Pathological left-handedness: is it familial?

Hand preferences and the incidence of familial sinistrality of developmentally retarded. Down's syndrome and non-retarded individuals were compared. The incidence of left-handedness in the retarded groups was higher than in the normal group. The incidence of familial left-handedness was also raised in the retarded groups. These results are inconsistent with the popular theoretical distinction between pathological left-handedness in special groups and familial left-handedness in the general population.