In almost every culture, on every continent, and in every historical period examined, most humans prefer to use their right hand. They write with it, eat with it, hold tools with it, and perform delicate tasks with it. Now a new study suggests that the explanation is not found only in the classroom, at home, or in tradition, but in two dramatic developments that made us human: The transition to walking upright and the significant growth of the brain.
For years, researchers tried to understand why humanity is so asymmetrical. Among monkeys and apes, one can find individuals who prefer the right hand or the left hand, but at the population level, there is usually no overwhelming majority in one direction. In humans, on the other hand, the picture is almost uniform: About 90% of the population use primarily their right hand, and only about 10% are left-handed. This is one of the strongest and strangest behavioral biases in nature.
The new study, led by researchers from the University of Oxford and the University of Reading in the UK, attempted to test several of the central hypotheses proposed over the years. Did right-handedness emerge due to tool use? Is it related to diet, habitat, body structure, walking upright, or brain size? To test this, the researchers conducted a broad analysis of data on more than 2,000 individuals from 41 primate species, including monkeys, apes, and humans.
But the important stage arrived when two variables were added to the model: Brain size and the ratio between the length of the forelimbs and hindlimbs. This ratio serves as an indirect measure of the animal's mode of locomotion. In species that move primarily on all fours or climb, the ratio between the arms and legs is different from that of creatures that walk upright. When the researchers included these two factors, the anomaly of humans almost disappeared. The meaning: It is possible that these two characteristics – a large brain and walking on two legs – explain a large part of the way the human preference for the right hand was formed.
According to the explanation proposed in the study, the story likely unfolded in two stages. In the first stage, our ancient ancestors began to walk on two legs. Walking upright freed the hands from their central role in locomotion, and allowed them to engage in other tasks: Carrying, holding, food processing, caring for offspring, and later also tool use. Once the hands became independent work tools, a ground for specialization was created: One hand could become more precise, fast, and skilled.
In the second stage, the brain entered the picture. As the brain of human ancestors grew, a specialization developed within it between the two hemispheres, the two halves of the brain. In most humans, the left hemisphere is highly involved in language, planning, and the control of complex movements. That same hemisphere also controls the right side of the body, and therefore also the right hand. The researchers estimate that the development of this brain specialization may have gradually reinforced the right-hand preference at the population level.
The study also attempted to project the model onto ancient human relatives. According to the predictions, early species like Australopithecus afarensis probably displayed only a weak preference for the right. In later species within the Homo genus, including Homo ergaster and Homo erectus, the preference for the right had already strengthened. Neanderthals, our evolutionary relatives, are also expected according to the model to be predominantly right-handed, though slightly less so than modern humans.
An interesting anomaly is Homo floresiensis, the dwarf human species from Indonesia nicknamed "the Hobbits." According to the model, its preference for one of the hands was much weaker, similar to contemporary chimpanzees. The possible explanation is that this species had a relatively small brain, and it likely retained more significant climbing abilities than other species of ancient humans. This datum aligns with the idea that the combination of a large brain and walking upright is what turned human right-handedness into such a powerful phenomenon.
However, the researchers emphasize that this is an evolutionary model and not direct proof that every right-handed individual is so because their ancestors walked upright. Hand preference is also influenced by genetics, embryonic development, environment, learning, culture, and habits. Previous studies have shown that a preference for a certain side can appear already in very early stages of pregnancy, even before the baby is born. On the other hand, human culture certainly contributed to reinforcing the preference for the right, especially in a world where writing, tools, weapons, eating utensils, and devices have been designed for generations for a right-handed majority.
The question that remains open is why left-handed people still exist. If a preference for the right conferred a distinct evolutionary advantage, one could expect that left-handedness would disappear almost completely. But it is maintained at a relatively stable rate of about 10% of the population. One of the hypotheses is that left-handedness may have advantages in certain situations, for example in combat, sports, or tasks where the rarity of the left hand surprises the opponent. Other hypotheses point to complex genetics or to the advantages of variation within a population.
The social significance of left-handedness has also changed over the years. In the past, left-handed children were sometimes forced to write with their right hand, and left-handedness was perceived in certain cultures as an anomaly or undesirable. Today, it is clear that this is natural variation – not a disorder and not a sign of a developmental problem. The new study does not suggest that left-handed people are different in terms of brain quality or its size, but rather attempts to explain why at the level of the entire human species, such a large majority for the right was created.