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Left-handedness seems to run in families. Most lefties can name one or more family member who also prefers to use the left hand for writing and other complex motor activities, but is this anecdotal evidence actually backed up by scientific studies? Turns out, it is.
Family studies clearly show that children's handedness is linked to that of their biological parents. Two left-handed parents have a chance of slightly over 26% to raise a left-handed child. However, this percentage drops to 19.5% in couples with one left- and one right-handed parent and to 9.5% if both parents are right-handed (Annett, 2002). Interestingly, such a relationship exists only for biological, but not for adoptive parents. A study comparing the influence of handedness of biological and adoptive parents found that if both biological parents were left-handed, the chance of their child being left-handed, too, was 27% (Carter-Saltzman, 1980). In contrast, left-handedness in adoptive parents seems to be unrelated to left-handedness in children, since even if both adoptive parents were left-handed, the chance of the child also being left-handed was only 5% in this study.
These and further studies indicate that left-handedness is to some extent heritable, which suggests that it is likely influenced by genetic factors. Indeed, it was believed for a long time that a single gene would cause left-handedness. However, newer studies have clearly shown that this is not the case (Armour et al., 2014). Instead, it is now estimated that at least 30 to 40, but potentially up to 100 different genes influence handedness (McManus et al., 2013). Functionally, these genes are involved in areas like brain development, the formation of the left-right body axis and neurotransmitter systems, among others. But how much of left-handedness is actually genetically determined? A large-scale twin study that analyzed handedness in twins and their families in more than 25,000 Australian and Dutch families (Medland et al., 2009) came to a surprising answer: Only about a quarter of the individual variance in handedness can be explained by genes, while three quarters are determined by environmental influences. Which environmental influences these are, is currently not fully understood, but model learning, parental instructions and cultural pressure to write with one hand have been suggested.
Thus, in contrast to what has been thought for a long time, the factors that determine whether we are a leftie or a right are anything but simple.
References
Annett M, 2002. Handedness and brain asymmetry: the right shift theory. Psychology Press, Hove, UK.
Armour JA, Davison A, McManus IC. Genome-wide association study of handedness excludes simple genetic models. Heredity (Edinb) 2014;112:221-225.
Carter-Saltzman L. Biological and sociocultural effects on handedness: comparison between biological and adoptive families. Science 1980;209:1263-1265.
McManus IC, Davison A, Armour JA. Multilocus genetic models of handedness closely resemble single-locus models in explaining family data and are compatible with genome-wide association studies. Ann N Y Acad Sci. 2013 Jun;1288:48-58.
Medland SE, Duffy DL, Wright MJ, Geffen GM, Hay DA, Levy F, van-Beijsterveldt CE, Willemsen G, Townsend GC, White V, Hewitt AW, Mackey DA, Bailey JM, Slutske WS, Nyholt DR, Treloar SA, Martin NG, Boomsma DI. Genetic influences on handedness: data from 25,732 Australian and Dutch twin families. Neuropsychologia 2009;47:330-337.
