Why Symmetry Is a Health Signal
Bilateral symmetry in bilateral organisms (like humans) is the developmental ideal: both sides of the face should, in principle, develop identically under genetically identical instruction. Departures from perfect symmetry, called fluctuating asymmetry, arise when developmental processes are disrupted by genetic or environmental stressors during growth.
The connection to health runs through this logic: a face that developed with minimal asymmetry experienced minimal developmental disruption, which implies a robust genetic programme and a relatively benign developmental environment. This is why symmetry is a reliable (if weak) signal of genetic quality and early-life health, not because symmetry itself is valuable, but because it is an indicator of things that are.
The Magnitude of the Effect
The symmetry-attractiveness relationship, while real, is smaller than popular treatments suggest. In most studies, symmetry manipulations explain 5–15% of the variance in attractiveness ratings, meaningful but not dominant. When symmetry is statistically controlled for, other variables (averageness, sexual dimorphism, skin quality signals) continue to explain most of the variance in attractiveness.
For face recognition, the relevant point is that natural levels of facial asymmetry in the normal range have minimal effect on matching accuracy. Asymmetry becomes relevant for recognition only at clinical levels, pathological facial asymmetry resulting from developmental conditions or injuries.