Fluctuating asymmetry and general intelligence: No genetic or phenotypic association

Fluctuating asymmetry (FA) is the non-pathological left–right asymmetry of body traits that are usually left–right symmetrical, such as eye breadths and elbow to wrist lengths in humans, but which can be affected by developmental stressors. It is generally considered throughout biology to be an indicator of developmental instability and thus of lack of overall biological fitness. Several investigators have proposed that deficiencies in general intelligence (g) may be indicators of the same kind of instability in human brain development. If so, FA and g should be negatively correlated. Moreover, because g shows substantial genetic influences, FA should also show genetic influences, and the two sets of genetic influences should be correlated. We investigated these propositions in a sample of 263 adults that included 88 pairs of twins. Results indicated genetic influences on FA, but FA and g were not correlated at either the observed or genetic levels.