Left-handedness as a function of sex, maternal versus paternal inheritance, and report bias.

The right-shift (RS) theory suggests that sex differences for handedness are due to the displacement of a chance distribution of asymmetry farther to the right in females than males by about 20%. An analysis of studies in the literature shows that when handedness is assessed by self-report, paired samples of males and females differ for incidence of left-handedness as predicted, but for parents assessed by indirect-report, there are fewer left-handed mothers than expected. When handedness is assessed by self-report in both generations, the RS genetic model successfully predicts the distribution in families. It is also successful at different levels of criterion from left-writing to non-right-handedness. The RS predictions are not always consistent with the findings of studies that depended on indirect report of parental handedness. When parental incidences are low the proportion of left-handed children in the families of left-handed mothers is higher than expected. When parental incidences are high, predictions for the families of left-handed mothers are excellent, but the percentage of left-handed children in the families of left-handed fathers is lower than expected. Data for all indirect-report studies are combined to test the idea that the chief cause of poor fit is underreporting of left-handed mothers by right-handed children. Transfer of right-handed children from R x R to R x L families, thereby raising the percentage of left-handed mothers by about 1%, is sufficient to give good fits to RS predictions for both sexes.