A recent publication reviewed the current status of knowledge about the origins of handedness.¹ Handedness is defined as the consistent use of one hand, either the right or the left, to perform one-handed activities. Many consider handwriting to be the signature handedness task with 85-90% of people using the right hand and the remaining 10-15% using the left. This report reviewed papers appearing in the scholarly literature over a fifty-year period. The authors asked what if any of the traditional approaches to the study of handedness have current scientific credibility. Here are several of the areas considered in this article.

Genetics: It is a common belief that handedness has a strong genetic origin. Left-handers in particular point to the presence of left-handed relatives as evidence for the idea that left-handedness runs in families. Many studies of family handedness patterns have pursued the idea that handedness is caused by a single gene residing somewhere on one of the twenty-three human chromosome pairs. The current emphasis rejects a simple single-gene approach. Researchers now claim that as many as forty genes on different chromosomes are involved in handedness development. Researchers also concede that epigenetics can play a role. Epigenetic influences, external to the genome, work through chemical tags that turn gene expression on or off but do not alter the gene itself. Recent studies have identified several genetic sites on specific chromosomes that influence handedness development.

Brain differences: There are anatomical differences in the dominant hemisphere controlling the movements of the preferred hand (the left hemisphere for right-handers and the right hemisphere for left-handers) when compared to the non-dominant hemisphere. However, these difference have been found in research using exclusively adult participants. It is unclear if these nerve fiber asymmetries in the dominant hemisphere are the result of years of one-handed use or the original cause of the preference for one hand over the other.

Fetal environment: Two theories of left-handedness center on the fetal environment. One theory claims that a difficult birth results in oxygen deprivation that deferentially affects left hemisphere development. This effect shifts handedness movement control to the right hemisphere and the left hand. The second theory argues that high levels of fetal testosterone impair the development of the left hemisphere. Right hemisphere growth is favored along with the formation of left-handedness. There is little evidence to support either of these theories. There is more evidence supporting fetal thumb sucking preferences as a predictor of handedness side later in life. Fetal sucking of the right thumb is associated with right-handedness in children. Fetal left-thumb sucking is not predictive of later handedness.

Cultural influence: Ample research evidence indicates that handedness side, particularly left-handedness, is affected by the society in which a person lives. The use of the left hand for specific activities is taboo in certain cultures and left-hand writing is often targeted for change. For this reason, rates of left-handedness vary across the world. Cultures that discourage left-hand use have relatively few left-handers while more tolerant societies support the presence of more left-handers. The world is arranged to accommodate the right-handed majority. This also pressures left-handers to develop right-handed proficiency in order to adapt.

What has fifty years of research revealed about the origins of human handedness? First, researchers agree that multiple genes on different chromosomes are involved in handedness development. Second, studies of children as well as adults are needed to unravel the mystery of hemispheric neural asymmetries. Do these brain differences cause handedness side or are they result of the consistent use of one hand over time? Third, fetal behaviors are more predictive of later handedness than are factors related to the uterine environment or birth process. Fourth, cultural pressures can alter handedness behaviors. However, there has never been a left-handed majority society so most likely biological factors play an important role in determining a right-handed human majority.

¹Marcori, A.J., & Okazaki, V.H.A. (2020). A historical, systematic review of handedness origins. Laterality: Asymmetries of Brain, Behavior, and Cognition, 25, 87-108. DOI: 10.1080/1357650X.2019.1614597