Random Developmental Variation of Human Phenotypic Traits, as Estimated by Fluctuating Asymmetry and Twin Studies

Random developmental variation, or developmental noise, contributes to total phenotypic variation in the human species. Despite exhortations to examine it, especially with respect to human behavior and intelligence, there has been little research specifically devoted to doing so. Random developmental variation can be estimated in studies of fluctuating asymmetry and comparisons of monozygotic and dizygotic twins. Estimation of random developmental variation requires that both genotype and environment be held constant. In a small sample of bilaterally symmetrical traits (dermatoglyphic ridge counts, digit lengths, ear lengths and widths), I show how the random developmental component can be estimated. In these traits, the percentage of total phenotypic variation attributable to developmental noise ranges from 3 percent to more than 25 percent. Moreover, for dermatoglyphic ridge counts, fluctuating asymmetry and twin comparisons give essentially the same estimates.