Introduction: Piloerection, the phenomenon of hair standing on end, is commonly viewed as an emotional response in humans1, a perspective that potentially overlooks its broader biological significance. Unlike in other mammals where piloerection has clear functional roles, the human response has been labelled vestigial2 due to our lesser hairiness. This study challenges the notion, proposing piloerection as a complex response to environmental stimuli. Aim and Hypothesis: We aimed to explore whether tactile, thermal, and audio-visual stimuli could trigger human piloerection, hypothesizing it as a multifunctional response to environmental changes rather than a mere emotional or vestigial reaction. We further hypothesized that piloerection influences skin temperature and is more predictably triggered by objective stimuli than by subjective emotional states or perceived coldness.

Experimental Design: We exposed participants to a variety of tactile, thermal, and audio-visual stimuli to induce piloerection. Through observing 1198 piloerection events in eight participants, we examined cardiovascular (ECG, cardiac impedance, respiration) and skin and core temperature responses to diverse stimuli, alongside participants' self-reported emotions and subjective feelings of coldness vs warmness.

Results: Results indicated that audio-visual stimuli inducing a stronger sympathetic response and tactile stimuli inducing a parasympathetic response. Challenging its classification as vestigial, piloerection episodes were associated with changes in skin temperature (see Figure 1) and this change was stronger for more intense episodes of piloerection (B = -.98, p < .001). Finally, environmental conditions (e.g. ambient temperature: sr2 = .53, p < .001) were more reliable predictors of piloerection than participants' subjective reports (e.g. self-reported coldness: sr2 = .02, p = .107) , challenging its treatment as a purely affective response in humans.

Conclusion: This research fundamentally challenges the prevailing understanding of piloerection in humans, presenting it not as an emotional artifact but as a nuanced physiological response to a variety of environmental stimuli. By demonstrating that human piloerection mirrors its function in other species, this study highlights the evolutionary continuity of physiological responses across species. The implications of these findings suggest a need to reconsider the role of piloerection in human physiology and psychology, from a marker of emotional states3 to a broader indicator of sensory and environmental engagement.