Study Population
Established in 2001, the Millennium Cohort Study is a prospective cohort study investigating the long-term health effects related to military service both during service and after separation from the military (24). The sample for the Cohort was randomly selected from US military rosters, with over-sampling of selected subgroups of interest, as previously described (25, 26). Service members were enrolled into four panels between 2001 and 2013, resulting in a total of 201,619 participants across all service branches, including both active duty and Reserves and National Guard (Reserve/Guard) members (27.3% cumulative baseline response rate). Previous studies have found the Millennium Cohort to be a representative sample of service members in terms of health status, and analyses on weighting for nonresponse have not identified changes in metrics for mental disorders (27, 28). Furthermore, using data from the Millennium Cohort, only slight differences between weighted means and nonweighted means for numerous mental and physical health conditions have been found; therefore, nonweighted data were used for the current study (29, 30).
After enrollment, participants were requested to complete follow-up surveys that are accessed online or via postal mail approximately every three to five years, even after they leave military service. These surveys collected information about behavioral, physical, and mental health, as well as service-related experiences. A detailed description of the Millennium Cohort has been previously published (24).
Inclusion Criteria
Cohort members who completed both the 2011-2013 and 2014-2016 surveys, referred to as “baseline” and “follow-up” respectively for this study, were eligible (n = 92,614) for the current study. Of these participants, those who were missing 3 or more of the items (n=2,348) from the DAR-5 (21, 22) at follow-up were excluded from this study (analytic sample N = 90,266, 97.5% of the eligible sample).
Measures
Problematic Anger
Problematic anger was assessed at follow-up from the DAR-5, a validated instrument introduced on the follow-up survey (Cronbach’s alpha=0.91). Participants responded to the question “Indicate the degree to which each statement describes your feelings or behavior:” with a 5-point Likert scale ranging from 1 (Not at all) to 5 (Very Much) for each of the five items (e.g., when I get angry, I get really mad; when I get angry at someone, I want to hit or clobber the person). Responses were summed and problematic anger (no/yes) was determined using the established cut-off of 12 points or higher (22). The DAR-5 measure (21) has demonstrated robust convergent, concurrent and discriminant validity. In addition, the recommended cut off, aligned with the 75th percentile of the State Trait Anger Expression Inventory-2 (STAXI-2;(20)), has been associated with psychological distress and functional impairment (22, 23) and utilized in numerous studies (31-33).
Predictors of Anger
Demographics and military experiences
Marital status and educational attainment were assessed at baseline using self-reported survey data. The other demographic and military characteristics were obtained at baseline from Defense Manpower Data Center personnel files including age, sex, race/ethnicity, military service branch, military component, pay grade, and military separation status. Recent deployment experience in support of the operations in Iraq and Afghanistan was defined as occurring between baseline and follow-up. It was assessed using electronic deployment data from Defense Manpower Data Center combined with participants’ responses to 13 self-reported combat experiences (e.g., “being attacked or ambushed”, “receiving small arms fire”) at follow-up. Those without a deployment between baseline and follow-up were classified as not recently deployed. Using methods similar to prior studies (34), individuals with a recent deployment were classified as deployed with no combat (endorsed 0 combat events), or deployed with low combat (endorsed 1-3 combat events), medium combat (endorsed 4-10 combat events), or high combat (endorsed 11-13 combat events). Due to the small number of participants reporting high levels of combat experiences, the two highest categories (medium and high) were combined into one category for this study.
Life stressors
Childhood traumatic experiences (e.g. childhood sexual abuse) were asked for the first time of participants at the 2016 (follow-up) survey. These four items from the Juvenile Victim Questionnaire (35) assessed traumatic experiences that occurred prior to age 18, and were summed (0 to 4) (35). Sexual assault was ascertained at baseline based on positive endorsement to one self-reported item (i.e., suffered forced sexual relations or sexual assault within the last 3 years). Financial problems were ascertained at baseline based on endorsement to one self-reported item (i.e., financial problems or worries within the last 4 weeks).
Psychological health/well-being
Probable posttraumatic stress disorder (PTSD) was assessed at baseline using the PTSD Checklist−Civilian Version (PCL-C), used to rate the severity of 17 PTSD symptoms (36). Based on criteria from the Diagnostic and Statistical Manual of Mental Disorders 4th edition (DSM-IV), participants were classified as having probable PTSD if they reported a moderate or greater level of at least one intrusion symptom, three avoidance symptoms, and two hyperarousal symptoms (37). Probable major depressive disorder (MDD) was assessed at baseline using eight Patient Health Questionnaire (PHQ) items, which correspond to a depression diagnosis in the DSM-IV (37). Based on criteria from the DSM-IV, probable MDD was defined as endorsing at least 5 items as “more than half the days” or “nearly every day”, in which one symptom was depressed mood or anhedonia (38). To assess the joint impact of PTSD and MDD, PTSD and MDD were combined to create a 4-level variable (neither PTSD nor MDD, PTSD only, MDD only, and comorbid PTSD and MDD). Problem drinking was assessed at baseline from endorsements of any of the 5 PHQ alcohol items (e.g., drank while working or taking care of responsibilities, missed or were late for work or other activities because you were drunk or hung over, drove a car after drinking too much) more than once in the last 12 months (39).
Potential mitigating factors for anger were also assessed. A version of the Posttraumatic Growth Inventory-Short Form (PTGI-SF)(40) was included at baseline to measure positive perspective. Unlike the original PTGI-SF, which asks for retrospective assessment of positive changes in personal perception following a traumatic event, the 11-item version used in the present study assessed current perspectives and did not reference a specific trauma. This current standing version was composed of the PTGI-SF (41)) and an additional item about compassion for others. Mean scores were calculated based on participant responses to each item (e.g., “I know that I can handle difficulties,” “I have a religious faith,” “I have a sense of closeness with others”) on a 6-point Likert scale from 0 (Not at all) to 5 (To a very great degree). Self-mastery was ascertained at baseline using 3 items (e.g., I can do just about anything I really set my mind to do) from Pearlin and Schooler’s Self-Mastery Scale (42). Mean scores were calculated based on responses on a 5-point Likert scale from 1 (Strongly disagree) to 5 (Strongly agree).
Statistical Analysis
Descriptive analyses and chi-square tests were used to compare demographic, military, life stressors, and psychological health/well-being characteristics by problematic anger status. Bivariate logistic regression analyses were performed to assess the relationship between each factor (e.g., sex, age, marital status, educational attainment, race/ethnicity, military service branch, military service component, military separation status, recent combat severity, childhood traumatic experiences, sexual assault, financial problems or worries, mental health status, positive perspective, and self-mastery) and problematic anger. A multivariable logistic regression model was performed to determine which factors were significantly associated with problematic anger. In order to determine the influence of each factor above and beyond the other variables, all factors were included in the multivariable model regardless of statistical significance. To assess the percent of problematic anger that could be reduced in this population if certain exposures/factors were eliminated, the population attributable risk percent (PAR%) was calculated for each life stressor and psychological/well-being factor [the prevalence of the factor among those with problematic anger multiplied by the adjusted odds ratio (AOR) minus 1 divided by the OR multiplied by 100 (prevalence among cases × [(OR−1)/OR] × 100%)]. Multicollinearity was assessed using a variance inflation factor of four or higher. P-values of less than 0.05 were regarded as statistically significant. All analyses were completed using SAS statistical software, version 9.4 (SAS Institute, Inc., Cary, NC).
Missing Data
Among the study participants (n=90,266), each DAR-5 item had less than 0.4% missing. All model predictors had less than 3.1% missing with the exception of the childhood traumatic experiences. Each of the four items had approximately 3% missing and an additional 3% who responded “prefer not to answer.” These responses were therefore classified into their own category to maintain these participants in analyses. For all other items, we used multiple imputation to maintain participants. Discriminant functions estimated categorical variables to ensure imputed data were integers within the range of possible values. A total of 50 imputed datasets were generated (43).