ceived higher ratings than the same stimuli that had diagonal sym- metry. This finding suggests that perceptual fluency, and not sym- metry per se, influences liking. Taken together, preference for symmetry seems to be part of a broader preference for fluent pro- cessing of incoming stimuli. Processing fluency is hedonically marked because it indicates a positive state of affairs in the inter- action with the environment. For example, processing fluency may signal that a stimulus has been encountered before and is thus familiar, or that ongoing cognitive processing progresses toward successful recognition and interpretation of the target (see Win- kielman et al. 2003). If Homo erectus began to craft symmetric tools because of a per- ceptual preference, the question arises whether the change of form from Mode 1 technologies to symmetric tools might reflect a change in preference during the same time period. Is it possible that preference for symmetry evolved between the emergence of Mode 1 technology and the first appearance of symmetry in tools? This seems unlikely to be the case because preference for sym- metry has been observed in a wide variety of species, including in- sects (Giurfa et al. 1996; Møller 1995), fishes (Morris 1998), birds (Møller 1992), primates (Rensch 1957), and humans (Gangestad et al. 1994; Humphrey 1997; Rhodes et al. 1998). This suggests that preference for symmetry had evolved before humans emerged. Moreover, Rensch (1957) has found that primates prefer symme- try in nonmating contexts, suggesting that this preference is pres- ent in all primates and thus existed in Homo erectus before they began to form symmetric stone tools. Finally, the view that Homo erectus formed symmetric tools be- cause they found them beautiful does not entail the necessity that they had any intention to produce symbolic art. Preference for symmetry is a more basic affective reaction than an evaluation based on aesthetic or symbolic value, as demonstrated by symme- try preferences in subhuman species that have no inclination for symbolic art. Putting meat on the bones: The necessity of empirical tests of hypotheses about cognitive evolution. P. Thomas Schoenemann Department of Anthropology, University of Pennsylvania, Philadelphia, PA 19104. ptschoen@sas.upenn.edu http://www.sas.upenn.edu/~ptschoen/ Abstract: Reconstructing the evolution of cognition requires maximal ex- traction of information from very sparse data. The role that archaeology plays in this process is important, but strong empirical tests of plausible hypotheses are absolutely critical. Quantitative measures of symmetry must be devised, a much deeper understanding of nonhuman primate spatial cognition is needed, and a better understanding of brain/behavior relationships across species is necessary to properly ground these hy- potheses. Understanding how we came to be the creatures we are is a fasci- nating and important topic. Wynn believes archaeology should play a critical role in this process. While it is easy to be sympathetic with this view, there are some very real problems – some of which are acknowledged by Wynn – that have to be addressed before this can happen. A lot more can and should be said about how to em- pirically address the issues Wynn raises. First, the whole concept of symmetry in stone tools needs to be clarified. Although Wynn notes that there is controversy over the extent to which symmetry in certain kinds of stone tools is real and intended, he nevertheless argues that it does in fact exist and was in fact intended, but then offers little to support this contention. Wynn states that bifaces “do not have the best symmetry, but the economy of means by which the symmetry was achieved reveals that some idea of mirroring must have guided the knapper” (sect. 2.4.1). But how do we judge economy of means, and why is a con- cept of mirroring necessary to achieve that particular level of sym- metry? More fundamentally, how do we compare different levels of symmetry, or determine whether a particular artifact shows a level of symmetry greater than what we would expect from purely random processes? At present, we must rely on the judgments of experienced knappers. However, given that there is controversy even among cognitive archaeologists over these issues, symmetry in lithic artifacts needs to be formalized in some quantitative man- ner to allow for empirical testing. Dibble and Chase (1981) sug- gested a measure of symmetry for flakes (the “angle of symme- try”). A version of this measure adapted to bifaces would certainly be a step in the right direction, but an even more comprehensive measure might use radial measurements such as those taken by Wynn and Tierson (1990). A possible metric of symmetry could be obtained by squaring the differences between pairs of corre- sponding left and right radial measurements (at equal degrees of divergence from the center), summing these squared differences across all corresponding pairs, and finally dividing by the number of pairs of radial measurements. This would provide a single num- ber: the average squared deviation from perfect symmetry across all the different corresponding left and right side pairs of mea- surements for an individual artifact. Means and standard devia- tions for this measure could then be obtained for entire assem- blages. Different sites (or different time periods) could then easily be compared statistically. This would allow for empirical tests of increasing symmetry. Other of types of measures would have to be devised to quantify things such as the “S-twist” in some handaxes, but in principle this is possible, and is really the only way to re- solve disputes over symmetry. Wynn argues that “The mirrored sides [of bifaces] are not just qualitative reversals, but quantitative duplicates, at least to the de- gree that this is possible given the constraints of stone knapping” (sect. 2.5.1). How do we evaluate empirically how close the bifaces are to the theoretical limit of symmetry given the material and the techniques used? This would be possible to assess, once a quanti- tative measure of symmetry is decided upon. One could use the most symmetrical bifaces produced by expert knappers (using the same techniques and materials used by hominids) as the “gold standard” against which to compare the degree of symmetry in bi- faces at archaeological sites. A deeper problem remains, however. To what extent is biface shape the goal of the stone tool maker, instead of being simply the unintended side effect of a reduction sequence? McPherron (2000) argues that bifaces were likely modified with use, and that this very fact challenges the idea that the shapes we see today are the desired end-product. He also suggests that geographic differ- ences in artifact shape are more simply explained as being differ- ences in degrees of artifact reduction, rather than differences in underlying mental templates of the tool makers. Similarly, Dibble and colleagues (Dibble 1987; Rolland & Dibble 1990) have long argued that a large part of the variability in Middle Paleolithic tool assemblages is best explained by different degrees of artifact re- duction. If such factors strongly affect shape differences between sites and across time, then what we are seeing is not changes in mental concepts, but rather changes in how hominids made use of various resources available in their environments. However, if it can be shown that repeated lithic reduction – driven only by util- itarian usefulness – typically results in statistically significantly less symmetry than that shown among the gold standards produced by expert knappers, then there might be a way to demonstrate that some sort of mental template was involved in the production of as- semblages of bifaces: that is, determine whether the degree of symmetry of archaeological artifacts exceeds that shown in the utilitarian models. Another area in need of empirical testing concerns exactly what nonhuman apes can and cannot perceive and produce with re- spect to symmetry. The target article focuses on what kinds of be- haviors apes demonstrate naturally, rather than the equally im- portant question of what they can be trained to do. When an Commentary/Wynn: Archaeology and cognitive evolution 416 BEHAVIORAL AND BRAIN SCIENCES (2002) 25:3