Quantitatively assessed familial sinistrality in right-, mixed-, and left-handers

Numerous manifestations of cerebral asymmetry in man depend on the presence of non-right-handers (NRHs) among the subject's biological relatives – familial sinistrality, FS. This study proposes a method for a quantitative assessment of FS, based on the coefficient of relatedness of the subject with every single one of his/her relatives with known handedness. Such an approach conforms much better with the genetic principles than the methods ignoring the degrees of genetic relationship. Contacting and including as many as possible living relatives enlarges the category of subjects with FS, even relative to studies where non-immediate family members have been included and this enlargement might be especially important for the right-handers. The method was applied on a sample of 870 subjects (264 right-, 246 mixed- and 360 left-handers). Handedness, either distributed in the three classical categories or presented by the raw handedness score as continua, showed a highly significant relationship with the index. A considerable excess of females over males in their FS was found and its possible genetic and psycho-sociological reasons were discussed. It was suggested that the proposed method mirrors an existing biological reality and could be applied in further studies of handedness in relation to its genetic grounds.