The most recent challenge to Kohn and Mithen's (1999) attempt to link symmetry in handaxes with sexual attraction (Reference HodgsonHodgson 2009a) has itself been challenged by Hayden and Villeneuve (2009) who prefer functional explanations associated with core reduction and sharpening. However, as shown by Reference WynnWynn (2002), Weban-Smith (2004) and Reference MachinMachin (2009) functional criteria do not account for the most symmetric handaxes. This suggests that we are still searching for other, non-functional explanations.

Reference BurrissBurriss (2009) attempts a more systematic rebuttal of my arguments, suggesting that a strong link between symmetry and attraction has been accepted. In fact, this is not supported by recent literature (see for example, Gangestad & Scheyd 2005; Polak & Starmer 2005; Rhodes & Simmons 2007; Bronstad et al. 2008; Zaidel & Hessamian 2010). Such studies indicate that what makes a person attractive is extremely complex, involving multiple aspects of the face and body which can range from skin texture and tone to jaw/cheek structure to more dynamic factors (see, for example, Braun et al. 2001). This is also consistent with the finding that single measures for assessing developmental stability/instability are essentially unreliable (Gangestad & Thornhill 2003). The conclusion arising from this is that the influence of symmetry is at best weak to nil. In this respect, Swaddle et al. (2008) also found that the detection of fluctuating asymmetry in avian species is too weak to serve as a reliable means for the purpose of choosing a mate.

Other aspects of symmetry do not accord with Burriss's analysis. For example, facial, rather than body, characteristics seem to be unique to females judging males, and, contrary to what Kohn and Mithen suggest, the role of 'symmetry' has been found to be relevant only to males judging females (Peters et al. 2007). In addition, it has been established that the ability to discriminate symmetry in morphed figures becomes reduced the more a shape approaches perfect symmetry (Tjan & Liu 2005), which is opposite to what would be expected if the detection of symmetry is such an important factor in mate selection. Furthermore, it has been shown that, although asymmetric faces may sometimes be regarded as unattractive, very unattractive faces may not necessarily be asymmetric, and, correspondingly, symmetric faces may not necessarily be viewed as attractive and very attractive faces can actually deviate from symmetry (Braun et al. 2001; Zaidel & Hessamian 2010).

Irrespective of whether symmetry is equated with attraction, Burriss seems to agree with the more substantive issue regarding the fact that no relationship seems to exist between symmetry and health (see Reference RhodesRhodes 2006; Rhodes & Simmons 2007; Nowell & Chang 2009). Thus, if there is no evidence that symmetry is related to health and therefore to genetic worth, then Kohn and Mithen's (1999) thesis must be regarded as flawed. Crucially, a link has, however, been established between health and averageness (Kalick et al. 1998; Rhodes et al. 2001a & b; Valentine et al. 2004; Zebrowitz & Rhodes 2004 and see below).

Further evidence of a lack of correspondence between perceived attractiveness and the symmetry of either the body or face in relation to fitness, comes from a study showing that there is no relationship between the attractive ratings of women made during a fertile period with regard to the qualitative aspects of mate suitability (Peters et al. 2008). Thus, the phenotype-associated fertility hypothesis does not seem to be valid with regard to choice of mate. Burriss draws attention to a supposed increase in the sensitivity of women to symmetrical faces when conception is likely; but although earlier studies seemed to support this idea, more recent research has found that it is not the case (see, for example, Cardenas & Harris 2007; Oinonen & Mazmanian 2007). Peters et al. (2009) found no relationship between the ovulatory phase and low fertility for both facial and body symmetry. Regarding Burriss's point that more of a link has been demonstrated between health and body symmetry compared to facial features, the face is regarded as more crucial to humans (than the body) for the communication of both general and more subtle information (Reference HalberstadtHalberstadt 2006), not least because we are such a social species (Reference DunbarDunbar 1998), therefore clues to fitness are more likely to be favourably biased towards the face.

There are, moreover, other ways of appreciating symmetry than the sexual. Recent evidence from neuroscience shows how the detection of symmetry is important in terms of early to mid perceptual mechanisms. In this respect, the human brain seems to give special precedence to detecting symmetry with respect to a whole range of objects that serve as a way of parsing the world into meaningful units (Liu & Kersten 2003). The brain thus seems to respond to symmetry within particular areas of the visual cortex i.e., V3A, V4d/v, V7 and LO which supports the notion that symmetry serves as a marker for detecting many kinds of objects (Beck et al. 2005; Sasaki et al. 2005; Reference SasakiSasaki 2007).

Thus, a more productive approach to understanding the significance of the shape of Acheulean handaxes would be to regard the non-functional aspects of symmetry as a bi-product of general perceptual and recognition processes that serve to encode the various regularities of the world. In tool-making, symmetry may represent evidence of an explicit cognitive ability to impose form on inert matter (Reference HodgsonHodgson 2009b). The more refined symmetries of Acheulean handaxes can thus be regarded as a kind of stimulus generalisation that may be associated with a bias for prototypicality/symmetry where key aspects of form are exaggerated and intensified in order to produce what is referred to as a super stimulus. According to Halberstadt (2006: 178), 'prototypical stimuli feel familiar, regardless and independent of whether they have been seen before, and this feeling is also associated with positive affect'. This finding is compatible with the suggestion that averageness/prototypicality, when combined with a preference for symmetry, is closely tied to perceptual correlates (Rhodes & Simmons 2007; Enquist and Arak 1994). It is these correlates that may have given rise to the more nonfunctional aspects typical of Acheulean handaxe shape. It also dovetails with Reference MachinMachin's (2009) view that such bifaces may partially derive from an 'aesthetic' tendency, related to display, which does not involve sexual selection (see also Reference HodgsonHodgson, 2009a & b). The non-functional symmetry of Acheulean bifaces may thereby have initially been based on a bias of the human perceptual system for perceiving symmetrical/prototypical objects as part of a more consciously disposed creative engagement.