Left Brain vs Right Brain: What the Evidence Actually Shows

The claim, and why it feels true

The popular idea is tidy. Left-brained people are logical, analytical, verbal and orderly. Right-brained people are creative, intuitive, visual and emotional. Schools sort children by it, corporate away-days sort executives by it, and free online quizzes have sorted millions of people by it.

It feels true because two real things sit underneath it. The brain does have two hemispheres. Some functions really are lateralised. The error is in the step between those facts and the conclusion — the assumption that because a function leans one way, a person can.

The study that tested it directly

In 2013 Jared Nielsen and colleagues at the University of Utah set out to test the hypothesis on its own terms. If some people are left-brained and others right-brained, then across a large sample there should be individuals whose left-hemisphere networks are globally more strongly connected, and others whose right-hemisphere networks are.

The left-lateralised hubs were largely language regions and parts of the default mode network. The right-lateralised hubs were largely attention-control networks. Both sets were present in the same brains. What did not appear, anywhere in a thousand people, was a person whose whole left hemisphere or whole right hemisphere ran hotter than the other.

This distinction is the whole article. It is also the thing that most coverage of this study loses. Nielsen did not find that the hemispheres are the same. He found that everyone has the same asymmetries.

What actually is lateralised

Quite a lot, and the details are more interesting than the myth.

Language

This is the strongest and best-established asymmetry. Language is left-dominant in roughly 95 to 99 percent of right-handed people. In left-handers it is still usually left-dominant, but far less reliably — about 70 percent.

Knecht and colleagues quantified the gradient in 2000 using functional transcranial Doppler ultrasound. Right-hemisphere language dominance rose from 4% in strong right-handers, to 15% in ambidextrous people, to 27% in strong left-handers. Handedness does not determine language dominance. It shifts the odds.

Attention and spatial processing

Networks controlling spatial attention lean right. This is why damage to the right parietal cortex can produce hemispatial neglect — patients who eat food from only one side of a plate — while equivalent left-sided damage rarely does.

What is not lateralised

Creativity. Logic. Analytical thinking. Intuition. Mathematics. Personality. None of these map onto a hemisphere. Corballis put the imaging evidence plainly: creative thought activates a widespread network, favouring neither hemisphere. Arithmetic recruits parietal regions bilaterally. There is no analytic hemisphere and no artistic one.

Where the myth came from — and what Sperry actually said

It came from real science, badly extrapolated.

In the 1960s Roger Sperry and Michael Gazzaniga studied patients whose corpus callosum — the great bundle of fibres joining the hemispheres — had been surgically severed to control intractable epilepsy. Testing each disconnected hemisphere separately revealed the left to be specialised for language and the right for various nonverbal and emotional functions. The work was extraordinary, and it won Sperry a share of the 1981 Nobel Prize in Physiology or Medicine.

The popular press arrived quickly. The New York Times Magazine ran “We Are Left-Brained or Right-Brained” in 1973. Time followed. By the 1980s the dichotomy had escaped into education, management training and self-help, where it has remained.

It is worth being precise about one thing, because it is almost always omitted: Sperry saw this coming and said so, in the Nobel lecture itself.

The left-right dichotomy in cognitive mode is an idea with which it is very easy to run wild.— Roger W. Sperry, Nobel Lecture, 8 December 1981

The myth is not a distortion the scientists failed to anticipate. It is a distortion the founding scientist explicitly warned against, in the most prominent venue available to him, and it spread anyway.

The evidence at the root of it is now contested

There is a further twist that almost no popular article covers.

The textbook account of split-brain patients says that each hemisphere becomes an independent conscious agent: the patient can respond to a stimulus in the left visual field only with the left hand, and to one in the right visual field with the right hand or by speaking. Two perceivers in one skull.

In 2017 Pinto and colleagues tested two patients with complete, radiologically confirmed transection of the corpus callosum across a wide range of tasks. They replicated the classic finding that stimuli cannot be compared across visual half-fields. But the patients showed full awareness of the presence of stimuli, and well above-chance recognition of their location, orientation and identity, anywhere in the visual field, regardless of which hand or mouth was used to respond.

What a left-brain/right-brain quiz is measuring

Nothing about your hemispheres. A questionnaire that asks whether you prefer lists or images, or whether you are more organised than spontaneous, is a short and unvalidated personality inventory. Its output is a label, and the label has no neuroanatomical referent.

That does not make the underlying differences imaginary. People genuinely do vary in how verbal or visual their thinking is, in how much structure they prefer, in tolerance for ambiguity. Those differences are real and measurable. They are simply not differences in which half of the brain is in charge, and describing them that way makes them harder to think about clearly rather than easier.

Myths and facts about brain lateralisation