The Impact of Passive Heat Stress on Aggression-Related Emotions and Covertly Measured Physical Aggression

Objective: It is well established in epidemiological literature that when heatwaves occur, violent crime rates increase. The mechanisms underlying these increases, however, are not well understood. This preliminary study seeks to quantify the effect of heat stress on emotional states in a controlled environment, with a specific focus on anger and physical aggression. Hypothesis: We hypothesized that passive heat stress would increase both aggression-related emotions and actual physical aggression (indexed covertly via physical force), but would not impact other emotions. Methods: On two separate occasions, six participants (four males: 26±7 y, 73.1.1±15 kg, 1.74±0.03 m; two females: 28± 7 y, 57.7±8kg, 1.72±0.16 m) donned water perfused suits and lay supine while either being heated with 50°C water (HEAT) or with a 32°C water control (CON). Measures (detailed below) were taken at baseline and at 0.2°C increase in rectal temperature increments until a total rise of 1.0°C was reached in the HEAT trial, and at baseline, 30 min, and then subsequently every 5 minutes in the CON trial, to attempt to match the measurement timing in the HEAT trial. Throughout both sessions, skin (Tsk) and rectal (Tre) temperatures were continuously monitored. Subjective measures of thermal sensation, thermal comfort, and an eight-item emotional scale (Afraid, Confused, Sad, Angry, Energetic, Tired, Happy, and Tense) were recorded on Visual Analog Scales ranging from 0 (neutral) to 200 mm (maximum effect). To covertly assess physical aggression, participants performed three rounds of a 'Whack-a-Mole' task (at baseline, mid-trial, and end-trial) to measure striking force, while being told their reaction time (but not striking force) was being assessed. All measures are presented as means with standard deviations. Tre and Tsk were analyzed at baseline and end-of-trial, striking force was analyzed as change from baseline to end-of-trial, and subjective end-of-trial measures were analyzed. Results: At baseline, Tsk (33.47°C±0.68°C; p=0.56) and Tre (36.6°C±0.49°C; p=0.64) did not differ between trials. However, both Tsk (CON: 33.97±0.41, HEAT 38.45±0.56; p< 0.001) and Tre (CON: 36.4±0.5, HEAT 37.7±0.5; p< 0.001) were higher in the HEAT compared to CON at end of trial. Thermal sensation (CON: 112±10 mm, HEAT 174±24 mm; p< 0.001), thermal comfort (CON: 121±43 mm, HEAT 41±44 mm; p=0.034), Anger (CON: 0±0 mm, HEAT 67±71 mm; p=0.050), Tension (CON: 4±10 mm, HEAT 57±56 mm; p=0.031), and striking force (CON: 88±44 N, HEAT 105±51 N; p=0.056) were elevated in the HEAT compared to CON trial, as well as Happiness (CON: 105±54 mm, HEAT: 41±58; p=0.047) being reduced in the HEAT trial. However, no differences were found in Fear (1±2 mm; p=0.363), Confusion (11±30 mm; p=0.233), Sadness (7±11 mm; p=0.274), Energy (53±54 mm; p=0.638), or Tiredness (58±42 mm; p=0.830). Conclusion: Statement Heating in a controlled laboratory environment increased emotions associated with aggression (anger and tension) as well as covertly measured use of force, but did not affect other emotions. Further analyses (with a larger sample size) will determine whether these effects differ by sex and whether heat stress also affects biomarkers associated with aggression. Funding This study was funded by the PI’s start-up funds. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.