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Loneliness

Lonely Leftie: Left-Handed in a Family Full of Right-Handers

Studies show how likely it is to be the only left-hander in a family.

Key points

  • It is uncommon, but not impossible, that two right-handed parents have a left-handed kid.
  • The chance of two right-handed parents having a left-handed kid is 11%.
  • Right-handed parents can have a left-handed kid since handedness is not only determined by genes but also influences from the environment.

Recently, a reader of my blog “The Asymmetric Brain” approached me and told me that she was the only left-hander in her family. Her mom, her dad, all four grandparents, all her aunts and uncles, and all her cousins were right-handed. She was wondering how common this was and how it could happen.

So let us have a look at the science of left-handedness and how it runs in families!

Being left-handed in a right-handed family is uncommon, but not impossible

The good news is: Being the only left-hander in a right-handed family does not mean that a person was switched at birth or worse. It is, in fact, not very common, but certainly possible.

A study from the 1980s found that two right-handed parents had an 11% chance of having a left-handed child (Carter-Saltzman, 1980). If at least one parent was left-handed, the chance of left-handedness in their child increased to 25.5%.

In addition to biological parents, the study also included data on adoptive parents. For adoptive parents, two right-handed parents had a chance of 14% to raise a left-handed child. If at least one adoptive parent was left-handed the chance of the child being left-handed was 15%. Importantly, it was possible to predict left-handedness in a child based on the handedness of biological parents, but not based on the handedness of adoptive parents.

How is it possible that two right-handed parents have a left-handed child?

The fact that it is possible to predict the handedness of a child based on the handedness of biological parents, but not that of adoptive parents, suggests that handedness is to some extent determined by genetic factors. Suppose handedness was determined only by environmental influences such as parents teaching their kids to write with one hand. In that case, the handedness of adoptive parents should have the same predictive power as that of biological parents, which is not the case.

However, handedness is not entirely determined by genes. A large-scale handedness study in twins and their families in more than 25,000 Australian and Dutch families found that about 25% of the individual variance in handedness is determined by genes (Medland et al., 2009). The factors that determine the remaining 75% are not well understood. It is thought that environmental factors like instructions to use one hand for writing by teachers or parents play a role. Moreover, it has recently been suggested the random biological processes during brain development may play a role (McManus et al., 2021). These factors may explain why in 11% of cases, two right-handed parents have a left-handed child.

References

Carter-Saltzman L. (1980). Biological and sociocultural effects on handedness: comparison between biological and adoptive families. Science, 209, 1263-1265.

McManus C. (2021). Is any but a tiny fraction of handedness variance likely to be due to the external environment? Laterality 26, 310-314.

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. (2009). Genetic influences on handedness: data from 25,732 Australian and Dutch twin families. Neuropsychologia 47, 330-337.

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