Plasticity of lateralization: schooling predicts hand preference but not hand skill asymmetry in a non-industrial society.

Considerable variation in the frequency of left-handedness between cultures has been reported, ranging from 0.5 to 24%. This variation in hand preference may have evolved under natural or cultural selection. It has been suggested that schooling affects handedness but as in most human societies only a selected and minor part of the population does not attend school this is difficult to test. We investigated to what extent schooling affects both hand preference and asymmetry in hand skill in a non-industrial population in the highlands of New Guinea. This provided unique opportunities because of the relatively recent establishment of a primary school in this population, and where people still live a non-industrial traditional life reflecting conditions in which handedness may have evolved. We interviewed 620 inhabitants (aged 5-70 y) to collect demographic data and school history, tested hand preference on 10 ecologically relevant activities, and measured performance of each hand on three tasks (pegboard, grip force, ball throwing). Schooled individuals were overall faster in fine motor performance, had greater grip strength and greater throwing accuracy. This suggests that there is implicit selection on the fitter part of the population to enter school. Schooling is associated with hand preference, as schooled individuals were more likely to be extremely right-handed and less likely to be strongly right-handed, but not with asymmetry of hand skill (controlled for sex and age). Developmental plasticity in hand preference but not skill asymmetry, and the weak correlations between hand preference and hand skill asymmetry indicate that they represent different aspects of brain lateralization. Furthermore, the weak correlations between hand preference and hand skill asymmetry leave room for moderating factors such as schooling, sex and age to have a differential effect on hand preference and hand skill, and each needs to be studied in its own right.