Is any but a tiny fraction of handedness variance likely to be due to the external environment?
I. C. McManus
Abstract
Non-shared environmental variance (NSEV) accounts for 76% of variance in genetic modelling of handedness. However, it is very misleading to suggest that NSEV, "highlights the importance of non-genetic factors for the ontogenesis of hemispheric asymmetries". NSEV is poorly named, is calculated only by subtraction, and provides no direct evidence for environmental effects in the sense of the external environment. Miller suggested that it would be better named as "residual effect". Mitchell has suggested that much or indeed most of NSEV is "developmental variance" and should be included under the heading of nature rather than nurture, and in handedness, "largely reflect[s] the outcome of randomness in brain development". Overall only a very small proportion of NSEV in handedness is likely to be related to external environmental factors in the usual sense of the term.