Maternal age and parity correlates of handedness: gender, but no parental handedness modulation of effects.

Mothers supplied information on the handedness of a total of 1079 children (238 left handed, 841 right handed), and also reported information on the handedness of the biological parents, parental ages when each child was born, parity, and birth complications. A MANOVA, employing offspring handedness and parental handedness (presence versus absence of at least one left handed) as the bases of classification, and parity, maternal age, paternal age, and a birth stress composite score as dependent measures, was applied to the data of female and male offspring separately. For females, there was a significant multivariate effect for handedness, but no effect of parental handedness nor any interaction of the handedness and parental handedness factors. Univariate ANOVAs, following the multivariate analysis, showed significant effects of handedness for the maternal age and parity measures, but not for paternal age or birth stress composite score. The analysis for males showed no significant multivariate or univariate effects. The data suggest, that in the absence of high risk parity and maternal age over 32, only about 7.8% of females are left handed. This implies that as many as 29% or so of female left handers may owe their sinistrality to factors associated with high risk parity and maternal ages over 32.