Preferences for symmetry in faces change across the menstrual cycle Anthony C. Little a,b,*, Benedict C. Jones c, D. Michael Burt d, David I. Perrett e a School of Psychology, University of Stirling, UK b School of Biological Sciences, University of Liverpool, UK c School of Psychology, University of Aberdeen, UK d School of Psychology, University of Durham, UK e School of Psychology, University of St. Andrews, UK Received 26 July 2006 accepted 17 August 2007 Available online 22 August 2007 Abstract Symmetry in human male faces may be a cue to heritable fitness benefits and is found attractive. Preferences for facial masculinity, another proposed marker of genetic quality, have been found to vary in ways that may maximise evolutionary relevant benefits and masculinity is found to be of increased attractiveness at peak fertility across the menstrual cycle. Here we show that women prefer more symmetric faces at peak fertility (Study 1) and that such shifting preferences may be potentially strategic preferences as we found them to occur only for judgements concerning short-term relations and when women already had a partner (Study 2). Such preferences potentially indicate a strategy that maximises the quality of extra-pair/short-term partners or a quality dependent response to hormones. Such strategic preferences for symmetry may support the role of symmetry in signalling potential good-gene benefits. # 2007 Elsevier B.V. All rights reserved. Keywords: Symmetry Facial attractiveness Menstrual cycle Preferences Hormones Mate choice Symmetry is found attractive by many animals (see review by M��ller and Thornhill, 1998). Studies of naturally occurring human facial asymmetries show that measured symmetry is positively correlated with attractiveness judgments (Grammer and Thornhill, 1994 Jones et al., 2001 Mealey et al., 1999 Penton-Voak et al., 2001 Rhodes, 2006 Scheib et al., 1999). Consistent with preferences for naturally occurring symmetry in real faces, computer graphic studies (Little et al., 2001 Perrett et al., 1999 Rhodes, 2006 Rhodes et al., 1998) have shown preferences for faces manipulated to increase symmetry, including cross-cultural (Rhodes et al., 2001) and cross-species (Waitt and Little, 2006) agreement on the attractiveness of symmetry. Preference for symmetric faces has been explained as an evolutionary adaptationtoidentify high-quality mates (Thornhill and Gangestad, 1999), though we note the issue is controversial and that some authors have argued that symmetry preferences may arise without recourse to adaptation (e.g., Johnstone, 1994). Symmetry in human faces has been linked to potential heritable fitness (���good-genes���) because symmetry is a useful measure of theabilityofanorganismtocopewithdevelopmentalstress(both genetic and environmental). Fluctuating asymmetry, asymmetry that has a normal distribution around a mean of zero (Valen, 1962), is thought to be particularly important. As the optimal developmental outcome of most characters is symmetry, deviation from perfect symmetry can be considered a reflection of challenges to development. Only high quality individuals can maintain symmetric development under environmental and geneticstressandthereforesymmetrycanserve asanindicatorof phenotypic quality as well as genotypic quality (e.g., the ability to resist disease, M��ller, 1997 M��ller and Thornhill, 1998, for reviews). In line with these ideas, morphological symmetry appears to be related to reproductive success in many species, including humans (Gangestad and Thornhill, 1997 M��ller and Thornhill, 1998). For example, more symmetric human males report more sexual partners than less symmetric men (Thornhill and Gangestad, 1994). Sexual dimorphism in human faces is another proposed marker of genetic quality (Thornhill and Gangestad, 1999) and has been much studied in terms of strategic preferences. The relationship between attractiveness and male facial masculinity www.elsevier.com/locate/biopsycho Biological Psychology 76 (2007) 209���216 * Corresponding author at: School of Psychology, University of Stirling, Stirling, FK9 4LA Scotland, UK. Tel.: +44 1786 467651 fax: +44 1786 467641. E-mail address: anthony.little@stir.ac.uk (A.C. Little). 0301-0511/$ ��� see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.biopsycho.2007.08.003

is not clear cut though, some findings showing attraction to masculinity (Cunningham et al., 1990 DeBruine et al., 2006 Grammer and Thornhill, 1994) and others showing attraction to femininity (Berry and McArthur, 1985 Cunninghametal.,1990 Little and Hancock, 2002 Perrett et al., 1998 Rhodes et al., 2000). Human males bring two factors to a parenting relation- ship: a level of paternal investment and potential heritable benefits (e.g., genes for high quality immune systems). The perceived high dominance and lower levels of co-operation point to lower paternal investment from the owners of masculine faces (Perrett et al., 1998). In the context of a short-term relationship the perceived cues to high paternal investment in the feminine faced male are of little value to a female. Females may therefore seek to maximise the genetic fitness of potential offspring if they are not extracting any other benefits from their mates. In long- term relationships, better parenting and increased co-operation may outweigh the benefits of genetic fitness, thereby enhancing the attractiveness of feminine-faced males. Indeed, studies have shown that masculinity in male faces and physical attractiveness are preferred more in short-term than in long-term contexts (Little et al., 2002 Scheib, 2001). An increased preference for genetic fitness over signs of parental investment would also be expected in extra-pair copulations when a woman has already acquired a long-term partner. Little et al. (2002) have also shown that women who have partners prefer masculinity in faces more than those without a partner. Linking to symmetry, women have been found to prefer pictures of men with symmetrical bodies as extra-pair partners (Gangestad and Thornhill, 1997). Shifting female preferences for masculine facial traits are also seenacrossthemenstrualcycle.Duringthefollicularphaseofthe menstrual cycle when conception is most likely, women prefer relatively masculine male faces (Frost, 1994 Johnston et al., 2001 Jones et al., 2005a,b Penton-Voak and Perrett, 2000 Penton-Voak et al., 1999). Women fantasise more about extra- pair relations at peak fertility (Gangestad et al., 2002) and report being less committed to their partners during the late follicular (i.e. fertile) phase of the cycle than at other times (Jones et al., 2005a,b). These data are suggestive of a possible mechanism wherewomenmaymaximisetheirchancesofbecomingpregnant with the offspring of males chosen for extra-pair affairs (Penton- Voak and Perrett, 2000 Penton-Voak et al., 1999). Interactions are also evident with other variables outlined above with there being trends for women who have partners and rating for short- term relationships showing the largest cyclic shifts (Penton-Voak et al., 1999), and that fertile women relatively prefer videos of dominant behaviour (Gangestad et al., 2004) and masculine bodies (Little et al., 2007) only when judging for short-term relationships.Suchmalesmaybeselectedforpossessingsuperior or alternative genes to the woman���s current partner. Of course a mechanism to attend to quality in males when most likely to become pregnant may also serve to maximise genetic benefits in offspring for women without partners. Studies do show that women prefer the odour of more symmetrical males at peak fertility (Rikowski and Grammer, 1999 Thornhill et al., 2003) but one previous study has shown no influence of the menstrual cycle on preferences for facial symmetry indicating that menstrual cycle shifts do not take place for symmetry (Koehler et al., 2002). This result has been subsequently replicated using the same methodology by the same group (Koehler et al., 2006). Another study has shown that symmetry detection is better at peak fertility but this advantage did not carry over to preferences (Oinorten and Mamanian, 2007), which does suggest the cycle plays a role in symmetry perception. Symmetry in male partners, like masculinity, potentially carries a cost to choosing females. It has been argued that high- quality males are less likely to invest in mates and instead pursue a strategy of maximising their number of lifetime mates (Gangestad and Simpson, 2000). Indeed, men with high body symmetry appear less inclined to provide paternal care than other men (Gangestad and Simpson, 2000) and symmetric men appear more inclined to engage in physical conflict with other men (Furlow et al., 1998). Thus we may expect similarities in potentially strategic preferences for symmetry and masculinity as symmetry is also associated with potential genetic benefits and decreased levels of parental investment in humans. 1. Study 1 Following studies showing preferences for facial masculinity are higher at peak fertility (Frost, 1994 Johnston et al., 2001 Penton-Voak and Perrett, 2000 Penton-Voak et al., 1999), in Study 1 we tested for change in preference for facial symmetry across the menstrual cycle using a powerful within-participant design. Koehler et al. (2002) previously found no difference in attractiontosymmetryinfaceswhenpreferencesduringtheearly and late follicular phases of the menstrual cycle were compared. Although the early and late follicular phases of the menstrual cycle differ in fertility they do not differ in progesterone level (Gilbert,2000).Changesinfacepreferencesduringthemenstrual cycle have been linked to change in progesterone level across the cycle in two different studies, however (DeBruine et al., 2005 Jones et al., 2005a,b), and changes in progesterone appear involved in menstrual cycle shifts in preferences for masculinity in both faces (Jones et al., 2005a,b) and voices also (Puts, 2005). Jones et al. (2005a) also found that predicted progesterone level during the menstrual cycle was associated with women���s commitment to their romantic partner. We therefore compared attraction to symmetry in male and female faces in the late follicular phase of the menstrual cycle and on the days between ovulation and onset of menses with the highest urinary concentration of progesterone metabolites. Since conception risk is higher in the late follicular phase than luteal phase, and the late follicular phase and highest progesterone test session from thelutealphaseshouldalsodiffermarkedlyinprogesteronelevel, this comparison captures differences in both fertility and progesterone level (Gilbert, 2000). By contrast, comparing the early and late follicular phases (Koehler et al., 2002) will capture differences in fertility but not progesterone level. Following previous findings implicating change in progester- one level for cyclic variation in preferences for putative cues to men���s genetic fitness (facial and vocal masculinity, Jones et al., 2005a Puts, 2005), we predicted that women would demonstrate strongerpreferences for symmetry when at the more fertile phase A.C. Little et al. / Biological Psychology 76 (2007) 209���216 210