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Fluctuating asymmetry (FA) is a phenomenon present in adolescent idiopathic scoliosis (AIS; Dangerfield et al., 1997;Goldberg et al., 1995Goldberg et al., , 1997b, which includes all random deviations between the left and the right sides of the human torso. It has been described in the literature as an expression of developmental instability and a consequence of environmental and/or genetic factors (Klingenberg, 2003a(Klingenberg, , 2003bValen, 1962). Hypothetically, the left and the right sides of the torso share the same genome, and as far as development is considered to be regulated by genetics, a perfectly symmetrical phenotype should be expected in the frontal plane. In the practice, the FA comprises the random deviations from the mean asymmetry of the shape in the sample subjected to study, a type of asymmetry that has been termed 'directional asymmetry (DA)' (Klingenberg, 2003a(Klingenberg, , 2003b. Nevertheless, the exposition to non-genetic factors like nutrition, gravity or lifestyle could modify the way an organism grows by perturbating its developmental system (Klingenberg, 2003a(Klingenberg, , 2003b. These environmental factors may cause the asymmetries in the human torso

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