Your body’s bilateral symmetry statistically predicts your health, probability of schizotypy and depression, number of sexual partners, and resting metabolic rate (particularly if you are male). Bodily symmetry may reflect “developmental stability” – i.e., influences like disease, mutation and stress may cause a developmental divergence from DNA’s symmetric blueprint. Not only do individuals differ in their environmental exposure to these things, but also in their sensitivity to them: a recent Intelligence article claims that “some individuals grow adaptive phenotypes under almost any conditions, whereas others show disrupted development given the slightest perturbations.” In other words, bodily symmetry reflects a combination of developmental history and genetic quality.
Prokosch et al. further suggest that general intelligence – Spearman’s g – is the neural equivalent of bodily symmetry. In other words, cognitive functioning reflects “neurodevelopmental stability“, whereas bodily symmetry reflects “morphodevelopmental stability“.
If true, then cognitive performance and bodily symmetry should predict one another. To verify this prediction, Prokosch et al. measured the right and left foot width, ankle width, wrist width, elbow width, ear width, ear length, index finger length, middle finger length, third finger length, and little finger length of 78 male subjects.
Each subject also completed standard tests of general intelligence, including Raven’s matrices, WAIS III vocabulary, Shipley vocabulary, and digit span (forwards and backwards).
The results showed that bodily symmetry was correlated with all the measures (except for digit span), and that the size of each correlation was related to how strongly each of these measures predits general intelligence. In other words, body symmetry more strongly predicts intelligence than brain size, nerve conduction velocity, reaction time reliability, and a number of other measures. (This result was replicated this year in a study where symmetry of finger length and palm width had the strongest relationship with intelligence.)
The authors suggest that human intelligence may therefore be a “fitness indicator,” like a male peacock’s tail is a sexual advertisement to female peacocks. Prokosch et al argue this is one explanation for why intelligence is so quickly, easily and accurately assessed in social situations, and why it has such a strong relationship with reproductive success. Furthermore, a new study – currently under review – suggests that the heritability of executive functions (which are closely related to gF) may be close to unity.
Intelligence is more than the peacock’s tail, of course – it provides a variety of advantages. But the important thing is that intelligence is not easily faked: the brain is a metabolic “money pit,” consuming a large percentage of our daily calories and requiring a lot of fat. The very fact that intelligence is so difficult to fake, or even to manifest in the first place, makes it a trustworthy signal of evolutionary fitness for the opposite sex.
Edit: I replaced the link related to the ease with which people can assess intelligence in social situations – I found something better than what was cited by Prokosch et al.