Visual experience affects handedness.

In birds, a lateralised visual input during early development importantly modulates morphological and functional asymmetries of vision. We tested the hypothesis that human handedness similarly results from a combination of inborn and experience-driven factors by analysing sidedness in children suffering from congenital muscular torticollis. These children display a permanently tilted asymmetric head posture to the left or to the right in combination with a contralateral rotation of face and chin, which could lead to an increased visual experience of the hand contralateral to the head-tilt. Relative to controls, torticollis-children had a higher probability of right- or left-handedness when having a head-tilt to the opposite side. No statistical significant relation between head position and direction of functional asymmetries was found for footedness and eye-preference, although the means show a non-significant trend in the same direction as was observed for handedness. Thus, an increased visual control of the hand during early childhood seems to modulate handedness and possibly other lateral preferences to a lesser extent. These findings not only show that human handedness is affected by early lateralised visual experience but also speak in favour of a combined gene-environment model for its development.