Threat exposure elicits physiological and psychological responses, the frequency and intensity of which, and concordance between, has implications for survival. Ethical and practical limitations on human laboratory fear inductions make it essentially impossible to measure response to extreme threat. Furthermore, ecologically valid investigations of group effects on fear are lacking in humans. The current preregistered study measured tonic and phasic electrodermal activity in 156 human participants while they participated in small groups in a 30 minute sequence of threats of varying intensity (a haunted house). Results revealed that (i) friends increased overall arousal, (ii) unexpected attacks elicited greater phasic responses than expected attacks, (iii) subjective fear increased frequency of phasic spikes, and (iv) startle had dissociable effects on frequency and amplitude of phasic reactivity. Findings show that etiology of emotional contagion varies depending on relationship type (increased among friends) and subjective fear is associated with temporal aspects of physiological arousal
