Arm folding: The myth
Most people have a strong preference when they fold their arms; they either have the left forearm on top (L) or the right forearm on top (R). Arm folding is sometimes used to illustrate basic genetics; the myth is that it is controlled by a single gene with two alleles, and the allele for R is dominant over the allele for L. I do not know how the myth got started. I don't know of any scientific papers that make this claim; the first paper to look at arm folding, Weiner (1932), clearly concludes that it has little or no genetic basis.
Arm folding as a character
Arm folding is easy to score; most people only fold their arms in one way, with the opposite way feeling very unnatural. In most populations, slightly more than half of people are L (McManus and Mascie-Taylor 1979). Some people, however, fold their arms either way; Weiner (1932) found that 3 out of 22 people folded their arms either way, while Reiss and Reiss (1998) reported that about 4 percent of their subjects had no preference.
Wiener (1932) was the first to examine the genetic basis of arm folding by comparing parents and offspring, with the following results:
|Parents||R offspring||L offspring||Percent R|
|R x R||31||50||38%|
|R x L||81||99||45%|
|L x L||58||70||45%|
Each of the three kinds of matings has about the same proportion of R and L offspring, so Weiner (1932) concluded that there is no genetic basis for arm folding preference. If the myth were true, two L parents could not have an R child, but close to half of the children of LxL matings are R. For some reason, people kept doing family studies of arm folding, so that Reiss and Reiss (1998) were able to summarize the numbers from 12 studies:
|Parents||R offspring||L offspring||percent R|
|R x R||731||672||52%|
|R x L||1230||1497||45%|
|L x L||629||1038||38%|
There is some association between parents and offspring, in that R x R parents have a higher proportion of R offspring than do L x L parents. All studies have found many R offspring of L x L parents and L offspring of R x R parents, so even if there is some genetic influence on arm folding, it is not a simple one-locus, two-allele genetic trait.
Reiss and Reiss (1998) summarized the results of two twin studies, which gave nearly identical results. About half of the pairs of monozygotic (identical) twins consist of one R twin and one L twin, and the same is true for dizygotic twins. This is further evidence for a lack of genetic influence on this trait.
|R + L||109||78|
Family and twin studies clearly demonstrate that there is little genetic influence on arm folding, and it certainly is not the simple one-gene, two-allele trait described in the myth. You should not use arm folding to demonstrate basic genetics.
McManus, I.C., and C.G.N. Mascie-Taylor. 1979. Hand clasping and arm folding: A review and a genetic model. Annals of Human Biology 6: 527-558.
Reiss, M., and G. Reiss. 1998. Arm-folding in families: A review of literature and some new data. Psychologia 41: 81-87.
Wiener, A.S. 1932. Observations on the manner of clasping the hands and folding the arms. American Naturalist 66: 365-370.
This page was last revised December 8, 2011. Its address is http://udel.edu/~mcdonald/mytharmfold.html. It may be cited as pp. 6-7 in: McDonald, J.H. 2011. Myths of Human Genetics. Sparky House Publishing, Baltimore, Maryland.
©2011 by John H. McDonald. You can probably do what you want with this content; see the permissions page for details.