Sexual selection shaped psychological mechanisms in both sexes to assess potential mates for evidence of mate quality (Buss, 2005). Attraction preferences are one such mechanism (Sugiyama, 2005) and physical attraction preferences are sensitive to fitness-promoting traits present in a potential mate. Physically attractive traits are thought to act as signals of good genetic quality (Neff & Pitcher, 2005) and are preferred because of the advantage such quality bestows towards reproductive success. Specifically, genetic quality is proposed to be a biological requirement necessary for physically attractive traits to develop and be maintained (Johnstone & Grafen, 1993). Furthermore, genetic quality is heritable to offspring, thereby increasing offspring reproductive success (Orr, 2009). All mating decisions inherently involve trade-offs due to costs inflicted on time and resources when choosing a long-term mate. Assessing a mate for genetic quality is imperative to ensuring one selects a quality mate with heritable fitness benefits towards offspring reproductive success (Buss, 2005). In order to minimize costs and maximize benefits when making mate selection decisions, humans use multiple and redundant signals of mate quality (Fink & Penton-Voak, 2002; Møller & Pomiankowski, 1993). Accordingly, this study supposed that siblings act as redundant signals of genetic quality that would factor into mating decisions. Because genetic quality is heritable (Houle, 1991) and visible through physical attractiveness (Thornhill & Gangestad, 1999), this study explored the possibility that knowledge of relatedness influenced attractiveness judgments of human faces. Supporting the main hypothesis of the current study, siblings affected judgments of physical attractiveness for target faces. Analyses showed this effect to be driven entirely by female raters for both male [t(62)=3.87, p<.001] and female [t(61)=2.24, p=.029] target faces. Secondary analyses examining the effects of sibling pair attractiveness differences (low vs. high) showed that relatedness significantly increased female ratings of facial attractiveness for both low and high facially attractive male and low facially attractive female target faces. Results offer two possible conclusions as to the role relatedness may serve in mate quality assessments that align with parental investment as well as kin selection assumptions.

[Truncated abstract]Judgements of physical attractiveness are thought to reflect evolved preferences for a high quality mate. The central aim of this thesis was to investigate the hypothesis that female preferences are adaptations for finding good quality mates and that faces and bodies signal honest information about mate quality. To date, most human mate preference studies have examined face or body attractiveness alone, and many have created stimuli using computer graphic techniques. Throughout these studies, I endeavoured to maximise the biological relevance of my studies by incorporating both face and body attractiveness, and using photographs of individual participants. Most research on attractiveness has focused on faces or bodies separately, while our preferences have evolved based on both seen together. A fundamental requirement of studying face and body attractiveness independently is that there is no interaction between the two. My first study confirmed that the face and body did not interact when an overall attractiveness judgment was made. I also investigated the independent contributions of rated attractiveness of the face and the body to ratings of overall attractiveness. Face and body attractiveness each made significant independent contributions to overall attractiveness in males and females. For both sexes, facial attractiveness predicted overall attractiveness more strongly than did body attractiveness, and this difference was significant in males. ... This study indicates that although current levels of T covary with male mating success, this effect may not be mediated by women's preferences for visual cues to T levels conveyed in static face or body features. The fourth study in this thesis investigated the phenotype-linked fertility hypothesis, which proposes that females obtain reliable information on male fertility from male expression of sexual traits. A previous study of Spanish men reported that facial attractiveness was positively associated with semen quality. I aimed to determine whether this effect was widespread by examining a large sample of Australian men. I also extended my study to determine whether cues to semen quality are provided by components of attractiveness: masculinity, averageness, and symmetry. I found no significant correlations between semen quality parameters and attractiveness or attractive traits. While male physical attractiveness may signal aspects of mate quality, my results suggest that phenotype-linked cues to male fertility may not be generalised across human populations. Together, these studies challenge current methodologies and theories of preferences for secondary sexual traits as honest signals of mate quality. The findings show that it is important to study human mate preferences in biologically relevant contexts, for example by using photographs of both faces and bodies, to maximise the real life application of results. In addition, the findings suggest that male attractiveness does not signal cues to testosterone or semen quality, although testosterone is associated with mating success. The implications of these findings and possible avenues for future research are discussed.

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