THE FIGHTING HYPOTHESIS: STABILITY OF POLYMORPHISM IN HUMAN HANDEDNESS
Most people exhibit hand preference for their daily activities and object manipulations, but it has long been a mystery to scientists as to how and why human handedness exists. Studies from different areas of science, including neurobiology, psychology, developmental biology, anthropology, evolutionary biology, genetics, and other fields, have all come together to try and solve the puzzle behind human handedness. Yet, there is not a single model that is sufficient to provide an universally accepted explanation. Interestingly enough, human handedness has been reported to exist since the Upper Palaeolithic, for more than 10,000 years; however, the proportion of left-handers in the human population, approximately 10% with slight cultural variations, has not changed since that time (Faurie & Raymond 2004).
Furthermore, left-handedness has been shown to be a heritable trait (McManus 1991) that associates with different types of fitness costs, such as higher susceptibility to neural disorders, lower height, and shorter life expectancy (Gangestad and Yeo 1997). If this is the case, handedness meets all the requirements for natural selection to act on, and left-handedness should have been wiped out by natural selection a long time ago. Why is there stability in the frequency for left-handedness? What could be the underlying cause for the polymorphism in handedness existing today?
In 1996, Raymond et al provided a new perspective to explain the stability of left-handedness in humans, which they named “the fighting hypothesis”. In their paper, Frequency-dependent maintenance of left handedness in humans, the fighting hypothesis suggested that “left-handers have a frequency advantage when they engage in combat”, due to the fact that right-handers are much more numerous than left-handers; this implies that right-handers will be in a relatively unfamiliar situation when facing left-handed opponents as compared to left-handers who are “more accustomed to encountering other right-handers” (Raymond et al 1996). This is described as a frequency-dependent situation because the fewer left-handers in the population, the more advantage they would have when fighting with right-handed opponents.
To test this hypothesis, Raymond and his colleagues performed a series of surveys and data collections related to handedness on sporting students and sporting elite, as they reasoned that “sporting performance is likely to be a good indicator of fighting abilities” in today’s world. The functional handedness of their subjects was determined either from the hand that holds the sporting equipment such as a racquet (for sporting students and elite), or from the hand that writes or throws (for the general population). They were expecting to see a percentage difference of left-handers between individuals in interactive sports (those sports involving two or more opponents and reflecting some fighting abilities, for example fencing, soccer) and individuals in the general population or non-interactive sports (sports in which no direct opponent is involved, for example swimming, gymnastics). As it turned out, the global average proportion of left-handers in interactive sports was found to be 32%, which was significantly higher than the 11% for non-interactive sports. This observation suggests that left-handedness is advantageous at the international sporting levels, and is in consistence with the fighting hypothesis.
The stable polymorphism between left- and right-handedness, as suggested by the fighting hypothesis, might actually be a consequence of the equilibrium between the fitness costs and the fighting advantage associated with left-handedness (Raymond et al 1996). Without the deleterious effects of left-handedness, provided that there are no other selection pressures, the fighting advantage of left-handedness should allow a higher frequency of left-handers, possibly in a 1:1 ratio with the frequency of right-handers; in contrast, if the fighting advantage of left-handers does not exist, the fitness reducing aspects of left-handedness should have driven left-handers extinct under natural selection (Raymond et al 1996). If there happens to be an event that disrupts the polymorphic equilibrium between left- and right-handedness, the frequency-dependent nature of the fighting advantage of left-handers shall adjust the frequency of left-handers accordingly back to its equilibrium value, which is about 10%.
Historically, the advantage that left-handed fighters enjoyed could have led to fitness-related consequences, including their own survival, higher social rank or status, or higher reproductive success (Raymond et al 1996). As a result, the higher the level of violence, the greater fitness advantage left-handers would experience (Faurie & Raymond 2004). In 2004, Faurie and Raymond performed an experiment that further supports the fighting hypothesis, which studied the correlation between homicide rate of cross-cultural traditional societies and the frequency of left-handedness. It turned out that there is indeed a positive correlation between homicide rate and frequency of left-handedness, leading to their conclusion that left-handedness is maintained by a negative frequency-dependent selection mechanism, as predicted by the fighting hypothesis earlier. Their study also provided some explanations for the cross-cultural variability of the frequency of left-handers.
In addition to the implications of the fighting hypothesis on maintenance of left-handedness, Raymond and his colleagues also claimed that “the fighting hypothesis is consistent with the greater frequency of left-handed males than females”, as they reasoned that “male/male fights are far more frequent than other combinations”. In other words, the fighting advantage is more apparent in male fights than female fights, resulting in a higher frequency of left-handers in males than females. However, if the costs of being left-handers are equal in both sexes, the fact that the frequency of left-handed females (approximately 8%) is not much lower than that of left-handed males (approximately 10%) is rather peculiar (data adapted from Raymond et al 1996). The existence of this relatively high frequency of left-handed females cannot be attributable to the fighting hypothesis alone.
Therefore, there is still room for refinement of the fighting hypothesis regarding this issue. For instance, the authors could possibly look into whether there is any sex-specific genetic effect in left-handedness (Raymond et al. 1996). Finally, as stated at the end of Raymond et al’s paper, even though the fighting hypothesis provides a good explanation for the stability of left-handedness, it “does not formally exclude other alternative hypotheses”. The fighting hypothesis also does not eliminate the existence of other advantages associated with left-handedness.
References
1. Gangestad, S.W. and Yeo, R.A. Behavioral genetic variation, adaptation and maladaptation: an evolutionary perspective. Trends in Cognitive Sciences 1(3), 103-108 (1997).
2. Raymond, M., Pontier, D., Dufour,A., and Pape, M. Frequency-dependent maintenance of left-handedness in humans. Proceedings of the Royal Society of London B 263, 1627-1633 (1996).
3. Faurie, C. and Raymond, M. Handedness frequency over more than ten thousand years. Proceedings of the Royal Society of London B 271, S43-S45 (2004).
4. Faurie, C. and Raymond, M. Handedness, homicide and negative frequency-dependent selection. Proceedings of the Royal Society B 272, 25-28 (2005).
5. McManus, I.C. The inheritance of left-handedness. Ciba Foundation Symposium 162, 251-267; discussion by Bock, G.R. & Marsh, J., 267-281 (1991).