People are left-brained (logical) or right-brained (creative).

Roger Sperry received the Nobel Prize in Physiology or Medicine in 1981, in part for his research on split-brain patients. Beginning in the early 1960s, Sperry and his colleagues at the California Institute of Technology had studied patients who had undergone corpus callosotomy, the surgical severing of the corpus callosum, the thick bundle of nerve fibers connecting the brain's two hemispheres, as a treatment for severe epilepsy. With the connection cut, the two hemispheres could be studied in relative isolation. The findings were striking. Language was processed predominantly by the left hemisphere. Spatial processing, the ability to arrange blocks and perceive geometric forms, was weighted toward the right.

Sperry was careful about what these findings meant. He described lateralization of specific cognitive functions, not lateralization of personality. But the translation from laboratory to culture was not careful.

By the 1980s, the split-brain research had been absorbed into a broader popular framework. Business consultants ran workshops on "right-brain creativity." Self-help books offered to unlock the underused right hemisphere. Teachers were told to balance left-brain analytical tasks with right-brain creative ones. The left-brain, right-brain formulation appeared in textbooks, management training manuals, and newspaper columns on cognitive styles. The framework's appeal was obvious: it gave a neurological vocabulary to the intuitive sense that some people were systematic and analytical while others were spontaneous and artistic. It seemed to explain real differences.

The problem was that the framework rested on a misreading of what split-brain research had actually shown. Split-brain patients were experimental subjects with a surgical procedure that no intact person had undergone. Their two hemispheres were literally disconnected. The behavior of their disconnected hemispheres said nothing straightforward about how intact brains divided cognitive labor, because intact brains do not divide it the same way. In a normal brain, the corpus callosum continuously integrates information across both hemispheres, and both hemispheres participate in nearly all cognitive tasks, participation that shifts in degree, not in kind.

In 2013, neuroscientist Jared Nielsen and colleagues at the University of Utah analyzed resting-state fMRI data from 1,011 individuals between the ages of 7 and 29, looking for evidence that people showed systematically stronger connectivity in one hemisphere's networks than the other. They found none. Lateralization of individual brain connections existed, some specific pathways leaned left or right, but there was no evidence that the overall strength of a person's left-brain network exceeded their right-brain network in any consistent way. The phenomenon the popular framework described, a person having a dominant hemisphere that shaped their personality, had no representation in the imaging data.

Specific lateralization of function, on the other hand, was real. Language is processed predominantly on the left in about 96 percent of right-handed people, a finding Sperry's work had helped establish. The left hemisphere is more involved in sequential processing; the right tends to handle more holistic spatial tasks. But these are tendencies of specific circuits, not the basis for classifying individuals as left-brained or right-brained personalities. The brain functions as an integrated organ; its hemispheres communicate constantly through a corpus callosum transmitting billions of signals per second.

The popular version endured in workplace seminars and classroom discussions well past the 2013 study. Part of the reason is that the underlying personality distinction the framework describes, systematic versus intuitive, analytical versus creative, captures something real about how people differ. What neuroscience corrected was not the observation that people differ, but the story about where those differences live in the brain.