Why most humans favour their right hand, according to a landmark new Oxford study

Regardless of cultural background or geographic origin, approximately 90 percent of people across the globe are right-handed. Scientists have long sought to understand why — and a new study may finally offer the most comprehensive answer yet.

According to the research, the preference traces back to our distant hominin ancestors. The key drivers appear to be bipedalism — the ability to walk on two legs — and the development of larger brains, which together appear to have cemented the right hand as the dominant one for most of humanity.

Crucially, this tendency is not exclusive to modern humans. The research team estimated that Neanderthals were also predominantly right-handed, and found that the further a species sits from Homo sapiens on the evolutionary family tree, the weaker this hand preference generally becomes.

First study to test multiple hypotheses together

"This is the first study to test several of the major hypotheses for human handedness in a single framework," said Thomas Püschel, evolutionary anthropologist at Oxford University in the UK.

"By looking across many primate species, we can begin to understand which aspects of handedness are ancient and shared, and which are uniquely human."

Prior research has established that left- or right-handedness is largely determined by genetics, with foetuses demonstrating a preference as early as eight weeks in the womb.

However, the roots of this bias extend far deeper into our evolutionary history. Archaeological evidence, first reported in 2016, indicates that hominins were favouring their right hands as far back as 1.8 million years ago.

A meta-analysis across 41 primate species

For the new study, scientists from Oxford and the University of Reading in the UK set out to investigate how, when and why this preference first emerged.

Among the hypotheses previously put forward: that right-handedness arose when our ancestors began using tools, descended from the trees, adopted upright walking, or underwent biological changes such as increases in body mass or brain size.

To explore these possibilities, the team conducted a meta-analysis drawing on data from more than 2,000 individuals across 41 species of monkeys and apes, including humans.

These data were then assessed using models that account for the evolutionary relationships between species, with researchers looking specifically for signs of a bias towards one hand and the strength of that preference.

Most species showed little evidence of any such bias. Humans, however, stood out as a clear anomaly, displaying a strong and consistent preference for the right hand.

The East Javan langur (Trachypithecus auratus) was the only species to show a stronger rightward bias. Intriguingly, orangutans and snub-nosed monkeys displayed a slight preference for the left hand.

Brain size and limb length as key factors

When the researchers examined which hypothesised factors were most strongly associated with handedness, they identified brain size and the relative length of arms and legs as the most significant variables.

Using this model, the team then extrapolated their findings to extinct human relatives — including Neanderthals — to determine whether those species may also have exhibited a hand preference.

A compelling evolutionary pattern emerged. Earlier ancestors, such as Australopithecus afarensis, showed only a slight rightward preference.

As the Homo genus emerged, however, that bias intensified. H. ergaster and H. erectus showed progressively stronger rightward preferences, and Neanderthals — our closest evolutionary cousins — displayed an even more pronounced bias, second only to modern humans.

The hobbit exception

One lesser-known relative may serve as the exception that reinforces the rule. H. floresiensis — the so-called "hobbits" of Indonesia — exhibited only a very slight preference for either hand, roughly on a par with present-day chimpanzees (Pan troglodytes).

Researchers suggest this may support their hypothesis, given that these hominins had comparatively small brains and had not entirely abandoned tree-climbing behaviour.

A two-stage process

Taken together, the findings point to a two-stage process through which strong hand preference developed in humans.

In the first stage, the shift to upright walking freed the upper limbs from locomotion, allowing hands to evolve into the refined instruments of fine motor control that humans rely on today.

Other animals exhibit preferences for one side of their bodies over the other, and research indicates that those which do tend to perform better at survival-related tasks — suggesting early human ancestors may have gained a similar advantage.

The second stage relates to brain development. As the human brain grew larger and its two hemispheres became increasingly specialised for different functions, a right-hand preference appears to have become neurologically embedded.

"The initial locomotor change prompted by bipedalism can be seen as providing ecological and anatomical opportunities for manual specialization, while encephalization may have later reinforced and further canalized population-level patterns of lateralization," the researchers write.

"Furthermore, culture may have acted concurrently with or amplified the effects of this emerging trajectory of hominin right-handedness."

Questions that remain

The scientists acknowledge that open questions remain, including why left-handed individuals exist at all, and whether comparable evolutionary patterns can be observed in other animals with limb preferences, such as parrots and kangaroos.

The study was published in the journal PLOS Biology.