Historical evidence indicates that the human population percentages of 90% right-handers and 10% left-handers have existed for millennia. The fighting hypothesis is an influential theory attempting to explain the ongoing presence of a left-handed minority over thousands of years. The classic fighting hypothesis states that left-handers are a minority because their handedness type is riskier and associated with poorer health outcomes when compared to right-handers. Left-handers are less fit than right-handers. However, left-handedness persists and does not become extinct because its rarity provides an advantage in fighting situations. Left-handers have a survival edge of surprise when fighting right-handers because right-handers rarely fight against them. Historically, this benefit has maintained the left-handed minority within human populations.
A recent paper took issue with the assumptions of the classic fighting hypothesis and proposed modifications to the original theory. ¹ The authors reviewed the research literature on the association between left-handedness and health. The results of the literature survey concluded that left-handers are as healthy as right-handers. Given the equal health status of left- and right-handers, the authors turned their attention to combat where they differentiated two types of fighting situations.
Right-handers have the fighting advantage when personal combat involves sharp weapons and protection of the heart is vital for survival. Seventy-three percent (73%) of the heart is located on the left side of the chest and penetration wounds of the left chest are the most likely to be lethal. When a weapon is held in the right hand and pointed toward an opponent, the right side of the chest is exposed. The right-hand stance protects the heart by exposing the right side of the chest not the left. The photo above illustrates this right-handed fighting posture in a fencing competition.
Left-handers are favored to win hand-to-hand fights where vulnerability of the heart is not at stake. Left-handedness is not a simple reversal of right-handedness. Left-handers tend to be mixed in their hand use (can use both the right and left hands) when compared to the consistent sidedness of right-handers. This versatility along with their rarity in the population gives them an advantage in close combat situations as illustrated in the photo to the right.
The authors summarize their modified fighting hypothesis as follows. The population rates of left- and right-handedness are not based on health or fitness differences. Rather, early hominids fought with sharp weapons. This style of combat favored the survival of right-handers because the right-handed stance protects the heart when confronting an opponent with a sharp weapon. Modern humans rarely fight with sharp weapons, but they do engage in hand-to-hand combat. Here, left-handers have an edge based on their trait of mixed-handedness and the fact that right-handers lack experience in fighting against them.
Handedness researchers have puzzled over explanations for the strong skew in human population rates of left- and right-handedness (10% versus 90%) for many years. Other species show handedness behaviors, but even species that are close genetic relatives of humans, like chimpanzees, do not show the dramatic human population asymmetry toward the right side. The modified fighting hypothesis proposes a novel explanation for this issue. The authors argue that behaviors involving social interactions among members of a species contribute to population levels of handedness types. Tool and weapon uses are examples of such interactions.
¹ Larsson, M., Schepman, A., & Rodway, P. (2023). Why are most humans right-handed? The modified fighting hypothesis. Symmetry, 15, 940. https://doi.org/10.3390/sym15040940