The left brain is logical, analytical and verbal. The right brain is creative, intuitive and holistic. You are one or the other, and it explains why you are good at spreadsheets or good at painting.
This is probably the most successful piece of neuroscience folklore ever produced, and it is built on a genuine Nobel-winning discovery that says something quite different.
What the split-brain work actually showed
Roger Sperry won the 1981 Nobel Prize in Physiology or Medicine for work with patients whose corpus callosum — the bundle of fibres connecting the hemispheres — had been surgically severed to control intractable epilepsy. With the hemispheres disconnected, Sperry and his colleagues could present information to one side at a time and observe what each could do alone.
The findings were extraordinary and they hold up. Language production was overwhelmingly a left-hemisphere function in these patients; certain spatial and configurational tasks were handled better by the right. The hemispheres processed information differently, and in the disconnected brain those differences became visible in a way they never are otherwise.
Sperry saw the popularisation coming. In his Nobel lecture he explicitly cautioned against the emerging tendency to divide people into left-brained and right-brained types, noting that the experimental situation — a surgically disconnected brain, information restricted to one visual field — bore no resemblance to how an intact brain operates. The whole point of the corpus callosum is that in everyone else, the two sides are in constant communication.
The warning was published in 1981. It did not travel.
The study that looked for left-brained people
In 2013, Jared Nielsen and colleagues at the University of Utah published a paper in PLOS ONE that tested the popular claim directly rather than the scientific one.
They analysed resting-state functional MRI from 1,011 people aged 7 to 29, dividing the brain into more than 7,000 regions and mapping the connectivity of each. The question was not whether functions are lateralised — that was already established — but whether individuals differ in global hemispheric dominance. Does a given person’s left hemisphere network run more strongly, overall, than their right?
They found strong, consistent lateralisation of particular networks — language on the left, attention networks on the right — exactly as expected. What they did not find was any evidence that individuals possess a global bias. People did not sort into left-dominant and right-dominant groups. The connectivity strength of one hemisphere’s networks did not predict the other’s in the way the popular claim requires.
The hemispheres are specialised. The people are not.
How the myth was built
The route from Sperry’s laboratory to the personality quiz is traceable and instructive.
A rare surgical population was treated as representative. Split-brain patients had severe epilepsy and a severed corpus callosum. Their brains were, by design, doing something no intact brain does.
A tendency became a dichotomy. “Language production is predominantly left-lateralised in most right-handed people” is a statistical statement with meaningful exceptions. “The left brain does language” is a rule. The second is much easier to remember.
Anatomical location became personality. This is the leap with no support at all. Even granting that analytic tasks engage certain left-hemisphere regions more, nothing follows about whether a person is an analytic type. Region-level activation does not scale up to disposition.
The 1970s wanted it. The myth arrived in a cultural moment receptive to the idea that schooling suppressed a creative faculty and that the remedy was to unlock it. That framing sold books, workshops and curricula, and it has never stopped selling them.
What lateralisation really looks like
The real picture is more interesting than the folklore and less useful for sorting people.
Language is left-lateralised in roughly 95% of right-handed people, but in a meaningful minority of left-handers it is right-lateralised or bilateral. Lateralisation varies with the task, develops with age, and shifts after injury — children who lose a hemisphere early can relocate language function to the other side, which is not something a fixed dominance model accommodates.
Almost any real cognitive act recruits both hemispheres. Understanding a sentence involves left-lateralised syntactic processing and right-lateralised prosody and pragmatics, operating together. Reading a face, doing arithmetic, playing an instrument — all bilateral, all constantly exchanging information through the corpus callosum at a rate the split-brain experiments interrupted precisely in order to observe.
Why it matters a little
It would be easy to file this as harmless. Mostly it is. But the myth is the load-bearing premise for a category of educational and corporate products — “right-brain training,” hemisphere-based teaching programmes, personality instruments sold to employers — that involve real money and, occasionally, real decisions about people.
It also does the thing that bad folk-neuroscience always does: it converts a difficulty into an identity. A student who has decided they are right-brained has acquired an explanation for why mathematics is not for them, backed by what sounds like biology.
The part Sperry got right twice
Sperry’s discovery was that the two hemispheres, when separated, reveal genuinely different competencies. His warning was that this tells you almost nothing about intact people, because intact people do not have separated hemispheres.
Both statements were in the Nobel lecture. Only one of them became famous.
Sources
- Nielsen JA, Zielinski BA, Ferguson MA, Lainhart JE, Anderson JS. An Evaluation of the Left-Brain vs. Right-Brain Hypothesis with Resting State Functional Connectivity Magnetic Resonance Imaging. PLOS ONE 2013;8(8):e71275.
- Sperry RW. Some Effects of Disconnecting the Cerebral Hemispheres. Nobel Lecture, 8 December 1981.
- Corballis MC. Left Brain, Right Brain: Facts and Fantasies. PLOS Biology 2014;12(1):e1001767.
