Intra-familial learning is only a minor factor in manifest handedness

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From the information provided by 2257 university students and faculty members, both the incidence of handedness types (right, left, non-right) and the degree of handedness (strength and consistency) were analyzed as a function of five familial sinistrality variables. In addition, the handedness characteristics of a separate group of subjects ( N=80) who had switched handedness were compared with those of the major sample. The frequency data showed that the incidence of nondextrality in respondents was significantly higher when there were nondextral siblings in the family than when all relatives were dextral, and higher still when a parent, rather a sibling, was the nondextral relative. The strength and consistency data showed that the presence of familial sinistrality was associated with miniscule reductions in degree of handedness not only in dextrals but, contrary to prediction, in sinistrals as well. The data from the hand-change subgroup showed that the incidence of ambilaterality was strikingly higher than among no-change subjects and that the degree of right- and left-handedness was lower than among their like-handed no-change counterparts. The finding that familial sinistrality was associated with large increases in the incidence of nondextrality but with very small changes in the degree of handedness suggests that incidental intra-familial learning is a negligible factor in human handedness.