Date of Award


Degree Name

MS in Psychology


Psychology & Child Development


Jason Williams


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.