Left-handedness in twin families: support of an environmental hypothesis

Information on handedness, assessed as writing-hand, was collected from 197 MZ twin pairs and 203 DZ twin pairs and from their parents, spouses, and children. Associations for pairs of relatives were studied by 2 X 2 tables, computing chi squared-values and tetrachoric correlations. Correlations of about .3 were obtained for mother-offspring and sibling pairs, while for cousins the correlation was .25 (in a small sample). No other significant associations were found, not even for twins. These results suggest only a small, if any, genetic effect, and only a small environmental between family effect, most of which seems to be a maternal effect. The lack of cotwin correlations and correlations for one of the twins with her/his ordinary brothers/sisters suggest the existence of an effect specific to twins, since the correlation for pairs of ordinary siblings is significant. Conclusions drawn from twin studies alone may be biased. The frequency of left-hand writers has increased from 1% to 10% in the different age groups born during the last century in Norway. This age effect is present mainly in hand writing and not so much in handedness generally, suggesting a decrease in cultural repression against left-hand writing.