Evolutionary advantage view of symmetry preference: Symmetry is expected be higher for potential partners (here human faces) and higher post-puberty when partner choice becomes more relevant

Preferring and Detecting Face Symmetry: Comparing Children and Adults Judging Human and Monkey Faces. Anthony C. Little and Jack A. F. Griffey. Symmetry 2020, 12(12), 2112; December 19 2020. https://doi.org/10.3390/sym12122112

Abstract

Background: Visual symmetry is often found attractive. Symmetry may be preferred either due to a bias in the visual system or due to evolutionary selection pressures related to partner preference. Simple perceptual bias views predict that symmetry preferences should be similar across types of stimuli and unlikely to be related to factors such as age.

Methods: The current study examined preferences for symmetry across age groups (pre-puberty vs post-puberty) and stimuli type (human face vs monkey face). Pairs of images manipulated for symmetry were presented and participants asked to choose the image they preferred. Participants repeated the task and were asked to detect symmetry.

Results: Both age of observer and stimuli type were associated with symmetry preferences. Older observers had higher preferences for symmetry but preferred it most in human vs monkey stimuli. Across both age groups, symmetry preferences and detection abilities were weakly related. 

Conclusions: The study supports some ideas from an evolutionary advantage view of symmetry preference, whereby symmetry is expected be higher for potential partners (here human faces) and higher post-puberty when partner choice becomes more relevant. Such potentially motivational based preferences challenge perceptual bias explanations as a sole explanation for symmetry preferences but may occur alongside them.

Keywords: symmetry; asymmetry; face preference; detection; development

4. Discussion

This study demonstrated that symmetry preference and detection varied according to the type of stimuli being judged and between the children and adult age groups. Preference and detection of symmetry was highest for the adult group, with preference for symmetry higher for human than monkey faces in adults. In the adult group, detection was higher for female faces of both species than male faces, while the pattern was mixed for children. The correlations between preferences for symmetry and its detection were generally positive but low (highest r = 0.25). The dissociation between preference and detection can be seen in adults judging female monkey faces, for which detection of symmetry was highest of all stimuli but preference for symmetry was lowest. The dissociation can also be seen in the children group where detection of symmetry was highest for monkey male faces, but preference was lowest.

Significant preferences for symmetry in human face stimuli for adult observers is consistent with previous work using manipulated stimuli [8,9,21,49]. The finding that symmetry preferences in adults were stronger for human than for monkey faces is also consistent with a previous study showing that symmetry was preferred more in human faces than in monkey faces and abstract art images [39]. Other studies have shown that symmetry is preferred more in salient biological images than in more abstract visual stimuli [36,50], and the results here suggest species can impact preference within the broad category of biological stimuli. Preferences across stimuli were intercorrelated for adults, but the correlations were generally low (the highest for human male and female faces was r = 0.35). This is consistent with previous work showing limited correlations between symmetry preferences for human faces and more abstract stimuli [36]. Taken together, the results here do not generally a support a simple perceptual bias view of symmetry preference that posits preference is a basic process of the perceptual system [22,23,24]. Such a view would predict that symmetry preferences would be similar across stimuli types that vary in symmetry in the same way and that preferences for symmetry across stimuli would be similar within each observer (i.e., if each observers’ preference for symmetry was generated via a basic process, we would expect that process to apply in a similar way across stimuli).

Children did not show significant preferences for symmetry in this study. From a simple perceptual bias view this is surprising as the basic processes of perception should be the same between children and adults. Experience could play a role in the difference as adults have been exposed to more faces than children; however, it seems likely that they will have been exposed to very large number faces by age 8 and this would be likely enough to generate a symmetric internal prototype if that explained preferences for symmetry [21,27,28,29,30]. Preferences for face symmetry is seen when controlling for rated distinctiveness [8], again suggesting that attraction to symmetry is at least partially independent of an individual’s representation of the prototypical face. Given there were also no linear effect between detection and age within the adult sample, greater experience appears an unlikely reason for the difference. Motivation is an alternative explanation, with adult men being more interested in symmetry associated with some aspect of mate quality than male children. While this would be consistent with an evolutionary advantage view for human female faces, it is difficult to apply to preferences for male and monkey faces, which are not relevant for heterosexual male mate preferences. Neither experience or mating motivation appear to explain the pattern of difference between preferences for symmetry in adults and children. Future work can usefully test changes across puberty or across time in younger children to examine mating motivation and experience effects on symmetry preferences.

There was significant detection of symmetry across stimuli for both adults and children, with higher accuracy in the adults. The pattern of detection did not follow the pattern for preference for either adults or children. In adults, preferences were highest for human faces but for detection preferences were highest for female faces, irrespective of species. In children, detection was highest for monkey male faces while preference was lowest for this stimulus type. It is unclear why detection ability varied across both species and sex in both adults and children. That children were better at detecting symmetry in monkey male than human male faces is surprising and suggestive that experience with a stimuli type does not underpin symmetry detection as, if experience were important, we would expect detection to be higher for stimuli with which observers have more experience. The same transforms were applied to human and monkey faces, keeping variation in asymmetry similar. Differences in judging these stimuli are then not dependent on asymmetry present and reflect some other aspect of processing by the observer. Future work can further examine how detection ability varies both with age and different types of stimuli.

Children had lower preferences and lower accuracy in detection of symmetry than adults, and it is possible poor detection explains the lack of preference in children. Controlling for detection, however, a significant difference in preference between adults and children was still seen. This indicates that differences in preference between children and adults is not completely dependent on detection ability. Further, weak correlations between preference and detection also support the idea that this ability does not underpin symmetry preferences. Similarly, the pattern of means for detection and preference noted above suggest the two measures can be decoupled. Overall, results here support previous findings [49] suggesting a dissociation between symmetry preference and detection, but contrast with other studies suggesting preferences can be explained by detection ability [10]. More research is needed to examine the circumstances under which detection and preference are more or less related. For example, it is possible greater deviations in symmetry may lead to a stronger relationship between preference and detection and that, with small deviations, different processes for the two judgments lead to different generated responses.

Overall, the results of this study reveal a complex pattern of preferences across type of stimuli and age group. These preferences appear decoupled from detection ability. The results from adults partially support predictions of an evolutionary advantage view because preferences were higher for human faces than monkey, and marginally higher for human female faces than male faces. This is consistent with ideas that symmetry may be used as a guide to mate quality, e.g., [21,31,32] or general partnerships (including male–male friendship). Higher preferences for symmetry in adults, for who these partnerships are more relevant, than for children is also consistent with the evolutionary view. Indeed, perceptions of health appear to be important in attraction to symmetric human faces [7,51]. The evolutionary view, however, does not explain why symmetry preferences are also higher in adults for monkey faces than in children. The results do not support the idea that simple perceptual bias views explain all of symmetry preference. Previous studies present findings that are difficult to account for, such as symmetry preferences being higher for opposite-sex compared to same-sex faces [33,34,35], also seen in the adult sample here. These results only suggest that the perceptual bias view may not fully account for symmetry preferences and some of symmetry preference could be due to perceptual bias. Indeed, multiple mechanisms may determine symmetry preference that include basic visual system processes and higher order motivational processes acting together. The motivational processes may drive differences in preferences between stimuli types [21,49].