Why are 90% humans right-handed? Scientists finally find a fascinating answer

Pick up the object in front of you. Which hand did you use? Right hand? Throw a ball, scroll through your phone, or stir your morning coffee, chances are you instinctively use your right hand. In fact, nearly 90% of humans across cultures are right-handed, making it one of the strongest behavioural patterns seen in our species. Scientists have been puzzled for decades, wondering why humans have developed this overwhelming preference for right-handedness. However, scientists may finally have an answer.

A major new study suggests the answer may lie in two dramatic evolutionary turning points that shaped human history itself. A new study led by researchers at the University of Oxford suggests that upright walking and the rapid expansion of the human brain may explain why humans became overwhelmingly right-handed over time.
The findings were published in PLOS Biology after researchers conducted one of the largest comparative analyses on primate handedness to date.

Data analysis

To investigate the mystery, scientists analysed behavioural data from 2,025 monkeys and apes representing 41 different primate species. The team used Bayesian modelling techniques that also considered how closely species are evolutionarily related to one another.

Rather than focusing only on genetics or brain structure, the researchers explored several possible explanations behind handedness. Their analysis examined factors such as diet, habitat, body size, social behaviour, movement patterns, tool use, and brain size. Initially, humans appeared to stand completely apart from every other primate species in terms of right-hand dominance. No other primate showed such a strong population-level preference for one hand.

Two important biological traits

However, that changed once researchers introduced two important biological traits into their models. The first was brain size. The second was the ratio between arm length and leg length, which scientists commonly use as a marker for bipedal movement or upright walking. Once those factors were included, humans no longer appeared to be such an evolutionary anomaly.

What did the research find?

According to the researchers, this suggests that the combination of walking upright and developing larger, more complex brains may have gradually pushed humans toward stronger hand specialisation, eventually leading to the extreme right-hand dominance seen today.

The study also offered insights into extinct human ancestors and how handedness may have evolved over millions of years. Researchers estimated that early hominins such as Ardipithecus and Australopithecus probably displayed only mild right-hand preferences, similar to what modern great apes show today.

Significant change in right-hand preference

But things appear to change significantly with the rise of the genus Homo. Species such as Homo erectus, Homo ergaster, and Neanderthal likely developed increasingly stronger right-hand preferences over time. The researchers believe these points point toward a two-stage evolutionary process.

In the first stage, walking upright freed the hands from their earlier role in locomotion. Once the hands were no longer constantly needed for movement, humans may have faced new evolutionary pressures favouring more specialised and asymmetric hand use.

In the second stage, the rapid expansion and increasing complexity of the human brain may have strengthened this preference even further, gradually creating the strong right-hand dominance that now defines modern humans.

The findings are especially fascinating because handedness has remained one of the most difficult human traits to fully explain. Scientists have long studied its links to genetics, language development, brain lateralisation, and evolution, but no single theory has fully answered why humans became so overwhelmingly right-handed compared to every other primate species.

This latest research now suggests that the answer may not come from one isolated factor, but from a combination of major evolutionary changes that transformed how early humans moved, used their hands, and developed cognitively over millions of years.