Relationship between intelligence and posttraumatic stress disorder in veterans
Introduction
About 89% of the general population will experience a potentially traumatic event (Kilpatrick et al., 2013), but lifetime prevalence of posttraumatic stress disorder (PTSD) is estimated at 8%, which is well below the proportion of individuals who experience trauma (American Psychiatric Association [APA], 2013). In military and veteran populations, PTSD is consistently one of the most commonly diagnosed psychiatric disorders, though rates range from 5 to 40% across samples (Maguen, Madden, Lau, & Seal, 2012; Sayer, Nelson, & Nugent, 2011). A meta-analysis estimated 23% prevalence rate in Iraq and Afghanistan veterans (Fulton et al., 2015). Given the high rates of PTSD in this population, there are pressing clinical and systemic reasons to identify individuals at greater risk of developing PTSD following exposure to trauma. The aim of this study was to further examine the relationship between one risk factor, intelligence, and PTSD in combat veterans.
Low intelligence has shown to have a stronger association with PTSD than other pre-trauma factors such as age, sex, minority status, and psychiatric history (Brewin, Andrews, & Valentine, 2000). A few studies suggest that a high intelligence quotient (IQ; high defined as one standard deviation above the mean) is potentially protective against developing PTSD (Breslau, Chen, & Luo, 2013; Breslau, Lucia, & Alvarado, 2006; McNally & Shin, 1995). There is also evidence for lower intelligence among individuals with PTSD, suggesting a directional relationship in which low IQ may increase vulnerability for developing PTSD following a traumatic event, whereas high IQ may be protective (Breslau et al., 2006; Breslau et al., 2013; Kaplan et al., 2002; Macklin et al., 1998; McNally & Shin, 1995).
In contrast, the converse relationship has been proposed in which the development of PTSD lowers intellectual functioning. For example, studies have found that decreased attention or short-term memory may affect outcomes of general intelligence testing following onset of PTSD (Emdad & Söndergaard, 2006; Koso & Hansen, 2006). It is also possible that effects vary depending on the cognitive variable such that some variables (i.e. premorbid intelligence) may serve as risk factors for PTSD, while others (e.g., attention, working memory, verbal memory) may decline following the development of PTSD (Dolan et al., 2012).
Despite how common PTSD is in military and veteran populations, the relationship between intelligence and PTSD has been largely understudied (Kessler et al., 2014) and is still poorly understood. Kaplan and colleagues (Kaplan et al., 2002) collected neurocognitive data on participants prior to their draft into the Israeli Defense Forces (IDF) and then compared the baseline data of veterans ultimately diagnosed with PTSD to veterans without a PTSD diagnosis. Results indicated that participants later diagnosed with PTSD had significantly lower intelligence, less formal years of education, and less motivation to serve compared to those who were not diagnosed with PTSD. Effect sizes were small, and after adjusting for motivation on the behavioral measure, all differences became non-significant. Twin studies of combat-exposed veterans have indicated a link between cognitive abilities and risk for developing PTSD, in which lower performance on cognitive tests (i.e. verbal, mathematical, attention, and IQ) were associated with increased risk for developing PTSD, and higher performance on cognitive tests were associated with decreased risk (Gilbertson et al., 2006; Kremen et al., 2007). A recent study of female veterans (Stricker, Keller, Castillo, & Haaland, 2015) also found lower estimated intelligence using the Wechsler Test of Adult Reading among veterans with a PTSD diagnosis. It has been suggested that lower intelligence may increase risk of PTSD due to other associated factors such as poorer coping resources, resilience, or motivation to serve (Dolan et al., 2012; Gilbertson et al., 2006; Kaplan et al., 2002; Kremen et al., 2007).
There are a number of limitations to the existing literature. First, most studies did not formally evaluate the validity of cognitive performance. A recent study summarizing 50 military and veteran studies found the average failure rate on performance validity tests (PVTs) was 30% (Denning & Shura, 2019). Not accounting for PVT failure seen in such a high percentage of individuals arguably reduces the internal validity of a study using cognitive outcomes, potentially leading to unreliable results. Similarly, symptom validity tests (SVTs), referring specifically to indices of symptom overreporting (i.e., symptom magnification or exaggeration), have also not been commonly employed in the cited studies, confounding diagnostic determinations and self-reported symptoms. By failing to measure and account for validity issues, previous conclusions about low intelligence as a risk factor for PTSD may be questionable. Second, previous research used narrow definitions of IQ. For example, a performance-based estimate may reflect effects of PTSD on cognition, whereas a demographic-based estimate may reflect premorbid IQ as a risk for developing PTSD. Using more than one type of estimate may better inform the question of directionality of the relationship between intelligence and PTSD. Finally, how PTSD is defined (e.g., chart diagnosis, structured interview, self-report measure) may also affect outcomes. A meta-analysis by Brewin et al. (2000), found that results differed when comparing continuous to categorical methods and interview to questionnaire methods for assessing PTSD. Further evaluation of those differences is warranted to better understand how methodological variance across studies affects outcomes.
The purpose of this study was to improve on prior methods and comprehensively evaluate the relationship between intelligence and PTSD in a sample of post-deployment veterans. First, we hypothesized that a lifetime history of PTSD would be associated with lower scores across several IQ estimates. Using lifetime diagnoses removes the confound of individuals who may have developed PTSD in the past and recovered prior to study participation. Second, we hypothesized that the relationship between IQ and PTSD would no longer be present after accounting for symptom and performance validity. Performance validity has been shown to relate to performance in a variety of cognitive domains in prior research (Rowland, Miskey, Brearly, Martindale, & Shura, 2017). Therefore, invalid performance may also be driving the relationship of lower IQ to PTSD in prior studies that did not account for performance validity. Third, we hypothesized that current PTSD symptom severity based on self-report measures would be related to lower IQ scores. If there is a relationship among IQ and PTSD, then a dose effect might be expected when using continuous variables. Finally, given the design of the TOPF, we predicted that current IQ scores would be significantly lower compared to the demographic estimate in those with PTSD compared to those without.
Section snippets
Participants
Data for these analyses were collected as part of a study on the cognitive and behavioral effects of primary blast exposure that enrolled participants between February 2016 and March 2019. Participants were identified either from the patient population of a Mid-Atlantic VA Medical Center or from re-contact lists of prior research studies. Eligibility criteria were: at least one combat deployment after September 11, 2001, English speaking, 18 years of age or older, able to comply with
Measures
All tests were administered in a fixed order and in a standardized manner in accordance with respective test manuals. Tests and interviews were administered by a trained master's-level research coordinator, neuropsychology post-doctoral fellow, or a neuropsychologist. Several methods for IQ estimation have been developed. All available methods of estimating IQ were evaluated to provide a comprehensive review of available estimates. The Wechsler Adult Intelligence Scale, 4th edition (WAIS-IV) (
Analysis
All analyses were conducted using SAS Enterprise Guide 7.1 (SAS Institute Inc., Cary, NC). Mean differences in IQ scores between presence and absence of lifetime PTSD diagnosis were evaluated using independent samples t-tests on the total sample (N = 338). Lifetime PTSD diagnosis was represented by CAPS-5 diagnostic scores. For analyses accounting for validity, individuals who failed the MSVT, b Test, and/or the SIMS (n = 90) were removed from the sample and t-tests were repeated. The final
Results
Characteristics of the full sample and by lifetime PTSD diagnosis are presented in Table 1. Given these data were derived from a study on primary blast injury, we checked to ensure blast was not a confound. First, t-tests were run comparing FSIQ scores across those with blast TBI (n = 136, M = 99.48, SD = 12.92) and those without blast TBI (n = 212, M = 99.50, SD = 13.65), which was not significant: t (336) = 0.02, p = .987, d = −0.00. Similarly, there were no significant differences in FSIQ
Discussion
A number of prior veteran studies suggest a relationship between intellectual functioning and PTSD (Gilbertson et al., 2006; Kaplan et al., 2002; Kremen et al., 2007; Macklin et al., 1998; McNally & Shin, 1995; Stricker et al., 2015; Vasterling et al., 2002). Our results do not consistently support findings of previous studies suggesting that low intelligence is a risk factor and/or high intelligence is a protective factor for the development of PTSD. Findings reported in previously published
Disclaimer
The views expressed in this article are those of the authors and do not reflect the official policy of the Department of Veterans Affairs, Department of Army/Navy/Air Force, Department of Defense, or U.S. Government.
Declaration of Competing Interest
This work was supported by grant funding from Department of Defense, Chronic Effects of Neurotrauma Consortium (CENC) Award W81XWH-13-2-0095 and Department of Veterans Affairs CENC Award I01 CX001135. This research was also supported by the Salisbury VA Health Care System, Mid-Atlantic (VISN 6) Mental Illness Research, Education, and Clinical Center (MIRECC) and the Department of Veterans Affairs Office of Academic Affiliations Advanced Program in Mental Illness, Research, and Treatment. The
Acknowledgements
The authors would like to thank the Veterans and Servicemembers who contributed their time and effort to this research. We would also like to thank David J. Curry, MSW, Christine Sortino, MS, Alana M. Higgins, MA, G. Melissa Evans, MA, and Mary Peoples for their contributions to this project.
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