"Snälla killar är egentligen mesiga kvinnohatare"-kretsloppet:

Key Words facial attractiveness, face perception, evolutionary psychology, mate choice, adaptation
■ Abstract What makes a face attractive and why do we have the preferences we do? Emergence of preferences early in development and cross-cultural agree- ment on attractiveness challenge a long-held view that our preferences reflect ar- bitrary standards of beauty set by cultures. Averageness, symmetry, and sexual di- morphism are good candidates for biologically based standards of beauty. A critical review and meta-analyses indicate that all three are attractive in both male and fe- male faces and across cultures. Theorists have proposed that face preferences may be adaptations for mate choice because attractive traits signal important aspects of mate quality, such as health. Others have argued that they may simply be by-products of the way brains process information. Although often presented as alternatives, I ar- gue that both kinds of selection pressures may have shaped our perceptions of facial beauty.

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Symmetry
Over the past decade, research on the attractiveness of facial symmetry has been prolific, motivated by the idea that symmetry advertises mate quality (e.g., Gangestad & Thornhill 1997, Gangestad et al. 1994, Palmer & Strobeck 1986, Parsons 1990, Polak 2003, Thornhill & Gangestad 1999, Thornhill & Møller 1997, Watson & Thornhill 1994). Fluctuating asymmetries (FAs) are nondirectional (ran- dom) deviations from perfect symmetry in bilaterally paired traits. In nonhuman animals, FA in body traits reflects developmental instability (inability to withstand stress during development), increasing with inbreeding, homozygosity, parasite load, poor nutrition, and pollution (Møller & Swaddle 1997, Parsons 1990, Polak 2003). In humans, body FA increases with inbreeding, premature birth, psychosis, and mental retardation (Livshits & Kobylianski 1991). If similar relationships exist for facial FA, then it could signal mate quality.

Symmetric bodies are attractive to many animals, including humans (Brooks & Pomiankowski 1994, Concar 1995, Gangestad & Simpson 2000, Thornhill &Gangestad 1994, Watson & Thornhill 1994). But are symmetric faces attractive? Early studies suggested that they were not, with normal (slightly asymmetric) faces preferred to perfectly symmetric versions (Kowner 1996, Langlois et al. 1994, Samuels et al. 1994, Swaddle & Cuthill 1995). However, more recent stud- ies found that perfectly symmetric faces were more attractive than the original, slightly asymmetric, faces (e.g., Perrett et al. 1999; Rhodes et al. 1998, 1999a,b) and that their appeal could not be explained by any associated increase in averageness (Rhodes et al. 1999b) or change in skin texture (Perrett et al. 1999, Rhodes et al. 1999a).

The discrepancy seems to reflect differences in how the perfectly symmetric faces were made (Rhodes et al. 1999b). In the early studies, symmetric faces were made by reflecting each hemiface about the vertical midline to create two sym- metric chimeras (Kowner 1996, Samuels et al. 1994). However, these chimeras typically display structural abnormalities in aspect ratios and the sizes of midline features. For example, if the nose bends sideways, then the nose will be abnormally wide in one chimera and abnormally narrow in the other. Slight deviations from front-on views in the original photographs result in abnormally wide or narrow chimeras and abnormal eye spacing. Attractiveness decreases with deviations from average facial configurations, so these abnormalities will offset any preference for symmetry per se. When perfectly symmetric faces are made by blending normal and mirror-reversed images (Figure 2), they are more attractive than the original, slightly asymmetric faces (e.g., Perrett et al. 1999; Rhodes et al. 1998, 1999a,b). The only exception is a study by Swaddle & Cuthill (1995), but failure to control expression and remove blemishes before morphing could have contributed to fail- ure to find a symmetry preference in this study. The meta-analysis confirmed that symmetry is attractive when blends are used (0.43 ± 0.32, N = 16) but not when chimeras are used (−0.62 ± 0.30, N = 3), t(17) = 5.71, p < 0.0001.

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CONCLUSIONS AND FUTURE DIRECTIONS
An evolutionary perspective in psychology is not new, as the William James quote at the outset of this chapter indicates. However, the past decade has seen evolu- tionary psychology emerge as a distinct field within psychology (Barkow et al. 1992, Barrett et al. 2002, Pinker 1997). In this chapter, we have seen how an evo- lutionary perspective has shaped research on facial attractiveness. We have seen that averageness, symmetry, and sexual dimorphism are attractive in both male and female faces (contrary to recent claims that feminine male faces are attractive). We have seen some evidence that attractive traits may signal health, which is an important aspect of mate quality, although the evidence is far from compelling. And we have seen that the way our brains process information also shapes our preferences.

There are many exciting directions for future research. More studies are needed on whether facial attractiveness and its components signal health and other as- pects of mate quality. Recently, male facial attractiveness has been linked to ge- netic heterozygosity at sites involved in immune function. Future studies should determine which components of male attractiveness (masculinity, averageness, symmetry) mediate this link, and whether female attractiveness is also linked to heterozygosity at these sites. A more direct test of a link between attractive- ness and immunocompetence could also be done by challenging the immune system.

We know little about whether preferences generate heritable genetic benefits as proposed by the good genes model. We know little about the heritability of attractive facial traits and face preferences, as required by both Fisherian and good genes models. We know little about how facial attractiveness interacts with body attractiveness to determine overall attractiveness. We know that newborn infants prefer to look at faces that adults find attractive but know little about what traits they prefer. We know little about whether preferences change during development (e.g., at puberty). We know little about individual differences in face preferences, and whether they reflect different optimal strategies for individuals of differing mate value (Little et al. 2001, Penton-Voak et al. 2003) or self-similarity preferences (Buston & Emlen 2003, De Bruine 2004). We know that experience affects what we find attractive (Perrett et al. 2002, 2003; Rhodes et al. 2003b), but we know little about the temporal dynamics of these effects, including whether there are sensitive periods in which experience has stronger effects and whether sexual imprinting occurs in humans (Little & Perrett 2002). Clearly, the evolutionary psychology of facial attractiveness is just beginning!