I recently found a blog post with an interesting theory about why there are more male than female left-handers.¹ The incidence of left-handedness among men is 11 to 12% as opposed to a 9 to 10% rate among women. This sex-related rate difference is robust and found across different cultures and different generations of participants.
The most often-cited theory to explain the sex difference in rates of left-handedness was formulated in the 1980’s by Norman Geschwind and his associates. They claimed the male hormone, testosterone, slows maturation of the left hemisphere of the brain during fetal development. The left hemisphere controls the movements of the right hand so the right hemisphere compensates for the delay and handedness control shifts from the left to the right hemisphere. This switch leads to higher rates of left-handedness among males. The Geschwind theory has been scrutinized and criticized over the years but contemporary studies still refer to it as an explanation for the sex difference in left-handedness rates.
The new theory offers a genetic rather than an hormonal explanation for the sex difference. The author claims there are genetic rules for embryo development and some trajectories are more likely than others when it comes to the occurrence of traits. One rule signals the development of differences in the left and right sides of the human embryo, including the brain. Any genetically-encoded process has variation. In the case of handedness, the two variations are left and right with the right-sided trajectory being more probable than the left-sided. The most common handedness developmental pathway goes right about 90% of the time but a lower probability channel (10%) is followed by some embryos to the left.
The author’s further claim is that males are more variable in their trait development than are females. This variation is found in several traits besides handedness including birth weight, body features, blood parameters and behavioral measures such as athletic and academic performance. For example, more males than females test at the extreme high and low ends of the distribution of IQ scores. The chromosomal basis of human sex determination is the reason for the difference in variability. Females have two X chromosomes while males have only one; the male genome is XY. Any trait with a genetic connection to the X chromosome is more likely to show variation in males because of the single X chromosome. Females need an exceptionally high dose of genetic influence since the second X chromosome can mitigate any effects associated with its partner X. Two X chromosomes impart genetic stability while one X chromosome contributes to genetic variation. The left-handed genetic pathway is less probable than the right-handed one. Male embryos show greater variability and, therefore, are more likely to follow the lower probability left-handed genetic pathway than are female embryos.
The author cites cross-species data as evidence for the link between the male/female sex chromosome differences and trait variability. Examining data from different species of mammals, birds, butterflies and insects, he finds females show more trait variability in species where the female has two different sex chromosomes. Trait variability is associated with genomes with two different sex chromosomes. In humans, this type of genome is found in the male of the species. In some birds and butterflies it is found in the female.
In summary, this theory has two parts. The first part assumes a high probability (right) and a low probability (left) genetic trajectory for handedness development. The second part argues that males are more variable in their trait development because of their dissimilar sex chromosomes, XY, as compared to the XX of females. Males are more variable in their trait development and females are more consistent. This variation difference leads to a higher percentage of male embryos taking the lower probability leftward trajectory. Higher rates of left-handedness among human males are the result. To quote the 1916 poem, The Road Not Taken, by Robert Frost, male embryos are more likely to take the “path less traveled”, in this case left, than are female embryos.
¹Mitchell, K. (2020). A sinister attractor-why males are more likely to be left-handed. Wiring the Brain, November 22. www.wiringthebrain.com/2020/11/a-sinister-attractor-why-males-are-more.html?spref=tw