When the outside temperatures rise, people tend to lose their cool. That connection is well known, but a Washington State University-led study found that emotional responses to heat are highly individualized and only one factor moderated it -- age.

Overall, researchers found that the actual temperature at which the majority of people felt uncomfortable during a hot summer depended on the individual. When they did feel discomfort, it often negatively affected their mood. The exception: older adults on average became more uncomfortable in high heat more quickly, but it did not affect their mood as much as it did younger adults.

"Older adults in general have worse thermoregulation, so this makes them more vulnerable to heat -- so that was not surprising -- but what was really interesting is that on average, older adults showed low levels of negative emotional states, even though they experienced more discomfort in the heat," said Kim Meidenbauer, a WSU psychology researcher and lead author on the study published in the journal BMC Psychology.

While the reason for this difference could not be pinpointed from this study, Meidenbauer said that other research on personality traits across the lifespan found that emotional stability tends to increase with age.

The goal of the current study was to try to better understand why high outside temperatures are associated with increases in violent crime and mental health hospital admissions. This connection has long been noted by scientists, but the causes remain unclear.

Since negative emotional states are linked to people acting out aggressively, Meidenbauer and her colleagues sought to investigate the connection among outside temperature, physical discomfort and "negative affect," such as feeling irritable, anxious or gloomy. They recruited about 400 participants in the Chicago area who used an app to report levels of comfort and emotional states while outside during the summer of 2022. The researchers used geolocation to determine the actual temperature at the time and place when the participants logged their self-reports.

The study found no direct connection between the actual temperature outside and people's emotional states. Perceived temperature, or how hot they felt it was, was more important, but even then, it depended on the individual whether the temperature caused discomfort leading to a negative mood.

"People really varied in the extent to which they found consistently extreme temperatures as hot or uncomfortable. Some people were experiencing 100-degree days, and they were still feeling good," said Meidenbauer.

When the participants did feel that discomfort though, more of them, and especially the younger adults, had an associated negative emotional state.

"This research is suggesting that for some people there is a really strong relationship between heat and negative affect working through discomfort," she said. "Because there is also an association between being in a particularly angry or irritable emotional state and then acting out aggressively -- this is a plausible mechanism at play."

Because the thermal discomfort is so variable, Meidenbauer said that it would likely be hard to link objective temperature to individual psychological experiences and behaviors without directly measuring them. As a next step, she plans to test the emotional response to heat in a laboratory setting.