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Continued existence of left-handedness cannot be explained by combat theory

22 December 2011

Sara Schaafsma studied Papuans to discover why after years of evolution left-handedness can still occur despite being related to certain health problems. She concludes that the prevailing hypothesis that left-handedness continues to exist because it is advantageous in a fight is incorrect. Good health care seems to be a more likely reason for its continued existence. Schaafsma will defend her thesis on 6 January 2012 at the University of Groningen.

There are left-handed people in all societies but they are always in the minority. Furthermore, a link has been demonstrated between left-handedness and certain diseases. Sara Schaafsma therefore wondered why after years of evolution left-handedness still occurs in humans. The combat hypothesis is a possible explanation for it.

Combat hypothesis

According to the combat hypothesis, left-handedness still exists because left-handers have an evolutionary advantage in societies where man-to-man combat is still important in terms of status and survival. As there are more right-handed than left-handed people the right-hander lacks the experience to know how to fight a left-hander. His left-handed opponent therefore has the greatest chance of winning.
Winning decreases the likelihood of dying as a result of the fight and yields status, which increases the overall likelihood of having children. As right-handedness and left-handedness is partly genetically determined, left-handedness, according to the combat hypothesis, continues to exist in these societies despite the disadvantage of its association with certain diseases.


Schaafsma researched the combat hypothesis in the unindustrialized Eipo community in the Indonesian province of Papua.  The Papuans do no yet have access to good contraceptives and only few modern medicines are available.  Their society is therefore one of the few in which this sort of research is still possible.


Schaafsma says, 'Until recently there was a lot of man-to-man combat in the EIPO community due to tribal wars and fights within the community. According to the combat theory we expected there to be more left-handers in this society than in an industrialized one. But actually there were relatively fewer.' Schaafsma believes that the lower percentage of left-handers than average makes the combat theory extremely unlikely as an evolutionary explanation for the occurence of left-handedness.

Health care

She believes that good health care is a more likely reason for the continued existence of left-handedness in humans. Until recently no modern health care was available in Papua and there was less left-handedness than average. Schaafsma wondered – partly due to the relationship between left-handedness and health problems – whether a lack of good health care could be linked to a low number of left-handers in a society.


She therefore compared the percentage of left-handers in twelve Western countries with public spending on health care. As the combat hypothesis was originally supported by demonstrating that more murders take place in a country with lots of left-handers, she also included the murder/manslaughter rates of each country.

Hypothesis supported

Schaafsma did indeed find a positive relationship between the number of left-handers in a country and the spending on health care but no relationship between the number of left-handers and the number of people killed with intent.
Schaafsma says, 'This result supports – at the expense of the combat hypothesis – the hypothesis that a low level of health care can lead to few left-handers in a society such as the Eipo community. It is unclear, however, what the precise role of health care is.'

Curriculum Vitae

Sara Schaafsma (Amsterdam, 1980) studied biology at the University of Amsterdam. She has conducted PhD research at the Centre for Behaviour and Neurosciences at the University of Groningen since October 2006. She will defend her thesis Hows and whys of left and right – Ontogeny of lateralization and its functional relevance on 6 January 2012. Her supervisor was Prof. A.A.G. (Ton) Groothuis. From 1 February Schaafsma will work as a postdoc at the Department of Neuroscience & Behavior of the Rockefeller University in New York.

Note to the editor

Contact: Sara Schaafsma, tel. 06-17648264 or via s.m.schaafsma

Last modified:13 March 2020 01.54 a.m.
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