Across the world, more people are right-handed than left-handed
Across all cultures and all countries of the world, many more people are right-handed than left-handed. There is not a single country in the world in which more people are left-handed than right-handed. On average, about 90% of people are right-handed. While non-human animals often also show handedness (or maybe rather “pawedness”), their rates of right-sided preferences are typically much lower than 90%. Why humans show such a strong preference for the right hand all over the planet that is not found in any non-human animal remains an enduring scientific mystery.
A new study on the evolution of handedness compared humans to non-human primates
While many studies on the evolution of handedness have tried to solve this question by just looking at psychological research on handedness in humans, a recent study published in the scientific journal PLoS Biology took a different approach (Püschel and co-workers, 2026).
In the study, the research team led by scientist Thomas A. Püschel from the University of Oxford, used a so-called comparative approach, in which handedness data from human volunteers were compared to handedness data from 41 different ape species. Overall, data from more than 2000 individuals across all species were included in the study.
While handedness in humans is often assessed using questionnaires, this approach can, of course, not be used in animals. Therefore, the scientists analyzed data from the so-called tube task. In the tube task, peanut butter or another type of food is placed into a rigid plastic tube. The animal (or human) must stabilize the tube with one hand and then use the fingers of the other hand to get the food out of the tube. Typically, a right-handed ape or human would use the right hand to retrieve the food and the left to stabilize the tube. Based on the result of the tube task from many previous studies in apes and humans, the scientists conducted a so-called Bayesian phylogenetic comparative meta-analysis, a highly advanced statistical method, to assess patterns of handedness across all species included in the dataset. They assessed two main handedness variables:
- Handedness direction: Indicates whether a species on average shows a left- or right-sided preference, or no preference at all.
- Handedness strength: Indicates how strong average hand preference is within a species, but ignores the direction of the preference in individuals
To assess which factors have driven the evolution of handedness, the scientists also included information on several different variables that could potentially affect handedness based on scientific theories on the evolution of handedness.
These included:
- Body mass
- Brain size
- Percentage of fruits in the diet
- Percentage of fruits and animals in the diet
- Intermembral index: the ratio between the length of the forelimbs (like arms) and the hindlimbs (like legs). Bipedal organisms that walk upright on two feet (like humans) typically have longer legs than arms.
- Body mass sexual dimorphism: Differences in body mass between male and female animals
- Tool use
- Substrate preference: information about whether the species lives in the trees or on the ground
- Social system: information about whether the species is solitary or lives in pairs or groups
- Intra-sexual competition level: Information on whether the species shows competition between male individuals with each other or female individuals with each other
- Extractive foraging: Information on whether the species forages difficult-to-access foods, such as termites in mounds
- Social learning: Information about whether individuals of the species learn skills from each other
Results of the study: Human handedness is an evolutionary outlier
For handedness direction, there was no statistically significant preference to one side across all investigated species. As was to be expected, the strongest preference of all was observed in humans, who, in contrast to most other species, showed a clear rightward preference. A few ape species, like chimpanzees and gorillas, also showed weak right-sided preferences, but not as strongly as in humans.
Interestingly, some species, like the brown spider monkey, showed weak leftward preferences. Thus, humans were a clear evolutionary outlier. When the scientists included the factors that could potentially affect the evolution of handedness, an interesting pattern emerged. Two factors were especially relevant: Brain size and walking on two feet. For the strength of handedness, humans showed one of the strongest preferences of all species that were included in the study. Again, brain size and walking on two feet showed up as key factors for the evolution of human handedness.
Take-Away
The results of the study showed that two factors that distinguish humans from apes are likely drivers for the evolution of handedness: The bigger brains of humans and the fact that we walk on two feet.
Why are these factors relevant for handedness?
Handedness is a form of brain asymmetry, e.g., left-right differences between the two halves of the brain. It has been argued that brain size is a crucial factor for the emergence of brain asymmetries. The bigger a brain is, the longer it takes to exchange information between the two halves of the brain. Therefore, an animal with a bigger brain benefits more from strong hemispheric asymmetries. If a brain function can be conducted by networks in one hemisphere due to strong hemispheric asymmetries, even a big-brained organism can react fast to the environment, since not much time-consuming transfer of information is needed.
This explains why humans, as the species with the biggest brains in the study, also have the strongest hand preference. In addition, walking on two and not on four feet may be a basic prerequisite to develop strong handedness, as it allows for sophisticated tool, given that the hands are not needed for movement.
