Ontogenesis of Left-Handedness
"Right handedness is the price which we have to pay for a higher civilization and greater mental acquirement; while left handedness would seem to be a step backward...a reversal toward primitive man."
I want to preface this article by saying that if you’re left-handed, it does not necessarily mean something is wrong with you. If general behavioral and biological patterns exist, I think they are worth exploring regardless of their popularity or hilarity. Concerning ambidexterity, you can argue that using an increased number of fingers, especially in fine motor activities, creates a more whole experience fueled by an abundance of metabolic energy.
The phenomenon of left-handedness, if you will, has been observed and recorded for a very long time. Nowadays its mean prevalence in society seems to lie around the 8-10% range, being more frequently expressed in males than females. This parallels the previous observation that male fetuses are more sensitive to environmental change, which I’ll touch upon later. There are a few theories as to why right-handedness has been the dominant “mode” or “choice” for people, and I will specifically focus on the relationship between left-handedness and various disorders that seem to be metabolic in origin (aren’t all disorders a product of failure in ordered energy?).
Handedness can be observed during gestation; babies usually suck the right thumb and move the right arm more than the left (Gutwinski et al., 2011). To my surprise, there is a wide range of disorders that seem to be more prevalent in people with left-handedness, but to go through every single study would take forever so I have picked the ones I’ve found to be the most interesting. A cohort study conducted in Sweden found men who were exposed to ultrasound during fetal life had an increased prevalence of left-handedness (n=6858 for exposed, n=172,737 for unexposed) (Kieler et al., 2001). As a quick side note, ultrasound has been shown to be both therapeutic and damaging, depending on severity and length of exposure; stimulating the repair of hard tissues and organs (bones, liver) as well as damaging them. Ultrasound is not a biologically neutral procedure, and a developing fetus is especially sensitive to strong alterations in its environment. Another study found a positive correlation between birth stress (premature birth, multiple births, prolonged labor, C-section, breathing difficulty, blue baby) and left-handedness (Bakan et al., 1973). In a sample of 1298 subjects, similar results were found (Coren, 1995).
One way epigenetic change is passed on from parent to offspring is through DNA methylation, which blocks gene expression and can alter the course of brain development. Metabolism is closely related to this, with nitric oxide being an active mediator of the methylation process (Bovee et al., 2017). Interestingly, the role of DNA methylation has been linked to handedness. Several specific genes are asymmetrically expressed in the fetal brain; one of them has recently been implicated in the expression of handedness. The authors found some evidence that environmental factors promote DNA methylation of these asymmetrically expressed genes which can be used to predict handedness direction; people who scored high in birth stress scores had a higher prevalence of left-handedness and an increase in DNA methylation of specific gene sites related to handedness (Schmitz et al., 2017).
“The main finding of this analysis was that DNA methylation of CpG sites in three asymmetrically expressed genes (LMO4, MT2A, and STK35) significantly predicted handedness direction while DNA methylation in LRRTM1 and NEUROD6 tentatively predicted handedness direction. Our findings suggest that not only genetic variation within these genes but also epigenetic modulation of their expression is relevant for handedness.“
One of the studies that shocked me was one conducted on pregnant mothers who were scanned to check for the handedness of the fetus: there were more left-handed self-touches in stress-exposed mothers (Reissland et al., 2014).
In my previous article regarding the relationship between homosexuality and metabolism, I talked about the influence of prenatal stress on the ontogenesis of same-sex behavior. In boys, left-handedness was significantly associated with gender dysphoria and homosexuality (Zucker et al., 2001, Lindesay, 1987). Wood found an increased frequency of left-handedness, dyslexia, and learning disorders in patients with autoimmune thyroid disease (Grave’s and Hashimoto’s thyroiditis) (Wood et al., 1992).
The most sensible explanation as to why intrauterine stress tends to favor the development of left-handedness is that the left cerebral hemisphere takes longer to develop and mature than the right (which you can also argue is more sensitive to stimuli since anesthesia, for example, tends to damage the frontal lobe rather than the brain stem) and since left-handedness is associated with right hemisphere control, damage to the left hemisphere development can shift motor function over to the opposite side of the brain causing the preference for left-hand use. This may also explain why left-handedness is more common in people born in the spring or summer, having been conceived during the stressful winter months. (Gutwinski et al., 2011).
I think it helps to think of left-handedness as the mildest symptom of prenatal stress exposure, although it does tend to occur alongside other symptoms like gender dysphoria, alcoholism, or autoimmune thyroid disease.
References
Bakan, P., Dibb, G., & Reed, P. (1973). Handedness and birth stress. Neuropsychologia, 11(3), 363–366. doi:10.1016/0028-3932(73)90050-x
Bovee, R. C., Hickok, J. R., Vasudevan, D., & Thomas, D. D. (2017). Mechanisms of Epigenetic Regulation by Nitric Oxide. Nitric Oxide, 255–270. doi:10.1016/b978-0-12-804273-1.00020-x
Coren, S. (1995). Family patterns in handedness: Evidence for indirect inheritance mediated by birth stress. Behavior Genetics, 25(6), 517–524. doi:10.1007/bf02327575
Kieler, H., Cnattingius, S., Haglund, B., Palmgren, J., & Axelsson, O. (2001). Sinistrality—a side-effect of prenatal sonography: A comparative study of young men. Epidemiology, 12(6), 618–623. doi:10.1097/00001648-200111000-00007
Gutwinski, S., Löscher, A., Mahler, L., Kalbitzer, J., Heinz, A., & Bermpohl, F. (2011). Understanding Left-Handedness. Deutsches Aerzteblatt Online. doi:10.3238/arztebl.2011.0849
Lindesay, J. (1987). Laterality shift in homosexual men. Neuropsychologia, 25(6), 965–969. doi:10.1016/0028-3932(87)90100-x
Reissland, N., Aydin, E., Francis, B., & Exley, K. (2014). Laterality of foetal self-touch in relation to maternal stress. Laterality: Asymmetries of Body, Brain and Cognition, 20(1), 82–94. doi:10.1080/1357650x.2014.920339
Schmitz, J., Kumsta, R., Moser, D., Güntürkün, O., & Ocklenburg, S. (2017). DNA methylation in candidate genes for handedness predicts handedness direction. Laterality: Asymmetries of Body, Brain and Cognition, 23(4), 441–461. doi:10.1080/1357650x.2017.1377726
Wood, L. C., & Cooper, D. S. (1992). Autoimmune thyroid disease, left-handedness, and developmental dyslexia. Psychoneuroendocrinology, 17(1), 95–99. doi:10.1016/0306-4530(92)90080-q
Zucker, K. J., Beaulieu, N., Bradley, S. J., Grimshaw, G. M., & Wilcox, A. (2001). Handedness in Boys with Gender Identity Disorder. Journal of Child Psychology and Psychiatry, 42(6), 767–776. doi:10.1111/1469-7610.00773
I'll occasionally listen to Ray's interviews, but that's the extent of it. I always figured eyelid twitchings were a magnesium/thyroid deficiency, but there may be something more to it!
Fascinating! I’m a highly ambidextrous lefty, and had/have a very stressed mother. I had multiple surgeries before I was learning handedness. Ears and hernia and endless antibiotics until the end of highschool. I shoot a gun with my right draw and paint left, guitar right… prune roses either hand. Drums took me forever to figure out which hand was dominant and every time I’d play a drum set I would switch the high hat and snare to the other side of the kick drum. I always kicked a ball left foot. But skateboard right foot back, so “normal” not “goofy footed.”
To me I’ve come to think of it as power vs control. My right hand and leg is stronger definitely. But my left hand and leg I express better and have finer motor skills with.
Side note… do you watch youtube podcast interviews etc? I have increasingly noticed that peoples left eye is squinting, half opened and blink a lot more often then their right eye! It is crazy when you see it. It is very often when a podcast host tries to start reading something while they’re on camera. They go right eye dominant and their left eye just shuts off! Also when someone is more alarmed or stressed for sure. I’ve even seen it in an tv show when an actress got really pissed off in a scene with a fight. She gets all intense and mad and does a full on blink with just her left eye! It was crazy! Not a wink but a blink. Fatigue stress… we start down regulating parts of our bodies for sure. But we also squeeze and intensify the areas or muscles that get us through the “stressor” and in turn maybe we are weakening those muscles too? Lactate?
Thanks!