Humans Are the Only Mammals That Favor One Hand Nine to One
Researchers estimate the strong preference for one hand arose with upright walking and big brains, with the tilt to the right coming after our genus, Homo, appeared.
Quanta Magazine · Why Am I Left-Handed?People favor the right hand by about nine to one, a population-wide tilt that researchers have not found in other mammals. The detailed comparison comes from primates, where many individuals prefer one hand but left preferring and right preferring animals roughly balance out within each species. Only humans lean so hard in one direction.
In an April 2026 analysis, Chris Venditti, an evolutionary biologist at the University of Reading, with Thomas Püschel and Rachel Hurwitz of the University of Oxford, concluded that the strong preference for one hand started about seven million years back, when ancestors became upright walkers with bigger brains. Their idea is that standing freed the hands for specialized work, and that it was more efficient for a growing brain to put fine motor control mainly in one hand.
The authors estimate that the added tilt toward the right came still later, after the genus Homo emerged 2.8 million years ago.
Questions people ask
Do chimpanzees and other primates show any handedness at all?
Many individual primates do have a preferred hand for tasks like reaching into a tube for food, and the preference tends to be stronger in bigger-brained species. What they lack is a shared direction. Within most species, left-preferring and right-preferring animals roughly balance out.
Why would walking upright encourage using one hand?
On four limbs, hands and paws are busy with locomotion and must stay fairly symmetrical. Once early hominins stood up, the hands were free for carrying, making tools, and precise manipulation. Specializing the brain and practice on one hand could then make delicate work faster and more efficient.