Handedness as a function of twinning, age and sex.

Questions about twin birth, sex, age and handedness for writing were asked as part of a survey of hearing disability (Davis, 1989) in a large sample of the adult population. The findings show unequivocally that the prevalence of left handedness is higher in twins than in the singleborn, in males than in females and in younger than in older adults. There was a marked and regular decline in the percentage of left writers with increasing age, but the effects for twinning and sex were evident in all of 7 age bands from 18 years to 80+. The findings are consistent with the assumption of the right shift theory of handedness (Annett, 1972, 1985) that the rs+ gene is expressed more strongly in females than in males, and in the singleborn than in twins.