Why 90% of humans are right-handed: New study points to walking upright

A staggering 90% of the human population favors their right hand, a trait so dominant it cuts across all cultures and historical eras. No other primate species displays such a lopsided preference. For years, scientists have wrestled with explanations rooted in brain structure, genetics, or culture, but the puzzle remained unsolved.

A fresh study from the University of Oxford now proposes that the answer lies in two monumental evolutionary shifts: the move to walking on two legs and the dramatic expansion of the human brain. The researchers argue that strong right-handedness emerged gradually through these changes, not from a single genetic mutation.

“This is the first study to test several of the major hypotheses for human handedness in a single framework,” said Thomas A. Püschel, a study author and anthropology professor at Oxford.

An evolutionary outlier no more

The team analyzed handedness data from 2,025 individuals across 41 monkey and ape species. Using Bayesian evolutionary models, they tested explanations including diet, tool use, social structure, habitat, body size, and brain size. Handedness was measured using the Mean Handedness Index (MHI), where positive numbers indicate a right-hand preference.

Humans scored a 0.76 on the MHI, a figure dramatically higher than other primates, which clustered near zero. “Humans display a pronounced right-handed bias (MHI = 0.76), which contrasts sharply with the phylogenetic prediction of the reduced model excluding humans (MHI = 0.0),” the authors noted. Humans were the only species with a strong, statistically credible rightward bias.

However, the picture changed when researchers added two variables: brain size and the ‘intermembral index,’ which compares arm length to leg length. Humans have unusually long legs compared to arms, a hallmark of bipedalism. Once these factors were included, humans no longer looked evolutionarily exceptional.

Bipedalism freed the hands

The researchers propose a two-stage explanation. First came bipedalism. As early human ancestors began walking upright, their hands were freed from locomotion. This likely created new evolutionary pressure for specialized hand use during tasks like carrying objects, manipulating tools, or gesturing. The study found that locomotion strongly influences handedness patterns across primates.

Tree-dwelling species, for instance, often show stronger hand preferences because moving through branches requires precise movements. Humans may have taken that pattern in a different direction, with upright walking allowing increasingly specialized use of one hand over the other.

Brain expansion strengthened the bias

Using evolutionary models, the researchers estimated handedness in extinct human relatives. Early hominins like Ardipithecus and Australopithecus likely had weak right-hand preferences similar to modern apes. The bias strengthened in species such as Homo erectus and Neanderthals before reaching its modern extreme in Homo sapiens.

“It is with the emergence of the genus Homo, and particularly the onset of significant encephalization, that we observe a marked increase in MHI values,” the authors added. One unusual exception was Homo floresiensis, the small-brained ‘hobbit’ species, where models predicted much weaker handedness, possibly because it retained adaptations for both climbing and upright walking.

Unanswered questions and future directions

The study suggests human right-handedness is deeply rooted in the same evolutionary changes that transformed how our ancestors moved and interacted with the world. Yet important questions remain. Scientists still do not know why left-handedness survived throughout human evolution, or how much human culture helped strengthen right-hand dominance over time.

“Humans are unique in displaying cumulative cultural evolution, which may amplify or stabilize behavioral asymmetries,” the study authors noted. The researchers also suggest future studies could investigate whether limb preferences in animals like parrots or kangaroos evolved through similar pressures, potentially revealing that handedness-like behavior emerged independently across different branches of the animal kingdom.

The study is published in the journal PLOS Biology.