Tests of the right shift genetic model for two new samples of family handedness and for the data of McKeever (2000).

Two new family samples are described for handedness in parents and children. In the first sample, drawn from the general population, questionnaires were completed for each individual (a self-report sample). In the second sample students completed a handedness questionnaire and described the handedness of their parents (indirect report sample). The percentages of left-handed children in the various family types (R x R, R x L etc., father x mother) were tested for goodness of fit to the predictions of the right shift genetic model, along with tests for the findings of McKeever (2000). Fits for the self-report sample were good for all family types. For the indirect report sample fits were good for many family types but there were more left-handed daughters than expected in R x L families and fewer than expected in L x R families. In the McKeever sample there was a shortfall of sons in L x R families but the fit for sons in R x L families was excellent. It is concluded that differences between samples here, and in the literature, are variable and probably due to sampling errors when the assessment of parental handedness depends on indirect report. Findings for sex differences do not suggest a need for hypotheses about X-linked mechanisms.