Performing When It Matters: The Effects of Acute Stress Exposure on Cognition and Decision-Making in Military Personnel

This dissertation examined how military personnel respond to acute stress and make decisions under pressure. What began as an attempt to induce and measure stress in simulated environments evolved into a broader investigation of the limits and possibilities of studying stress, cognition, and moral judgment in ecologically valid but controlled settings. Across four empirical chapters, the work highlights how difficult it is to elicit genuine stress responses and how strongly context and environmental demands shape human performance in military-relevant scenarios.

Chapter 2 tested whether a Virtual Battle Space (VBS) could serve as an effective stress induction tool. Using a within-subjects design, participants were exposed to both VBS combat scenarios and a validated virtual reality (VR) height exposure. While the VR task reliably elicited physiological and subjective stress, the more complex VBS environment did not. This showed that immersion and emotional salience—rather than scenario realism alone—are critical for triggering stress. Despite these differences, cognitive performance on attention and inhibition tasks remained stable, suggesting that brief stress exposure did not impair basic cognitive functioning.

Chapter 3 addressed these limitations by introducing heat stress as an added environmental burden. In a randomized controlled trial, participants completed VBS scenarios under either high heat or normal temperature. The goal was to determine whether physiological stress markers such as cortisol and alpha-amylase would respond to this combined load. Although heart rate and heart rate variability changed, reflecting physical strain, cortisol and alpha-amylase did not. This indicated that no true hormonal stress response was triggered, highlighting how difficult it is to activate the stress system even under physically demanding conditions, particularly in a trained military population.
Chapter 4 examined whether acute heat stress affected cognitive and behavioral performance in combat-related tasks. Working memory, inhibition, cognitive flexibility, situational awareness, and decision-making were measured. Most domains remained stable, showing substantial resilience to thermal strain. However, a significant group difference emerged in decision-making at follow-up: performance slightly improved in the heat-stress group while declining in the control group. This suggests that prolonged heat exposure may stabilize or enhance decision-making, possibly by increasing arousal or task engagement. Cognitive flexibility also improved, though this was likely due to a learning effect from repeated testing.

Chapter 5 shifted focus to moral decision-making. Using an online Moral Trade-off System, 670 military personnel and civilians completed war-related moral dilemmas involving trade-offs between lives. Participants were classified into moral phenotypes and these were examined in relation to psychological and demographic variables. Although group differences in phenotype distribution narrowly missed significance, civilians were more likely to save a larger number of soldiers than military participants. Military personnel, however, reported greater confidence in their choices. Importantly, moral phenotypes could not be reliably predicted from traits such as stress, anxiety, or trauma history, suggesting that ethical decision-making is strongly shaped by context rather than stable personality traits.

Overall, this dissertation shows that stress and decision-making in operational contexts depend on immersion, environmental demands, and situational meaning, supporting a more nuanced and context-sensitive approach to studying military performance under pressure.