Human fluctuating asymmetry in relation to health and quality: a meta-analysis

Abstract

Developmental instability (DI) reflects the inability of a developing organism to buffer its development against random perturbations, due either to frequent, large perturbations or to a poor buffering system. The primary measure used to assess DI experienced by an individual organism is fluctuating asymmetry (FA), asymmetry of bilateral features that are, on average in a population, symmetrical. A large literature on FA in humans in relation to measures of health and quality (close to 100 studies and nearly 300 individual effect size estimates) has accumulated. This paper presents the first quantitative meta-analysis of this literature. The mean effect size (scaled as Pearson r) was about 0.2. Effect sizes covaried negatively with sample size, consistent with effects of publication bias, the tendency for significant effects to be published. Conservative correction for this bias reduced the mean effect to about 0.1. Associations with FA underestimate effects of underlying DI due to imprecise measurement of the latter. A model-based best estimate of the mean effect of DI on outcomes is about 0.3, a theoretically meaningful, relatively large effect, albeit of moderate absolute size. The data are consistent, however, with a range of true effect sizes between 0.08 and 0.67, partly due to large study effects. Study-specific effect sizes in DI ranged between −0.2 and 1.0. A humbling and perhaps sobering conclusion is that, in spite of a large body of literature involving nearly 50 000 participants, we can only confidently state that there is on average a robust positive average effect size. An accurate estimate of that effect size was not possible, and between-study variation remained largely unexplained. We detected no robust variation across six broad categories of outcomes (health and disease, fetal outcomes, psychological maladaptation, reproduction, attractiveness and hormonal effects), though examination of narrower domains reveal some corrected effects close to 0.2 and others near zero. The meta-analysis suggests fruitful directions for future research and theory.