Theories of embodiment propose that our bodily states can influence affective processing. This thesis investigated the possibility that facial feedback (i.e., afferent signals from facial muscles) can influence the interpretation of affective stimuli. One study tested the effect of overt smiling and frowning on the interpretation of short descriptions of everyday events. Smiling, as compared to frowning, led to more positive evaluations, but only for participants who were aware of the emotional relevance of their expressions. A second study tested whether subtle changes in facial activation (elicited by unconsciously presented happy/angry facial expressions) led to changes in evaluations of ambiguous target symbols. While angry prime faces, as compared to happy prime faces, induced more frowning (as measured via electromyography), this change in facial activation did not translate into a behavioural effect on subsequent evaluations. A third study investigated the relation between naturally occurring facial reactions and interpretations of both clearly valenced and ambiguous facial expressions. Results indicate that facial reactivity predicts participants' self-reports of their own emotional reactions towards others' expressions (Experiment 1). A relation between facial reactions and interpretations of the expression senders' emotional states was only found in cases in which participants with high sensitivity towards their own bodily states (as measured with a test of interoceptive accuracy) tried to interpret ambiguous expressions (Experiment 2). In a last experiment, prolonged presentation of emotional prime faces led to expression-congruent facial reactions, but resulted in expression-incongruent behavioural reactions in both classification speed and interpretative tendency of emotional target faces. Overall, this thesis suggests that facial feedback is not generally involved in the interpretation of affective stimuli, but that it might contribute to evaluative processes only under special circumstances.

Though human nonverbal vocalisations are widespread, scientific consideration of their mechanisms and communicative functions has been largely overlooked. This is despite their close alignment with the vocal communicative systems of primates and other mammals, whose primary function is to signal indexical information relevant to sexual and natural selection processes. In this thesis, I examine human nonverbal vocalisations from an evolutionary perspective, with the central hypothesis that they are functionally and structurally homologous to nonhuman mammal calls, communicating evolutionarily relevant indexical information that is perceived and utilised by listeners. In Chapter 1, I introduce the methodological framework (source-filter theory) necessary to understand the production of vocal signals in mammals, before summarising the information contained within the acoustic structure of nonhuman mammals and human speech, and the effects these cues have on both vocaliser and listener. I then examine the current evidence for functional and structural homology between human and nonhuman nonverbal vocalisations. In Chapters 2 to 5, I quantitatively analyse the acoustic structure of a number of nonverbal vocalisations, and perform playback experiments to examine their functional effects on listeners. In Chapters 2 and 3, I investigate whether aggressive roars and distress screams communicate acoustic cues to absolute and relative strength and height. In Chapter 4, I analyse the acoustic structure of pain cries of varying intensity, and conduct playback experiments to explore the acoustic and perceptual correlates of pain. In Chapter 5, I examine whether the fundamental frequency of tennis grunts produced during professional tennis matches is dependent on the sex and body posture of the vocaliser, as well as the progress and outcome of the contest, and whether listeners can infer these cues. In Chapter 6, I tie these findings together, arguing that the acoustic structure of human nonverbal vocalisations, in continuity with nonhuman mammal vocalisations, has been selected to support the functional communication of indexical and motivational information.