Symmetry and Humans: Reply to Mithen's 'Sexy Handaxe Theory.'

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In reply to Machin's criticism of Kohn and Mithen's (1999) 'Sexy Handaxe Theory' in a recent Antiquity debate (Machin 2008: 761-6), Mithen (2008: 766-9) states that sexual selection is still relevant to the symmetry of Acheulean handaxes because this provides the only theory that can account for the various features typical of such artefacts. This conclusion may be misconceived, however, due to the conflation of the various factors relating to symmetry, attractiveness, and health. Crucially, recent studies have not found a genetic link based on sexual selection for physical traits based on symmetry. For example, Koehler et al. (2002) established that there was no difference in preference for the symmetry of male faces by females nearing conception compared to those females taking contraceptives. Similarly, Rhodes et al. (2001) found that, although there might be a link between facial symmetry and perceived health, there was no correlation between facial symmetry and actual health. So, although symmetrical faces may be attractive there have been no studies that support a link between facial symmetry and real health (Valentine et al. 2004; Rhodes 2006: 214). In addition, it has been established that it may not be symmetry but averageness that determines whether a face is regarded as attractive (Rhodes et al. 1999). Interestingly, a number of researchers reject the symmetry hypothesis because perfect symmetrical faces seem to be regarded as less rather than more attractive (Langlois et al. 1994; Swaddle & Cuthill 1995). Furthermore, facial attractiveness and symmetry, instead of signalling health, may have evolved more as a way of disguising health problems in order to manipulate perceivers and therefore may not serve as a reliable means of gauging fitness (Kalick et al. 1998). Crucially, symmetry in males does not prove that preference for symmetry evolved for the purpose of assessing genetic worth; rather, this preference may have arisen as a by-product of selection for the purpose of improved perception and recognition (Enquist & Arak 1994). The purported advantage gained by females in mating with more symmetric males may therefore be a fortuitous effect arising out of a more generalised sensory predilection deriving from a 'perceptual bias' for symmetry (Valentine et al. 2004). Anatomical symmetry may thus have little to do with adaptive fitness or genetic quality. This conclusion is reinforced by the fact that symmetry also does not seem to serve as a signal of health in animals (Polak 2003; Tomkins & Simmons 2003).

Mithen also criticises Machin for failing to suggest an alternative theory that could account for the different factors associated with Acheulean handaxes. Such a theory does in fact exist, based on the notion that symmetry and sexual selection are not necessarily linked. In this regard, Machin's finding that the enhanced symmetry of Acheulean handaxes goes beyond the functional needs of butchering may not imply, as Mithen proposes, that this supports sexual selection; rather, this may be evidence of an interest in symmetry for other, more fundamental, reasons. Reber (2002; Reber et al. 2004), for example, has put forward the concept of perceptual fluency for the fact that the preference for symmetry derives from a tendency predicated on more basic concerns than sexual selection that is consistent with a 'perceptual bias' account of why symmetry is important. From this perspective, symmetry provides a crucial means by which things in the world can be identified in that many biological objects are initially recognised by way of symmetry (Enquist & Arak 1994; Wagemans 1997; Beck et al. 2005). Thus, the ability to detect symmetry has been found to precede conscious awareness in that it is rapid and automatic, to the extent that 4-months old infants are able to discern such shapes (Beck et al. 2005) and brain-damaged patients suffering neglect have an implicit, yet unconscious, awareness of symmetry (Driver et al. …