy, Coast
Guard, Air Force, and Marines), service component (active
duty and Reserve/Guard), highest education level, pay
grade, Unit Identification Code, beginning date of active
service, date and reason for separation from service, social
security number, and recent deployment to southwest Asia,
Bosnia, or Kosovo (January 1, 1998 to September 1, 2000).
For this study, missing demographic data for marital status,
occupation, education, and pay grade were supplemented
with self-reported data from the survey whenever possible.
This reduced those missing data for at least one demographic characteristic from 1.8% to 0.4% of the cohort.
2.2. Mental health status
The Millennium Cohort Study questionnaire consists of
more than 450 questions and components regarding diagnosed medical conditions, reported symptoms, psychosocial assessment, physical status, functional status, alcohol
use, tobacco use, occupation, alternative medicine use, exposures, sleep patterns, and basic demographic and contact
data [16]. Mental health measures for this report were
based on two standardized instruments: the Primary Care
Evaluation of Mental Disorders Patient Health Questionnaire (PHQ) [17e19] and the posttraumatic stress disorder
(PTSD) Checklist-Civilian Version (PCL-C) [20,21]. Additionally, an aggregate measure of PTSD or one of the five
PHQ measures was analyzed.
Using standardized scoring algorithms [17e19], the
PHQ provides a psychosocial assessment based on scores
of several health concepts. Reliability and validity of this
instrument has been investigated in a study of 3,000 adult
primary care patients, comparing self-administered PHQ
to primary care physician diagnosis found to have a moderate level of reliability (Kappa 5 0.65), high overall accuracy (85%), and high sensitivity (75%) and specificity
(90%) [18]. Furthermore, sensitivity and specificity have
also been reported high for two PHQ defined threshold
disorders including major depressive disorder (sensitivity 5 0.93; specificity 5 0.89) [22] and panic disorder
(sensitivity 5 100%; specificity 5 0.63) [23]. Measured
for these analyses are threshold disorders (disorders that
correspond to specific diagnoses from the Diagnostic and
Statistical Manual of Mental Disorders, Fourth Edition,
DSM-IV), including major depressive syndrome (9 items)
[24], panic syndrome (15 items), other anxiety syndrome
(6 items), and bulimia nervosa (4 items), and subthreshold
disorders (disorders for which criteria encompass fewer
symptoms than are required for any specific DSM-IV diagnosis), including alcohol abuse (5 items) and binge-eating

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J.R. Riddle et al. / Journal of Clinical Epidemiology 60 (2007) 192e201

disorders. Other anxiety syndrome makes up a generalized
anxiety disorder category excluding anxiety about having
a panic attack (panic disorder), being publicly embarrassed
(social phobia), being contaminated (obsessive-compulsive
disorder), being away from home or close relatives (separation anxiety disorder), gaining weight (anorexia nervosa),
having multiple physical complaints (somatization disorder), or having a serious illness (hypochondriasis), and
the anxiety and worry do not occur exclusively during
PTSD. For the purposes of this report, binge-eating and bulimia nervosa disorders are combined into eating disorders.
The PHQ defines alcohol abuse as an indication of at least
one of five questions answered ‘‘yes’’ if in the past 6
months any experiences have happened more than once.
Defining experiences include drinking alcohol even though
a doctor suggested that you stop drinking because of a problem with your health; you drank alcohol, were high from
alcohol, or hung over while you were working, going to
school, or taking care of children or other responsibilities;
you missed or were late for work, school, or other activities
because you were drinking or hung over; you had a problem
getting along with other people while you were drinking; or
you drove a car after having several drinks or after drinking
too much [17e19].
The PCL-C is a 17-item self-report measure of PTSD
symptoms that requires respondents to rate the severity of
each symptom during the past 30 days on a 5-point Likert
scale (from 1 5 not at all to 5 5 extremely). Participants
were identified as possibly having PTSD if they reported
a moderate or above level of at least one intrusion symptom,
three avoidance symptoms, and two hyperarousal symptoms
[25], and had a total score of 50 or more on a scale of 17e85
[13,20,21,26]. Using this instrument, with a cutoff of 50 has
been reported to be highly specific (specificity 5 99%) in
comparison to other instruments and other cutoff values with
slightly lower sensitivity (60%), a positive predictive value
of 75% and a negative predictive value of 97% [27].
2.3. Statistical analyses
Initial investigation of population characteristics included univariate analyses with chi-square tests of association to assess significant differences in the composition of
the Millennium Cohort when compared with the demographic and military characteristics of the sampling frame
of U.S. military in October 2000. Weighted and nonweighted (data not shown) prevalence estimates of mental
disorders were calculated for demographic subgroups
within the cohort. Weighting was based on the inverse of
the sampling fraction for the three characteristics oversampled: female, past deployed, and Reserve/Guard member. A multivariable exploratory model analysis was
conducted to assess multicollinearity, significant associations, and possible confounding while simultaneously adjusting for all other covariates in the model. Multivariable
logistic regression was used to compare the differences in

adjusted odds of mental health morbidity while controlling
for possible confounders including sex, age, education,
marital status, race/ethnicity, short- and long-term service,
deployment status, pay grade, active-duty status, service
branch, and occupation. Using SAS software (version 9.1,
SAS Institute, Inc., Cary, NC, USA), prevalence, weighted
prevalence, odds ratios (ORs), and 95% confidence intervals (CIs) were calculated for personnel with complete
covariate data [28e30].
3. Results
Demographic data for this report were complete and
available for 76,476 of 77,047 (99.3%) Millennium Cohort
respondents. The cohort consisted of 73% men, 68% between 25 and 44 years of age, 51% with at least some college experience, 63% married, 70% white non-Hispanic,
30% recently deployed, 45% with less than 10 years of military service, 77% enlisted personnel, 57% active duty, 48%
Army, and 20% combat specialists (Table 1). When compared with the U.S. military demographic and military
characteristic distributions in October 2000, significant differences were observed for each population characteristic.
These differences between the cohort and U.S. military
population were largely due to oversampling to assure adequate representation of certain demographic groups. The
cohort is composed of proportionally more service members who are older, educated, married, female, recently
deployed, officer, in the Air Force, with longer time in
service, and who are health care specialists.
Alcohol abuse defined by PHQ was the most prevalent
mental health disorder identified in the cohort (nonweighted, 11.9% [data not shown]; weighted, 12.6% within
6 months) (Table 2). Weighted prevalence of other disorders in this cohort during the previous month was less than
4%; PTSD (2.4% within 1 month), major depressive disorder (3.2% within 2 weeks), panic syndrome (1.0% within 1
month), other anxiety syndrome (2.0% within 1 month),
and eating disorders (3.1% within 3 months). The prevalence of all disorders was higher in women when compared
with men except for alcohol abuse. In general, the six mental disorders were more prevalent in younger, less educated,
single, nondeployed, short-term service, enlisted, and Army
personnel. Weighted and nonweighted (data not shown)
prevalence estimates were consistent in magnitude for the
different subgroups of the cohort.
Model analyses began with investigation of multicollinearity using variance inflation factors of greater than four
to establish collinearity. Length of service, age, and pay
grade were modestly correlated but did not have variance
inflation greater than four. These variables were carefully
evaluated in the subsequent seven separate models constructed for each of the six individual mental disorders
and one for any disorder.
Table 3 reports the multivariable logistic regression
results. After adjusting for all variables in separate

J.R. Riddle et al. / Journal of Clinical Epidemiology 60 (2007) 192e201

195

Table 1
Characteristics of Millennium Cohort Study members (panel 1), June 30, 2001 to July 30, 2003, and the U.S. military in October 2000a
Variable

Cohort n (%), N 5 76,476

U.S. militaryb n (%), N 5 2,068,078

P value*

Sex
Male
Female

56,052 (73.3)
20,424 (26.7)

1,761,962 (85.2)
306,116 (14.8)

!0.0001

Age (years)
17e24
25e34
35e44
O44

14,466
26,829
25,254
9,927

(18.9)
(35.1)
(33.0)
(13.0)

672,919
702,523
517,703
174,933

(32.5)
(34.0)
(25.0)
(8.5)

!0.0001

Education
No high school diploma
High school diploma/equivalent
Some college
Bachelor’s degree
Master’s/PhD

4,706
32,832
19,594
12,503
6,841

(6.2)
(42.9)
(25.6)
(16.4)
(9.0)

169,583
1,110,558
431,868
233,039
121,878

(8.2)
(53.7)
(20.9)
(11.3)
(5.9)

!0.0001

Marital status
Single
Married
Divorced

22,914 (30.0)
48,312 (63.2)
5,250 (6.9)

855,488 (41.4)
1,109,238 (53.6)
103,352 (5.0)

!0.0001

Race/ethnicity
White non-Hispanic
Black non-Hispanic
Other

53,248 (69.6)
10,552 (13.8)
12,676 (16.6)

1,398,008 (67.6)
394,067 (19.1)
276,003 (13.4)

!0.0001

Recent deployment experience
No
Yes

53,536 (69.8)
23,182 (30.2)

1,861,270 (90.0)
206,808 (10.0)

!0.0001

Length of service (years)
0e4
5e9
10e14
O14

19,714
14,922
13,983
27,857

Military pay grade
Enlisted
Commissioned officer
Warrant officer

(25.8)
(19.5)
(18.3)
(36.4)

774,424
370,773
304,069
618,812

(37.5)
(17.9)
(14.7)
(29.9)

!0.0001

59,116 (77.3)
15,996 (20.9)
1,364 (1.8)

1,755,370 (84.9)
289,497 (14.0)
23,211 (1.1)

!0.0001

Service component
Reserve/Guard
Active duty

32,883 (43.0)
43,593 (57.0)

854,087 (41.3)
1,213,991 (58.7)

!0.0001

Branch of service
Army
Air Force
Navy and Coast Guard
Marines

36,357
22,339
13,858
3,922

(47.5)
(29.2)
(18.1)
(5.1)

961,242
499,122
427,077
180,637

(46.5)
(24.1)
(20.7)
(8.7)

!0.0001

Occupational category
Combat specialists
Electrical repair
Communications/intelligence
Health care specialists
Other technical
Functional support specialists
Electrical/mechanic repair
Craft workers
Service support
Students, prisoners, other

15,257
6,730
5,390
7,906
1,969
15,335
11,353
2,375
6,659
3,502

(20.0)
(8.8)
(7.1)
(10.3)
(2.6)
(20.1)
(14.9)
(3.1)
(8.7)
(4.6)

463,075
172,152
148,859
169,049
57,377
372,645
325,395
80,061
200,935
78,530

(22.4)
(8.3)
(7.2)
(8.2)
(2.8)
(18.0)
(15.7)
(3.9)
(9.7)
(3.8)

!0.0001

*P values based on Pearson chi-square test of association.
a
Only observations with complete Table 1 demographic data were used in this study.
b
Demographic and military characteristics based on U.S. military service rosters for active-duty and Reserve/Guard members in sampling frame in
October 2000.

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J.R. Riddle et al. / Journal of Clinical Epidemiology 60 (2007) 192e201

Table 2
Weighteda prevalence of PHQ and patient checklist defined mental health morbidity among Millennium Cohort participants

PTSD
(w%)

Major
depressive
disorder
(w%)

Panic
syndrome
(w%)

Other
anxiety
syndrome
(w%)

Alcohol
abuse
(w%)

Eating
disorders
(w%)

18.3

2.4

3.2

1.0

2.0

12.6

3.1

Sex
Male
Female

18.4
17.5

2.2
3.2

2.8
5.1

0.9
2.1

1.7
3.4

13.4
8.4

2.9
4.3

Age (years)
17e24
25e34
35e44
O44

29.9
17.9
14.4
12.5

3.8
2.2
2.1
1.8

5.5
2.9
2.4
2.2

1.6
0.9
1.0
0.9

3.0
1.9
1.7
1.2

22.8
12.4
9.0
7.8

3.7
3.3
2.9
2.5

Education
No high school diploma
High school diploma/equivalent
Some college
Bachelor’s degree
Master’s/PhD

25.0
22.9
15.8
12.8
8.8

3.8
3.3
1.9
1.1
0.6

5.1
4.4
2.3
1.5
1.0

1.8
1.2
1.1
0.6
0.4

3.1
2.8
1.6
0.8
0.6

17.9
15.9
10.4
9.2
5.3

3.6
3.6
2.9
2.3
2.7

Marital status
Single
Married
Divorced

26.4
14.6
19.1

3.2
2.0
2.8

4.5
2.5
3.5

1.3
0.9
1.5

2.5
1.7
2.1

20.5
9.1
12.4

3.2
3.1
3.0

Race/ethnicity
White non-Hispanic
Black non-Hispanic
Other

19.3
15.3
16.5

2.2
2.8
2.6

3.0
4.0
3.2

1.1
1.0
0.9

2.0
2.1
1.8

13.7
9.1
10.6

3.3
2.1
3.2

Recent deployment experience
No
Yes

18.4
17.0

2.4
1.9

3.2
2.8

1.1
1.0

2.0
1.7

12.7
12.0

3.2
2.9

Length of service (years)
0e4
5e9
10e14
O14

25.9
18.9
15.3
14.2

3.5
2.2
1.9
2.0

4.8
3.1
2.4
2.4

1.4
1.0
0.7
1.0

2.8
2.0
1.6
1.6

19.2
13.3
10.1
9.0

3.4
3.3
3.1
2.9

Military pay grade
Enlisted
Commissioned officer
Warrant officer

20.7
10.5
11.0

2.9
0.6
1.5

3.8
1.1
1.3

1.3
0.4
0.2

2.4
0.6
1.1

14.3
7.2
7.0

3.3
2.5
2.2

Service component
Reserve/Guard
Active duty

19.1
17.8

2.2
2.5

2.7
3.5

1.0
1.1

1.6
2.2

14.1
11.5

2.7
3.4

Branch of service
Army
Air Force
Navy and Coast Guard
Marines

19.5
13.0
19.7
25.3

3.0
1.2
2.3
2.8

3.7
1.8
3.2
4.0

1.2
0.9
0.9
0.9

2.5
1.2
1.7
2.4

13.1
8.8
14.0
19.2

3.4
2.3
3.5
3.2

Occupational category
Combat specialists
Electrical repair
Communications/intelligence
Health care specialists
Other technical
Functional support specialists
Electrical/mechanic repair
Craft workers
Service support
Students, prisoners, other

19.0
17.7
18.8
14.8
19.7
15.7
20.6
20.4
19.6
21.0

2.1
2.2
2.2
2.2
3.0
2.3
2.6
3.0
3.2
2.2

2.6
2.9
3.6
2.8
3.9
3.2
3.6
3.0
3.9
3.2

0.6
1.0
1.1
1.2
1.4
1.2
1.0
0.7
1.6
1.3

1.7
1.6
2.2
2.0
2.3
1.9
2.2
1.7
2.8
1.8

14.3
12.4
13.0
8.7
13.3
9.4
14.9
15.5
12.8
15.7

3.2
3.1
3.1
3.6
2.4
3.1
3.2
2.4
3.3
2.8

Variable

Any PHQ
or PTSD
(w%)

Full cohort

a

Weighted for sampling differences in sex, military component, and prior deployment for those with complete demographic data.

J.R. Riddle et al. / Journal of Clinical Epidemiology 60 (2007) 192e201

197

Table 3
Adjusted odds using multiple logistic regression of PHQ and patient checklist defined mental health morbidity among Millennium Cohort subgroups
Major
depressive
disorder

Panic
syndrome

Other anxiety
syndrome

Alcohol
abuse

Eating
disorders

CI

OR

CI

OR

CI

OR

CI

OR

CI

OR

CI

1.0
1.4

d
1.2e1.5

1.0
1.8

d
1.6e1.9

1.0
2.3

d
1.9e2.7

1.0
2.1

d
1.9e2.4

1.0
0.6

d
0.5e0.6

1.0
1.6

d
1.5e1.8

d
0.7e0.8
0.6e0.7
0.5e0.6

1.0
0.9
1.2
1.4

d
0.8e1.0
0.9e1.4
1.1e1.8

1.0
0.8
1.0
1.2

d
0.7e1.0
0.8e1.2
1.0e1.6

1.0
0.8
1.1
1.2

d
0.6e1.0
0.8e1.6
0.8e1.8

1.0
1.0
1.1
1.2

d
0.8e1.2
0.9e1.4
0.9e1.6

1.0
0.6
0.5
0.4

d
0.6e0.7
0.5e0.6
0.4e0.5

1.0
0.9
0.8
0.7

d
0.8e1.0
0.7e1.0
0.6e0.9

1.1
1.0
1.0
0.8
0.8

1.0e1.1
d
0.9e1.0
0.8e0.9
0.7e0.9

1.1
1.0
0.9
0.6
0.5

0.9e1.3
d
0.7e1.0
0.5e0.7
0.4e0.8

1.2
1.0
0.8
0.6
0.5

1.0e1.4
d
0.7e0.9
0.5e0.8
0.4e0.7

1.3
1.0
1.0
0.7
0.6

1.0e1.6
d
0.8e1.2
0.5e1.0
0.4e0.9

1.3
1.0
0.9
0.6
0.7

1.1e1.5
d
0.8e1.0
0.5e0.8
0.5e1.0

1.0
1.0
1.0
0.9
0.8

0.9e1.1
d
0.9e1.0
0.8e1.0
0.7e0.9

1.0
1.0
1.1
0.9
1.0

0.9e1.2
d
0.9e1.2
0.7e1.0
0.8e1.3

Marital status
Singleb
Married
Divorced

1.0
0.7
1.1

d
0.7e0.8
1.0e1.2

1.0
0.9
1.3

d
0.8e1.0
1.0e1.5

1.0
0.9
1.2

d
0.8e1.0
1.0e1.5

1.0
1.0
1.5

d
0.8e1.2
1.2e2.0

1.0
1.0
1.2

d
0.9e1.2
1.0e1.5

1.0
0.6
0.9

d
0.6e0.6
0.8e1.0

1.0
1.1
1.2

d
1.0e1.2
1.0e1.4

Race/ethnicity
White non-Hispanicb
Black non-Hispanic
Other

1.0
0.7
0.9

d
0.6e0.7
0.8e0.9

1.0
1.0
1.2

d
0.8e1.1
1.1e1.4

1.0
1.0
1.1

d
0.9e1.2
0.9e1.2

1.0
0.6
0.9

d
0.5e0.8
0.8e1.1

1.0
0.8
0.9

d
0.7e0.9
0.8e1.1

1.0
0.6
0.8

d
0.6e0.6
0.8e0.9

1.0
0.6
0.9

d
0.5e0.7
0.8e1.0

Recent deployment experience
Nob
Yes

1.0
1.0

d
0.9e1.0

1.0
0.9

d
0.8e1.0

1.0
0.9

d
0.8e1.0

1.0
1.0

d
0.8e1.1

1.0
0.9

d
0.8e1.1

1.0
1.0

d
1.0e1.1

1.0
1.0

d
0.9e1.1

Length of service (years)
0e4b
5e9
10e14
O14

1.0
0.9
0.9
0.9

d
0.9e1.0
0.8e1.0
0.8e1.0

1.0
0.8
0.7
0.7

d
0.7e0.9
0.6e0.9
0.5e0.8

1.0
0.8
0.8
0.7

d
0.7e0.9
0.7e0.9
0.6e0.9

1.0
0.9
0.8
0.9

d
0.7e1.1
0.6e1.0
0.6e1.2

1.0
0.9
0.8
0.8

d
0.7e1.0
0.6e1.0
0.6e1.0

1.0
1.0
0.9
0.9

d
0.9e1.0
0.8e1.0
0.8e1.0

1.0
1.0
1.0
1.2

d
0.9e1.2
0.9e1.2
1.0e1.4

Military pay grade
Enlistedb
Commissioned officer
Warrant officer

1.0
0.6
0.6

d
0.6e0.7
0.5e0.7

1.0
0.4
0.4

d
0.3e0.5
0.3e0.7

1.0
0.5
0.5

d
0.4e0.6
0.3e0.8

1.0
0.5
0.2

d
0.3e0.6
0.1e0.6

1.0
0.4
0.5

d
0.3e0.5
0.3e0.8

1.0
0.7
0.6

d
0.6e0.8
0.5e0.8

1.0
0.7
0.5

d
0.6e0.9
0.4e0.8

Service component
Reserve/Guardb
Active duty

1.0
0.8

d
0.8e0.9

1.0
1.3

d
1.1e1.4

1.0
1.4

d
1.3e1.5

1.0
1.3

d
1.1e1.5

1.0
1.6

d
1.4e1.8

1.0
0.6

d
0.6e0.7

1.0
1.3

d
1.2e1.5

Branch of service
Armyb
Air Force
Navy and Coast Guard
Marines

1.0
0.7
1.0
1.2

d
0.6e0.7
1.0e1.1
1.1e1.3

1.0
0.5
0.7
0.8

d
0.4e0.6
0.6e0.8
0.7e1.0

1.0
0.6
0.8
0.9

d
0.5e0.6
0.7e0.9
0.8e1.1

1.0
0.7
0.9
0.8

d
0.5e0.8
0.7e1.0
0.6e1.1

1.0
0.5
0.7
0.8

d
0.5e0.6
0.6e0.8
0.7e1.1

1.0
0.8
1.2
1.4

d
0.7e0.8
1.1e1.2
1.3e1.5

1.0
0.6
0.9
0.8

d
0.5e0.7
0.8e1.0
0.6e1.0

Occupational category
Combat specialistsb
Electrical repair
Communications/intelligence
Health care specialists
Other technical
Functional support specialists
Electrical/mechanic repair
Craft workers
Service support
Students, prisoners, other

1.0
0.9
0.8
0.9
0.9
0.8
0.9
0.9
0.9
0.8

d
0.8e1.0
0.8e0.9
0.8e0.9
0.8e1.0
0.8e0.9
0.9e1.0
0.8e1.0
0.9e1.0
0.7e0.9

1.0
1.1
0.8
1.0
1.0
0.9
1.0
1.3
1.2
0.9

d
0.9e1.3
0.7e1.1
0.8e1.2
0.8e1.4
0.8e1.1
0.9e1.2
1.0e1.7
1.0e1.4
0.7e1.1

1.0
1.1
1.1
1.0
1.1
1.0
1.1
1.0
1.1
0.9

d
0.9e1.3
0.9e1.3
0.8e1.2
0.9e1.5
0.9e1.1
0.9e1.2
0.8e1.3
1.0e1.3
0.8e1.2

1.0
1.4
1.2
1.5
1.4
1.2
1.3
1.0
1.8
1.3

d
1.0e1.9
0.9e1.7
1.1e2.0
0.9e2.1
1.0e1.6
1.0e1.7
0.6e1.6
1.4e2.4
0.9e1.9

1.0
0.9
0.9
1.0
0.9
0.8
1.1
0.9
1.2
0.9

d
0.7e1.1
0.7e1.1
0.8e1.2
0.7e1.3
0.7e1.0
0.9e1.3
0.6e1.2
1.0e1.5
0.7e1.1

1.0
0.9
0.8
0.8
0.9
0.7
0.9
0.9
0.8
0.7

d
0.8e0.9
0.7e0.9
0.7e0.8
0.8e1.0
0.7e0.8
0.8e1.0
0.8e1.0
0.8e0.9
0.6e0.8

1.0
0.9
0.8
1.0
0.9
0.9
0.9
0.7
1.0
1.0

d
0.7e1.0
0.6e0.9
0.9e1.2
0.7e1.2
0.8e1.0
0.8e1.1
0.6e1.0
0.8e1.1
0.8e1.2

Any PHQ or
PTSD

PTSD

Variable

ORa

CIa

OR

Sex
Maleb
Female

1.0
0.9

d
0.9e1.0

Age (years)
17e24b
25e34
35e44
O44

1.0
0.7
0.6
0.6

Education
No high school diploma
High school diploma/equivalentb
Some college
Bachelor’s degree
Master’s/PhD

a
ORs and associated 95% CIs for those with complete demographic data are adjusted for sex, age, education, marital status, race/ethnicity, short- and
long-term service, deployment status, pay grade, active-duty status, service branch, and occupation.
b
Reference category for measure of association.

198

J.R. Riddle et al. / Journal of Clinical Epidemiology 60 (2007) 192e201

multivariable logistic models, women were at statistically
significant increased adjusted odds of PTSD (OR 5 1.4),
major depressive disorder (OR 5 1.8), panic syndrome
(OR 5 2.3), anxiety syndrome (OR 5 2.1), and eating disorders (OR 5 1.6), whereas at statistically significant decreased adjusted odds of alcohol abuse (OR 5 0.6) when
compared with men. Older personnel, when compared with
personnel aged 17e24 years, were at significantly lower
odds of any PTSD or PHQ morbidity (OR 5 0.6), alcohol
abuse (OR 5 0.4), and eating disorders (OR 5 0.7), and
significantly higher odds of PTSD (OR 5 1.4) and major
depressive disorders (OR 5 1.2). Personnel with a bachelor’s degree or higher were at a significant decreased odds
of any PHQ or PTSD morbidity, and specifically PTSD,
major depressive disorder, panic syndrome, other anxiety
syndrome, and alcohol abuse. Married personnel were at
half the odds of alcohol abuse (OR 5 0.6) when compared
with single personnel. Black non-Hispanic personnel were
at a significant decreased odds of any PHQ or PTSD
(OR 5 0.7), and specifically panic syndrome (OR 5 0.6),
other anxiety syndrome (OR 5 0.8), alcohol abuse
(OR 5 0.6), and eating disorders (OR 5 0.6) when compared with white non-Hispanics. Personnel in service for
more than 14 years were at significantly lower odds of
PTSD (OR 5 0.7) and major depressive disorder
(OR 5 0.7) when compared with those in service for less
than 4 years. Being an officer was strongly protective
against mental health morbidity in all seven measures when
compared with enlisted personnel. When compared with
Reserve/Guard, active-duty personnel were significantly
more likely to have symptoms of PTSD (OR 5 1.3), major
depressive disorder (OR 5 1.4), panic syndrome
(OR 5 1.3), other anxiety syndrome (OR 5 1.6), and eating
disorders (OR 5 1.3) and less likely to have symptoms of
alcohol abuse (OR 5 0.6). Air Force personnel were at significantly lower odds of each of the seven outcomes when
compared with Army personnel, whereas Navy personnel
were at lower odds of PTSD (OR 5 0.7), major depressive
disorder (OR 5 0.8), and other anxiety syndrome
(OR 5 0.7), and at higher odds of alcohol abuse
(OR 5 1.2). Marines were at significant increased odds of
alcohol abuse (OR 5 1.4) when compared with Army personnel. There were no consistent statistically significant
findings among occupational categories, although combat
specialists appeared to be more likely to have symptoms
of alcohol abuse and less likely to have symptoms of panic
syndrome when compared with other occupations. Those
recently deployed to southwest Asia, Bosnia, or Kosova
were not at statistically different adjusted odds of mental
health morbidity when compared with those not recently
deployed.
4. Discussion
The worldwide prevalence of mental disorders has increased significantly during the past two decades and the

projected 15% prevalence by 2020 [3] will have farreaching public health implications [31]. The complex nature of identifying mental disorders, coupled with multiple
causal pathways and diverse populations in which these
conditions present, make this a likely underestimate of
the true burden of disease. The U.S. military represents
the diversity of the U.S. population with varying ethnic
groups, social backgrounds, occupations, and demographic
characteristics. Military personnel may be less likely to
seek medical evaluation and feel more alienated due to real
or perceived stigma regarding mental health conditions than
what might be seen in the general public. This underscores
the importance of understanding the burden of mental disorders in this population, which may be underreported in
medical databases. Mental health disorders in military service members have a broad impact on military readiness
through diminished organizational productivity and effectiveness. The results of this report suggest that although
there are subpopulations in need of further study and potential intervention, the mental health of this military cohort
compares favorably to the U.S. population in general.
The prevalence of the mental disorders, other than alcohol abuse, was found to be consistent with or less common
than that of other reports on military and nonmilitary populations [13,32e43]. For example, the prevalence of PTSD
in the weighted military sample was 2.4% compared with
3.5% in the recent National Comorbidity Survey Replication (NCS-R) of households [44], major depressive disorder
prevalence was 3.2% compared with 6.7% in the NCS-R
sample, and panic syndrome was 1.0% compared with
2.7% in the NCS-R sample. These studies are not directly
comparable because of different survey methodology, different population demographics, and the fact that the
NCS-R study focused on 12-month prevalence rather than
1-month prevalence rates.
However, the rates that we detected are well within
expected prevalence estimates from numerous studies
[13,32e43]. A previous study that extrapolated Epidemiologic Catchment Area (ECA) data to a military population
estimated the prevalence of panic syndrome in a military
population to be 0.9% [45]. This compared with 1.0% detected in this direct survey of a military population. Major
depressive disorder was estimated to have a prevalence of
3.8%, based on the extrapolated ECA data, compared with
3.2% that we detected. PTSD was not measured in the
ECA study.
As reported elsewhere [46e48], populations at greater
odds of mental disorders in general included women,
young, and single personnel, and individuals with lower socioeconomic status as measured by education, enlisted
rank, and length of service. Our finding of a significant reduction in adjusted odds among black non-Hispanics for
panic syndrome, anxiety, alcohol abuse, and eating disorders when compared with white non-Hispanics has not
been previously reported. Although this may be due to study
population size or composition, another possible

J.R. Riddle et al. / Journal of Clinical Epidemiology 60 (2007) 192e201

explanation may be that these results represent the true
baseline prevalence of a large subpopulation of U.S.
military members and may be of significant interest for
comparison to postcombat deployments in support of
Operation Enduring Freedom or Operation Iraqi Freedom.
Additionally, active-duty and Army personnel were at increased adjusted odds of many of these disorders when
compared with Reserve/Guard and other services.
Alcohol abuse, as defined by the PHQ, was the most
prevalent mental disorder in the Millennium Cohort and
was consistent with or slightly more prevalent when compared with other nonmilitary populations [49e52]. After
controlling for sex, age, education, marital status, race/ethnicity, short- and long-term service, deployment status, pay
grade, active-duty status, service branch, and occupation in
the model, personnel who were male, younger, single, less
educated, enlisted, active duty, Marine, and combat specialists were at the highest adjusted odds of this disorder and
should be considered for further investigation and
intervention.
It is interesting that there were no significant differences
in mental health morbidity due to recent deployment to
southwest Asia, Bosnia, and Kosovo. In a recent report
[13], PTSD symptoms consistent with PCL scoring in this report were found in 5.0% of the overall study population prior
to deployment to Iraq. After returning from deployment,
6.2% of a population deployed to Afghanistan and more than
12% of population deployed to Iraq were identified as having
PTSD symptoms. The baseline PTSD symptoms reported
here is nearly half of the baseline reported by Hoge et al.
and may be explained by differences in study population
composition. Specifically, the baseline PTSD symptoms in
Millennium Cohort members was 3.9% for those aged
17e24 years. However, this group makes up only 19% of
the cohort, whereas those ages 18e24 years comprised the
majority, with a very small proportion over 40 years of age
in the previous report [13]. The Millennium Cohort consists
of more highly educated personnel, more officers, and more
married personnel, all of which are associated with a reduced
prevalence of PTSD symptoms (Table 2). Lastly, the report
by Hoge et al. focused on combat units that were engaged
in direct combat, unlike the participants of this cohort who
may or may not have been directly engaged in combatrelated deployments.
There are notable limitations to these analyses that
should be mentioned. Although the original cohort was
a random weighted sample of the U.S. military, the study
population consists of a self-selected group representing
35.9% of the invited and contacted personnel. Because
we oversampled female, deployed, and Reserve/Guard
members, and because there were proportional differences
between responders and nonresponders including older,
more educated, married, and officer members [14], these
data may not be representative of the military population
in general. It is possible that those who were in ill health
may have believed that inclusion in a study of this type

199

might be of benefit to them, resulting in a possible overestimation of the true prevalence. Conversely, those who
might be severely ill may not choose to respond or may
not have the capacity to do so, which could potentially underestimate the true prevalence. Additionally, although the
Millennium Cohort is a longitudinal prospective study, the
cross-sectional design of these analyses did not permit us to
investigate temporality at this stage. However, additional
longitudinal studies of this cohort will provide us this opportunity. Furthermore, an enrollment period encompassing
the tragic events of September 11, 2001, may have introduced a temporal change in mental illness because military
personnel were called upon to respond to the terrorist attacks [53]. Lastly, although these mental health instruments
have undergone thorough testing and are believed to correlate well with a physician’s assessment of mental health
[17e20,54], the use of standardized instruments for selfreported data as a surrogate for mental health diagnosis is
imperfect.
Despite limitations, our study has a number of unique
strengths. The large study population, along with many
demographic variables, permitted robust risk estimates,
allowing for considerable statistical power to detect small
differences in the mental health outcomes measured. Additionally, we used multivariable modeling techniques to
model the odds of disease while adjusting for many covariates. Finally, the use of standardized instruments allows
for comparison to other populations, such as the U.S. population in general [18,20] or other military populations,
such as the Canadian [43,55], Australian [56], or United
Kingdom forces. This analysis represents a major step in
the effort to establish the baseline mental health status in
a large U.S. military cohort for prospective observation
of trends in exposures and outcomes temporally investigated over a 22-year follow-up period.
In summary, the U.S. military represents a complex and
dynamic group of Americans whose sensitivity to world
events is heightened due to the personal impact on their
lives. In this report, we document the baseline prevalence
of mental disorders in a large U.S. military cohort. With
the exception of alcohol abuse, our findings suggest a mentally healthier population than other comparison populations and should be reassuring to those serving and those
who depend on the readiness of our military forces. However, the higher prevalence of alcohol abuse and the disproportionally higher burden of mental disorders among
some subgroups should prompt further clinical investigation and intervention. When factors known to increase
the prevalence of mental health morbidity are combined
with physically and mentally demanding military occupations and lifestyle, extended periods of separation from
family or home life, high operational tempo during times
of war, relative lack of knowledge of mental illness symptoms or treatments, and social stigmas often associated
with treatment, there is reason for concern. Further coordination between primary care providers, mental health

200

J.R. Riddle et al. / Journal of Clinical Epidemiology 60 (2007) 192e201

specialists, and researchers is necessary to identify and
prioritize critical areas for research and improvement of
clinical services to protect the mental health of the U.S.
military. In the future, the Millennium Cohort Study
[14,16] will continue to address important issues of military health through prospective long-term follow-up of
individuals in the cohort.

Acknowledgments
We thank Scott L. Seggerman from the Management
Information Division, Defense Manpower Data Center,
Seaside, California, for providing a sample of military personnel and their demographic and deployment data. Additionally, we thank Isabel Gomez, Gia Gumbs, Sheila
Jackson, Cynthia Leard, Travis Leleu, Nick Martin, Robb
Reed, Steven Spiegel, Jim Whitmer, and Dr. Sylvia Young
from the Department of Defense Center for Deployment
Health Research, Naval Health Research Center, San Diego, California, and Dr. Nicole Bell and Laura Senier from
the Army Research Institute of Environmental Medicine,
Total Army Injury and Health Outcomes Database Project,
Natick, Massachusetts. We appreciate the support of
the Henry M. Jackson Foundation for the Advancement
of Military Medicine, Rockville, Maryland.
This represents report AFRL-WS 05-2003 and NHRC05-18, supported by the Department of Defense, under work
unit no. 60002. The views expressed in this article are those
of the authors and do not reflect the official policy or position
of the Department of the Navy, Department of the Army,
Department of the Air Force, Department of Defense, Department of Veterans Affairs, or the U.S. Government.
Approved for public release; distribution is unlimited. This
research has been conducted in compliance with all applicable federal regulations governing the protection of human
subjects in research (Protocol NHRC.2000.007).
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