Fluctuating asymmetry and masculinity/femininity in humans: a meta-analysis

Theory predicts that if particular morphological features correlate with "genetic" quality--i.e., mating with individuals bearing those characteristics increases fitness--selection favors preferences for these features. Both developmental instability (DI)--which emerges morphologically as small random deviations from perfect symmetry, i.e., fluctuating asymmetry (FA)--and sexually dimorphic traits due to variation in sex hormones, like facial masculinity and digit ratios, may reflect mate quality and/or relate to attractiveness. Therefore, FA and morphological expressions of sex hormones may represent interrelated measures/signals of individual quality. This article presents a meta-analysis of these associations, including 44 effect sizes from 16 studies. Mean effect size (Pearson correlation) was nearly zero (r = .04, SE = 0.05). Largest effect sizes occurred for the few studies measuring sex hormone levels directly. This result was, however, unexpected since there is little evidence that hormone levels determined at a single time point would accurately reflect masculinity/femininity. Facial masculinity/femininity, which contains information on the functioning of the endocrine system, did not relate to DI (average effect size: r = .01, SE = 0.04). In spite of a substantial body of literature, there is little evidence for a robust association between DI and sexual dimorphism. Nevertheless, estimates of average effect sizes in DI showed wide confidence intervals. It is, therefore, premature to draw general conclusions in any direction. Further research is clearly required both to study associations between DI and sexual dimorphic morphological traits and the link between these dimorphic traits and levels of reproductive hormones.