Articles

Special Twin Environments, Genetic Influences and their Effects on the Handedness of Twins and their Siblings

Sarah E. Medland^a1 c1, Margaret J. Wright^a2, Gina M. Geffen^a3, David A. Hay^a4, Florence Levy^a5, Nicholas G. Martin^a6 and David L. Duffy^a7

^a1 Queensland Institute of Medical Research, Australia; Cognitive Psychophysiology Laboratory, University of Queensland, Australia. sarahMe@qimr.edu.au

^a2 Queensland Institute of Medical Research, Australia.

^a3 Cognitive Psychophysiology Laboratory, University of Queensland, Australia.

^a4 Department of Psychology, Curtin University of Technology, Australia.

^a5 School of Psychiatry, University of New South Wales, Australia.

^a6 Queensland Institute of Medical Research, Australia.

^a7 Queensland Institute of Medical Research, Australia.

Abstract

It has been suggested that twinning may influence handedness through the effects of birth order, intra-uterine crowding and mirror imaging. The influence of these effects on handedness (for writing and throwing) was examined in 3657 Monozygotic (MZ) and 3762 Dizygotic (DZ) twin pairs (born 1893–1992). Maximum likelihood analyses revealed no effects of birth order on the incidence of left-handedness. Twins were no more likely to be left-handed than their singleton siblings (n = 1757), and there were no differences between the DZ co-twin and siblingtwin covariances, suggesting that neither intra-uterine crowding nor the experience of being a twin affects handedness. There was no evidence of mirror imaging; the co-twin correlations of monochorionic and dichorionic MZ twins did not differ. Univariate genetic analyses revealed common environmental factors to be the most parsimonious explanation of familial aggregation for the writing-hand measure, while additive genetic influences provided a better interpretation of the throwing hand data.

Correspondence:

^c1 Address for correspondence: Sarah Medland, Genetic Epidemiology Unit, Queensland Institute of Medical Research, 300 Herston Rd, Brisbane, Queensland 4029, Australia.