Body Symmetry and Attractiveness

Body symmetry refers to the degree to which an organism's left and right sides are identical. In evolutionary psychology, the focus is often on fluctuating asymmetry (FA), which represents small, random deviations from perfect bilateral symmetry in traits that are symmetrical on average at the population level. These deviations are thought to arise from an organism's inability to buffer environmental and genetic stressors during development. Consequently, lower FA is hypothesized to be an honest signal of genetic quality, developmental stability, and overall health, making it a potential cue for mate choice.

The Argument for Symmetry as a Signal

The theoretical basis for symmetry as an attractiveness cue stems from sexual selection theory. Hamilton and Zuk (1982) proposed the 'handicap principle' for sexual ornamentation, suggesting that elaborate traits can signal genetic quality if they are costly to produce and maintain, especially in the face of parasites and pathogens. Extending this, Møller (1990) and others argued that symmetry could serve as a more general indicator of an individual's ability to cope with developmental perturbations. Organisms with good genes, robust immune systems, and favorable developmental environments are expected to exhibit lower FA, as they can better maintain their developmental trajectory despite stressors.

From this perspective, individuals with lower FA are considered to possess 'good genes' that confer resistance to disease, efficient metabolism, and overall fitness. By choosing symmetrical mates, individuals are hypothesized to gain direct benefits (e.g., healthier partners who can contribute more to offspring care) or indirect benefits (e.g., offspring inherit good genes, leading to higher viability and reproductive success). This led to the prediction that humans, like many other species, should find symmetrical individuals more attractive.

Evidence for Symmetry Preferences

Research on body symmetry and attractiveness has employed various methodologies, including studies of facial symmetry, body symmetry (e.g., hand, foot, ear length differences), and digitally manipulated images. Early studies often focused on facial symmetry, finding that more symmetrical faces were rated as more attractive, healthier, and more dominant (e.g., Rhodes et al., 1998). Some studies have also reported correlations between facial symmetry and perceived health, intelligence, and personality traits.

Beyond the face, research has explored symmetry in other body features. For instance, some studies have found that more symmetrical bodies, as measured by various limb and digit lengths, are perceived as more attractive (e.g., Gangestad and Thornhill, 1997). Men with lower FA have been reported to have more sexual partners and earlier sexual debut, suggesting a link between symmetry and reproductive success (Thornhill and Gangestad, 1999). Similarly, women with lower FA have sometimes been found to have higher estrogen levels and greater fecundity.

Cross-cultural studies have provided some support for symmetry preferences, suggesting that the preference might be a universal human trait rather than a culturally learned one. However, the strength and consistency of these findings vary considerably across studies and methodologies.

Critiques and Limitations

Despite the theoretical appeal and initial empirical support, the relationship between symmetry and attractiveness has faced significant critiques and methodological challenges. One major concern is the reliability and validity of FA measurements. FA is often very small, close to measurement error, and can be difficult to assess accurately (e.g., Palmer, 1996). Different methods of measuring FA can yield inconsistent results, and the assumption that FA directly reflects developmental stability has been questioned.

Another critique centers on the strength of the observed effects. While some studies report statistically significant preferences for symmetry, the effect sizes are often small, suggesting that symmetry might be only one among many cues influencing attractiveness, and perhaps not the most dominant one (e.g., Perrett et al., 1999). Other factors, such as averageness, sexual dimorphism (e.g., masculinity in men, femininity in women), skin health, and body fat distribution, often account for a larger proportion of the variance in attractiveness judgments.

Furthermore, the link between FA and actual genetic quality or health has been debated. While theoretically plausible, empirical evidence directly connecting FA to robust measures of genetic fitness or immune function in humans is not consistently strong. Some studies have failed to find a robust correlation between FA and health outcomes, or between FA and reproductive success, leading some researchers to question the 'good genes' hypothesis as applied to human FA (e.g., van Dongen and Gangestad, 2011).

Open Questions

The field continues to investigate the precise role of symmetry in human mate choice. Future research aims to disentangle the relative contributions of various attractiveness cues, including symmetry, averageness, and sexual dimorphism, and to understand how these cues interact. There is also ongoing work to improve the measurement of FA and to establish more robust links between FA and underlying biological quality. Some researchers are exploring whether preferences for symmetry are condition-dependent, meaning they might be stronger in certain contexts (e.g., short-term vs. long-term mating) or for individuals with particular mating strategies.

Ultimately, while the concept of symmetry as a signal of genetic quality remains an important theoretical construct in evolutionary psychology, its empirical significance in human attractiveness appears to be more nuanced and complex than initially proposed. It likely functions as one cue among many, its influence potentially modulated by other factors and individual differences.