Formal models for the study of the relationship between fluctuating asymmetry and fitness in humans

Abstract

Objectives To evaluate three of the main verbal models that have been proposed to explain the relationship between fluctuating asymmetry and fitness in humans: the “good genes,” the “good development,” and the “growth” hypotheses. Materials and Methods A formal model was generated for each verbal model following three steps. First, based on the literature, a theoretical causal model and the theoretical object of inquiry were outlined. Second, an empirical causal model and the targets of inference were defined using observational data of facial asymmetries and life‐history traits related to fitness. Third, generalized linear models and causal inference were used as the estimation strategy. Results The results suggest that the theoretical and empirical assumptions of the “good genes” hypothesis should be reformulated. The results were compatible with most of the empirical assumptions of “the good development” hypothesis but suggest that further discussion of its theoretical assumptions is needed. The results were less informative about the “growth” hypothesis, both theoretically and empirically. There was a positive association between facial fluctuating asymmetry and the number of offspring that was not compatible with any of the empirical causal models evaluated. Conclusions Although the three hypotheses focus on different aspects of the link between asymmetry and fitness, their overlap opens the possibility of a unified theory on the subject. The results of this study make explicit which assumptions need to be updated and discussed, facilitating the advancement of this area of research. Overall, this study elucidates the potential benefit of using formal models for theory revision and development.

The age at first reproduction as a potential mediator between facial fluctuating asymmetry and reproductive success in women

“…One of the anthropometric features of the body that has been suggested as a signal of DS is fluctuating asymmetry which is defined as “small, random deviations from perfect, bilateral symmetry of the body” (Parsons, 1990; Tomkins & Kotiaho, 2001), and is very often used regarding the facial features. The putative role of fluctuating asymmetries in reflecting DS was recently discussed within the theoretical framework of “good genes” and “good development” hypotheses (Farrera, 2022). According to those hypotheses, low fluctuating asymmetry (which carries a high physiological cost) can only be afforded by individuals with good genetic quality and those who can resist harsh intrauterine environmental conditions (Farrera, 2022), for example, the exposure to pathogens or toxins (Livshits & Kobyliansky, 1991) or insufficient energy supply (Parsons, 1990).…”

“…The putative role of fluctuating asymmetries in reflecting DS was recently discussed within the theoretical framework of “good genes” and “good development” hypotheses (Farrera, 2022). According to those hypotheses, low fluctuating asymmetry (which carries a high physiological cost) can only be afforded by individuals with good genetic quality and those who can resist harsh intrauterine environmental conditions (Farrera, 2022), for example, the exposure to pathogens or toxins (Livshits & Kobyliansky, 1991) or insufficient energy supply (Parsons, 1990). However, recent observational analyses revealed potential constraints in this conceptual framework and a need of further exploration of other potential formal models underlying the link between DS, fluctuating asymmetries and fitness (Farrera, 2022).…”

“…Fluctuating asymmetry has been linked to several cues of potential fertility. Individuals with low asymmetry were suggested to have higher reproductive capabilities, but the results are not conclusive (Farrera, 2022). Levels of asymmetry were found to be related to conception probability and fecundity.…”

“…in this conceptual framework and a need of further exploration of other potential formal models underlying the link between DS, fluctuating asymmetries and fitness (Farrera, 2022).…”