From Genome: The Autobiography of a Species in 23 Chapters:

Some asymmetries in the body are consistent: the heart is on the left side of the chest, for example, in most people.  But other, smaller asymmetries can go randomly in either direction.  In some people the left ear is larger than the right; in others, vice versa.  The magnitude of this so-called fluctuating asymmetry is a sensitive measure of how much stress the body was under when developing, stress from infections, toxins or poor nutrition.  The fact that people with high IQs have more symmetrical bodies suggests that they were subject to fewer developmental stresses in the womb or in childhood.  Or rather, that they were more resistant to such stresses.  And the resistance may well be heritable.  So the heritability of IQ might not be caused by direct ‘genes for intelligence’ at all, but your ability to develop a high IQ under certain environmental circumstances.  How does one parcel that one into nature and nurture?