Handedness refers to a consistent asymmetry in skill or preferential use between the hands and is related to lateralization within the brain of other functions such as language. Previous twin studies of handedness have yielded inconsistent results resulting from a general lack of statistical power to find significant effects. Here we present analyses from a large international collaborative study of handedness (assessed by writing/drawing or self report) in Australian and Dutch twins and their siblings (54,270 individuals from 25,732 families). Maximum likelihood analyses incorporating the effects of known covariates (sex, year of birth and birth weight) revealed no evidence of hormonal transfer, mirror imaging or twin specific effects. There were also no differences in prevalence between zygosity groups or between twins and their singleton siblings. Consistent with previous meta-analyses, additive genetic effects accounted for about a quarter (23.64%) of the variance (95%CI 20.17, 27.09%) with the remainder accounted for by non-shared environmental influences. The implications of these findings for handedness both as a primary phenotype and as a covariate in linkage and association analyses are discussed. ; Sarah E. Medland, David L. Duffy, Margaret J. Wright, Gina M. Geffen, David A. Hay, Florence Levy, Catherina E.M. van-Beijsterveldt, Gonneke Willemsen, Grant C. Townsend, Vicki White, Alex W. Hewitt, David A. Mackey, J. Michael Bailey, Wendy S. Slutske, Dale R. Nyholt, Susan A. Treloar, Nicholas G. Martin, Dorret I. Boomsm