“If the right side of the brain controls the left side of the body, then only left-handed people are in their right minds.” ~Anonymous
Throughout history, the phenomenon of left-handedness has often been associated with misfortune and criminality, with some even going so far as to deem it the mark of the devil (Benningfield, 2019). During the Salem witch trials, individuals we so fondly refer to as “lefties” nowadays were accused of witchcraft and executed. While most of the accused and convicted individuals back then were women (about 78%), modern statistics indicate that among left-handed North Americans today, only about 11% of females are left-handed compared to 13% of males (Mastin, 2012). What accounts for this surprising difference? Researchers have suggested that exposure to differing amounts of prenatal (before birth) androgens (male sex hormones) like testosterone may play a role in determining whether a fetus will develop into a so-called “leftie” (Richards et al., 2021a).
A study investigating the connection between the androgen receptor gene (AR) and handedness obtained results that suggest individuals with longer CAG-repeats (base repeats in DNA) in the AR gene (resulting in less efficient AR function) have a greater instance of non-right-handedness (Arning et al., 2015). Think of it like a game of matching wooden pegs to their corresponding holes. Pretend you have a square-shaped peg that represents a molecule of testosterone. If the hole is also square-shaped (a normal AR gene), then inserting the peg into the hole is very simple and the individual’s cells can receive greater amounts of androgens. Now if that square-shaped hole happened to be circular (an AR gene with longer CAG-repeats), then inserting the same square-shaped peg becomes much more difficult – maybe even impossible. This scenario is similar to what happens in the cells of individuals with longer CAG-repeats: they receive less androgens due to an inability of specific androgens to bind to their corresponding receptors. Therefore, these findings seem to indicate that exposure to lower amounts of prenatal androgens may increase an individual’s likelihood of being left-handed. While this doesn’t account for the disparity in the percentages of male and female “lefties” today, it helps affirm that prenatal hormones may be involved in determining handedness. So, what’s the catch? Since this study was performed in adults and researchers didn’t directly measure the concentrations of androgens present in fetal cells, we must proceed with caution when interpreting these outcomes as indicative of the relationship between prenatal sex hormones and handedness.
Another study conducted on twins discovered that females in opposite-sex twin pairs have a significantly lower prevalence of left-handedness in relation to females in same-sex twin pairs (Vuoksimaa et al., 2010). Operating under the hypothesis that females in opposite-sex twin pairs may be exposed to more testosterone than females in same-sex twin pairs because of the formers’ close proximity to their male siblings in the womb, researchers measured individuals’ circulating testosterone and estradiol levels through saliva samples. Although statistically significant differences in the prevalence of left-handedness were discovered between females with twins of different genders, males in the study from opposite-sex twin pairs and same-sex twin pairs displayed no differences in handedness prevalence. Thus, despite the observed difference in females with twins, these results do not necessarily support the hypothesis that exposure to higher levels of prenatal testosterone increases the likelihood of an individual being right-handed.
A scientific analysis of available research emphasizes the association between the digit ratio and handedness (Richards et al., 2021b). The digit ratio, calculated by dividing the length of one’s index finger by the length of one’s ring finger on the same hand, is used as a “proxy measure of [testosterone]” in some studies on prenatal sex hormones and handedness. Due to the sexually dimorphic (A sexually dimorphic trait is a trait that appears differently in males and females.) nature of the digit ratio that arises prenatally, some scientists view the ratio as “determined in part by individual differences in exposure or sensitivity to foetal testosterone and oestradiol”. Extrapolating connections between handedness and prenatal hormone levels using the digit ratio, researchers discovered that a low digit ratio, indicating low prenatal testosterone levels, corresponded to left-handedness. These findings directly oppose those in previously mentioned studies. A shortcoming of studies that measure the digit ratio as an indicator of prenatal hormone levels is the lack of direct, concrete evidence supporting that the digit ratio reflects prenatal androgen levels.
So, who’s right? The answer: “Scientists aren’t so sure”. As with all scientific research, absolute certainty is simply not feasible, and assumptions are necessary in order to draw conclusions. The divide in the scientific community about whether lower or higher levels of prenatal androgen exposure correspond to left-handedness is more present than ever and which side prevails remains to be determined. For now, the only thing we can confidently assert is: your left-handed friend is definitely not a witch or an embodiment of evil so treat them with kindness!
Written by Emily Zheng and edited by Aldrin V. Gomes, PhD
Arning L, Ocklenburg S, Schulz S, Ness V, Gerding WM, Hengstler JG, et al. Handedness and the X chromosome: the role of androgen receptor CAG-repeat length. Sci Rep 2015; 5: 8325.
Benningfield B. A History Of How The Left Hand Became Associated With Evil And The Devil [Internet]. Ranker 2019 [cited 2023 Jun 28] Available from: https://www.ranker.com/list/left-hand-facts/bailey-benningfield
Mastin L. Handedness Statistics [Internet]. Right, Left, Right, Wrong! 2012 [cited 2023 Jun 27] Available from: https://www.rightleftrightwrong.com/statistics.html
Richards G, Beking T, Kreukels BPC, Geuze RH, Beaton AA, Groothuis T. An examination of the influence of prenatal sex hormones on handedness: Literature review and amniotic fluid data. Horm Behav 2021; 129: 104929.
Richards G, Medland SE, Beaton AA. Digit ratio (2D:4D) and handedness: A meta-analysis of the available literature. Laterality 2021; 26: 421–84.
Vuoksimaa E, Eriksson CJP, Pulkkinen L, Rose RJ, Kaprio J. Decreased prevalence of left-handedness among females with male co-twins: evidence suggesting prenatal testosterone transfer in humans? Psychoneuroendocrinology 2010; 35: 1462–72.