Fluctuating asymmetry and intelligence

The general factor of mental ability (g) may reflect general biological fitness. If so, g-loaded measures such as Raven's progressive matrices should be related to morphological measures of fitness such as fluctuating asymmetry (FA: left–right asymmetry of a set of typically left–right symmetrical body traits such as finger lengths). This prediction of a negative correlation between FA and IQ was confirmed in two independent samples, with correlations of − 0.41 and − 0.29, respectively. Head size also predicted Raven's scores but this relationship appeared to be mediated by FA. It is concluded that g along with correlated variables such as head size are in large part a reflection of a more general fitness factor influencing the growth and maintenance of all bodily systems, with brain function being an especially sensitive indicator of this fitness factor.