Photo by JSKruse

For the last few decades, the popular interpretation of research on human cerebral hemisphere lateralization has cast the left brain as Mr. Spock: logical, rational, adept at math and language, with Captain Kirk embodying the right brain: impulsive, intuitive, emotional, creative. You can take tests that purport to show which hemisphere dominates your thinking style.

Usually, they lionize the left hemisphere as the more reasonable half and the one that should be in charge. But a magisterial book by psychiatrist Iain McGilchrist has been substantially, if slowly, changing this perception, turning our older understanding of hemispheric differences on its head, so to speak.

McGilchrist taught literature at Oxford before attending medical school, becoming a psychiatrist, and launching a career in brain imaging research. In 2009 he published The Master and His Emissary, reshaping how we understand how the two hemispheres work together, with important implications not just for neuroscience, but for psychiatry, philosophy, history, and the comprehension of our current era as well.

McGilchrist presented his theory in exquisite detail (more than 450 pages) and with exhaustive references (at least 1870 footnotes) to back up his assertions. The following is my attempt to distill his ideas. Any faults with oversimplification or outright errors are mine.

Brain lateralization background

Roger Sperry won the Nobel Prize in 1981 for his work with “split brain” patients, individuals in whom the corpus callosum, the neuronal bundle connecting the two hemispheres, had been damaged, surgically severed, or failed to develop.

His work built on more than a century of lesion studies — examining the brains of people with focal damage to try to determine what essential functions had been impaired by the destruction of specific brain regions. Over the past forty years, Sperry’s students and colleagues, particularly psychologist Michael Gazzaniga, have advanced Sperry’s research and popularized the roles of the two hemispheres.

This body of research underpinned the pop psychology that designated certain functions to each hemisphere- language, logic, and linear thinking to the dominant left, and holistic, intuitive, impulsive reactions to the subservient right brain. The right brain was considered good at art and music and other subjects that American public education finds worthy of devoting a mere hour to each week if that.

The neuroscience in The Master and His Emissary

McGilchrist spends the first half of The Master and His Emissary summarizing not just human research, but looking at brain lateralization across the animal kingdom. Rather than just contemplating what each hemisphere does, and divvying up tasks assigned to one-half brain or the other, one of McGilchrist’s brilliant insights was that the hemispheres differ primarily in how they work, including how they apprehend the world, interact with each other, and create our understanding of life.

Designating different functions for each hemisphere was essentially an incomplete, left-brain way to think about the differences between the hemispheres.

After immersing himself in neuroscience for two decades, McGilchrist felt that the field had inadequately addressed two big questions. Why across species, is the brain a bilateral structure, with two very similar masses yoked in parallel where one would seemingly suffice? Nature didn’t go through the trouble to design and preserve such intricate biology in order to just have a spare. The very existence of mirror-image hemispheres implies some division of labor, some separate role for each half.

His second question was why, if the two hemispheres need to communicate to create a unitary experience of consciousness (or for some other reason) are most of the fibers in the corpus callosum carrying functionally inhibitory input to the other hemisphere?

The human corpus callosum carries neuronal projections that only reach about 2% of the neurons in the other hemisphere, but the majority of these are either directly inhibitory connections or activate inhibitory interneurons in the other hemisphere. Rather than sharing information, the corpus callosum primarily serves to silence parts of the contralateral hemisphere.

McGilchrist’s answer to these questions begins by focusing on focus. Animals face two tasks of utmost importance: they need to feed themselves, and they need to avoid becoming someone else’s food. These tasks require very different types of attention. Focusing on feeding requires a very narrow focus, usually just a few degrees of the visual field, to ascertain that one is identifying something familiar and edible, as well as grasping it and conveying it to the mouth.

Looking out for danger, in contrast, necessitates scanning the whole environment, and being attuned to what is new, different, and potentially threatening. One needs to understand the context and relationships between objects in the environment.

These two types of focus operate in mutually exclusive ways. Splitting attention between two or more tasks, or multitasking, involves, at a neuronal level, rapidly shuttling back and forth between the two tasks. Alternating between tasks results in slower completion times and increases the likelihood of missing information. Since procuring food and avoiding becoming prey to others are both essential for living, each of these tasks became the purview of separate hemispheres.

The left hemisphere evolved expertise in narrow focus, to identify and grasp food. The right hemisphere took the opposite tack, and specialized in scanning the whole sensory world for problems, to avoid being eaten or other disasters.

The right brain looks at the whole world, is attuned to context, and is concerned with what is possible, not just what is certain. It is immersed in the moment, engaged with what is living and animate, and attentive to relationships and connections. It holistically extracts implicit information about the world and what is important. The right brain also informs the left brain where to start looking.

The left brain focuses on what is safe, edible, and procurable. It experiences just a piece of the world at a time, and constructs, piecemeal, a map of that world and how to identify and grasp (physically and mentally) what it needs to survive. Its understanding is detached and abstract focused on certainty and utility. The left brain developed language and math as ways to codify, articulate, construct, and convey its comprehension of the world back to the right brain, and to other brains.

The right brain then accepts this processed information from the left brain and uses it to refine its own outlook on the world. While the left brain’s perception of the world is precise, useful, and clearly defined, it is less expansive, less inclusive, and less aware of relationships and contexts than the right brain. The right brain is able to accept contradictions as well as implicit information that the left just can’t even see.

The right brain’s job is to experience the world, the left to manipulate it. The right brain employs metaphors to create its understanding. The left views metaphors as imprecise, artsy, and even deceitful. The left brain uses algorithms to construct its pathway to truth and will reject observable “data” that conflict with its rules.

McGilchrist also makes clear that the two hemispheres share a lot of similarities, and that the human brain does possess significant redundancies. While language is often considered the ultimate achievement of the left hemisphere, the right brain can actually understand much that is spoken to it. So the left/right discrepancies are not absolutes, but rather subtle, pervasive tendencies. Nevertheless, the pervasive differences accrue to create hemispheres that operate in complex and significantly different ways.

Philosophy and history in The Master and His Emissary

In the second half of his treatise, McGilchrist addresses how the different modes of understanding the world and creating truth embraced by each hemisphere have influenced philosophy and history. The two hemispheres work best when cooperating, but with the right in charge, since only the right sees the whole picture.

But starting with the Renaissance, and greatly accelerated by the rise of capitalism and consumerism in the last two centuries, the left hemisphere has become increasingly dominant in Western societies. This imbalance has happened too quickly to reflect a structural evolutionary change in the brain, rather it arises from a cultural conquering.

For McGilchrist, the right hemisphere, rather than being the minor player, is the true master. It takes in the whole world of experience, apprehends it, and presents and delegates to the left what areas to focus on. The left dissects, rebuilds, understands and provides information back to the right. The left “makes a good servant, but a bad master”.

The two hemispheres complement each other. The right brain would compliment the right if it could speak but is mute, so this is moot. The left brain though, believes it sees clearly what is true. It disparages information that is inconsistent, contradictory, or that can’t be clearly stated or defined by rules and abstractions. It deems all that it misses — context, implicit information, holistic knowledge, embodied experience, and living relationships as non-essential, non-factual, and either erroneous or irrelevant. When the left brain dominates, important information and relationships regarding the world get discarded.

McGilchrist suggests at least five reasons for the recent subversion of the natural order of hemispheric dominance. The first is that the left brain’s mission statement is to grasp and manipulate (feed) the individual. Utilitarianism is seductive — it produces tangible results. Secondly, not only is the left brain grasping, it is more readily graspable.

The left brain simplifies life and features consistency and rules, which makes its stance easier to understand and appreciate than the nebulous, uncertain, and self-contradictions embraced by the right brain. Thirdly, the left brain is much more able to express and articulate its perspective, and win arguments about its superiority, because the very process of arguing is a left brain strength.

The fourth reason for the left brain’s recent supremacy is that science, technology, and bureaucracy are all left-brain productions and have powerfully shaped our world, particularly over the last 2–3 generations. So even if the right brain tries to immerse itself in the real world, (rather than just relying on a left brain map of the world), our physical world itself is increasingly a constructed, fabricated, left brain environment.

Finally, the left brain’s failure to be aware of what it is missing gives it an advantage in vying for dominance. It simply demeans, devalues, and ignores the parts of reality that it can’t grasp — none of that really matters. Someone who cuts off parts of the course wins the race. All five of these factors recursively strengthen left brain dominance at a social and individual level.

McGilchrist avoids becoming overtly political but makes a strong case that the inappropriate dominance of the left brain leads to many of the problems of the modern world. Aspects of global warming, racial and gender injustice, and political divisions arise from left-brain approaches that know the rules they have created for defining truth and blindly reject evidence from the lived world that might contradict their “knowledge”. Human lives themselves are diminished by rejecting and ignoring what we can’t fully comprehend or codify.

Throughout the book, McGilchrist refers to autism and schizophrenia as conditions in which the left brain inappropriately dominates. He sees in autism the emphasis on rules, the detachment from relationships, the emphasis on parts rather than the whole, and the inability to grasp context as a warning about the direction our left-brain dominated society is trending in.

Similarly, he interprets some of the bizarre delusions and hallucinations of schizophrenia as an unfettered left brain running amok — the left brain trying to make sense out of an isolated visual input or auditory sensation, and concocting stories based on the rules about the world it knows while ignoring how poorly that narrative fits with the actual world.

McGilchrist doesn’t have much to say about ADHD, which is rather interesting, given his prioritization of attention as foundational to cerebral hemispheric differentiation. Many features of ADHD would suggest a right brain bias — being drawn to what is new, and readily bored with what is old; challenges in extracting rules or learning from experience; a desire to engage and embed in situations, rather than just observing; and strong tendencies for non-linear thinking.

However, at least one ADHD tendency — missing social context because of hyper-focusing on a particular detail, would suggest an overemphasis on left-brain thinking. Maybe this speaks to ADHD being fundamentally a problem with regulating attention, and not being a “deficit” in attention.

McGilchrist himself would be among the first to point out that the right brain — left brain roles that he has identified don’t explain everything about the human cortex. A lot is going on within each hemisphere, and some deviations from normal might reflect communication differences between the two hemispheres rather than just differences in dominance.

A theory can be right even if some things get left out.