Fig. 1.

Types of asymmetry distributions. Asymmetry is characterized by the distribution of asymmetry values for particular characters in a sample (adapted from Palmer and Strobeck, 1986). Asymmetry is quantified as the signed difference between right and left sides (L–R) for a hypothetical characteristic of interest. (A) Fluctuating asymmetries deviate from perfect symmetry and are small and randomly distributed on each side so that the distribution of L–R values is unimodal and centered at zero. This definition assumes an underlying ideal form of perfect symmetry (i.e., L−R=0). (B) Directionally asymmetric measurements are consistently larger on one side than on the other side and the mean value is non-zero (i.e., L−R≠0). (C) Antisymmetry refers to measurements that are asymmetric but these measurements can be larger on one side in some individuals and larger on the other side in different individuals, which causes the distribution of signed differences between the left and right side to be bimodal.