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My research utilizes a behavioral key-press task adapted from the classic bar-press technique employed in many rodent studies of reward to explore the incentive salience of beauty among humans. In Chapter 2, I replicate previous findings indicating that gender differences exist for the incentive salience of beauty. I extend past findings with regard to the incentive salience of heterosexual beauty by investigating the role of additional aspects of facial appearance. Here, I find that apparent health holds incentive salience. This may serve an adaptive function by driving motivation to seek out healthy potential mates while avoiding infectious individuals. In Chapter 3, I explore gender differences in the incentive salience of adult and infant faces. I show that women demonstrate greater motivation, overall, to view infant faces while both men and women differentiate between the high-cute and low-cute versions of infant faces, suggesting that infant cuteness may hold incentive salience for both men and women but that infants in general have higher incentive salience for women. In Chapters 4 and 5, I investigate individual differences and variation across the menstrual cycle for women viewing adult faces. Women's own attractiveness was found to influence motivation to view attractive individuals, especially same-sex individuals. Within-subject variations in motivation across the menstrual cycle were apparent for the incentive salience of same-sex beauty. Taken together, the results of these experiments suggest that the incentive salience of same-sex faces among women may be partially driven by intrasexual competition – a novel explanation for women's motivation to view same-sex individuals. Overall my research has indicated that infant cuteness, adult attractiveness and apparent health influence the motivational value of faces, while individual differences also exist among women with respect to own attractiveness and fertility. The key-press paradigm offers an exciting new method for exploring inter- and intra-sexual behavior in humans.

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Information conveyed by the face can be used in social encounters to make fast decisions about another person. Recently, a new model of face perception has been postulated (Oosterhof & Todorov, 2009) suggesting that there are two basic judgements that underlie person evaluations from faces: dominance and trustworthiness. But on the basis of which cues are these judgements made, and do these cues have biological validity? In this thesis I investigate two putative facial cues to dominance and trustworthiness; namely, facial width-to-height ratio (fWHR) and skin yellowness. In men, fWHR has previously been linked to aggressive and dominant behaviour as well as the perception of these traits. Here I show that a more positive dominance related trait (achievement striving) is also related to this metric, indicating a general association of fWHR to dominance rather than simple aggression. Furthermore, I explore the biological underpinnings of this metric by showing 1) that contrary to initial findings and predictions fWHR is not sexually dimorphic, and 2) that fWHR is associated with testosterone, indicating a physiological link between appearance and behaviour. Additionally, I extend current work on fWHR by showing that it acts as a cue to dominance not only in humans but also in non-human primates. The second part of the current thesis firstly identifies skin yellowness as a novel cue used in trustworthiness judgement. It shows that this putatively carotenoid induced cue to current health is not only employed in mate choice context but may also play an important role in other social context and in judgements of who poses an adequate partner for social interactions. Secondly, I show that skin yellowness is inversely related to testosterone levels in men, showing for the first time an association between this carotenoid induced signal and testosterone in humans thereby extending previous work in birds.