The distribution of handedness in chimpanzees: Estimating right shift in Hopkins' sample

Abstract

The right shift (RS) theory of handedness and cerebral dominance suggests that hand preference depends on an underlying continuous normal distribution of asymmetry for hand skill. The distribution in humans is displaced towards the right, in comparison with the unbiased distribution expected for most nonhuman species. Reports that chimpanzees are more often right- than left-handed raise the question whether they also have a distribution of asymmetry shifted to the right. This paper shows how to estimate extent of RS from classifications of hand preference as left, mixed, and right. Whereas the human distribution is shifted by about 1.0 z, that of chimpanzees in the sample of Hopkins, Cantalupo, Freeman, Russell, Kachin, and Nelson (2005) is shifted by about 0.32 z.