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Fluctuating asymmetry, the random deviation from perfect symmetry, is a widely used population-level index of developmental instability, developmental noise, and robustness. It reflects a population's state of adaptation and genomic coadaptation. Here, we review the literature on fluctuating asymmetry of human populations. The most widely used bilateral traits include skeletal, dental, and facial dimensions; dermatoglyphic patterns and ridge counts; and facial shape. Each trait has its advantages and disadvantages, but results are most robust when multiple traits are combined into a composite index of fluctuating asymmetry (CFA). Both environmental (diet, climate, toxins) and genetic (aneuploidy, heterozygosity, inbreeding) stressors have been linked to population-level variation in fluctuating asymmetry. In general, these stressors increase average fluctuating asymmetry. Nevertheless, there have been many conflicting results, in part because (1) fluctuating asymmetry is a weak signal in a sea of noise; and (2) studies of human fluctuating asymmetry have not always followed best practices. The most serious concerns are insensitive asymmetry indices (correlation coefficient and coefficient of indetermination), inappropriate size scaling, unrecognized mixture distributions, inappropriate corrections for directional asymmetry, failure to use composite indices, and inattention to measurement error. Consequently, it is often difficult (or impossible) to compare results across traits, and across studies.

Many adverse environmental and genetic factors can affect stability of development during human growth. Although the level of fluctuating asymmetry (FA) may be influenced by environmental and genetic stress encountered during this period, directional asymmetry (DA) is largely attributable to differential mechanical loading during bone growth, for example, handedness. I assessed the effects of heavy working conditions and socioeconomic conditions on asymmetry levels in three groups of young human males: 1) individuals employed in the heavy industry sector (n = 104, mean age = 18.48 +/- 0.61 years), 2) individuals who had the same socioeconomic status as the laborers (n = 102, mean age = 18.39 +/- 0.58 years) but were not laborers, and 3) nonlaborers from the higher socioeconomic levels of society (n = 103, mean age = 18.43 +/- 0.67). For all subjects, hand length, hand width, elbow width, wrist width, knee width, ankle width, foot length, foot width, ear length, and ear width were measured. All measurements of the upper extremities in the labor group appeared to exhibit DA; in the other two groups only hand measurements exhibited DA. According to analysis of FA, subjects living in poor conditions exhibited more FA than their nonlaborer peers living in better conditions. In addition, biomechanical pressures due to heavy working conditions of the labor group appeared to cause increased DA in the upper extremities: DA increased with an increase in the number of years working.

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