A large-scale study of handedness and pregnancy/birth risk events: implications for genetic theories of handedness.

Whether left-handedness is due to genetic factors or to pregnancy and birth stress events is an important question for models that attempt to explain the origins and distribution of human handedness. Major genetic theories of handedness, such as those of Annett (1985) and McManus (1981), allow for a nondetermination of some left- or right-handedness by "chance", but they hold that "pathological left-handedness", specifically, is of minimal influence. On the other hand, theorists such as Bakan (1971) and Coren (1995) take the view that right-handedness is a universal human characteristic, presumably due to a polygenic influence, and that left-handedness is essentially always the product of pregnancy and birth risk factors. Research attempting to find associations of left-handedness and unfortunate pregnancy and birthing events has produced very inconsistent results. We report a large-scale study that employed handedness for writing as the criterion for handedness, mothers' reports regarding the occurrence or nonoccurrence of a large number of possible pregnancy and birth stress events, and mothers' reports regarding the handedness of the biological parents and of their offspring. Of the 25 potential stressors studied, only maternal age showed a significant association with left-handedness of offspring. Further, this relationship was quite weak and would account for no more than 1/84th of the approximate 11% incidence of left-hand-edness in the general population. Implications for theories of handedness are discussed.