5 Left and Right Brain Differences That Might Surprise You

This article is an excerpt from the Shortform book guide to "The Master and His Emissary" by Iain McGilchrist. Shortform has the world's best summaries and analyses of books you should be reading.

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What’s the difference between the left and right brain? Is one hemisphere more important than the other?

In his 2009 book, The Master and His Emissary, Iain McGilchrist cites an array of scientific evidence intended to dispel the myth that the right hemisphere is “minor.” He contends that the right brain is actually dominant over the left brain and explains five significant differences between the hemispheres.

Keep reading to learn about five left and right brain differences that speak to McGilchrist’s intriguing argument.

Left and Right Brain Differences

We’ll examine five of the left and right brain differences that McGilchrist discusses—differences in understanding meaning, perceiving objects, grasping coherent wholes versus individual parts, processing emotions, and thinking intuitively. These brain hemisphere differences illustrate one of McGilchrist’s key claims: The myth of left-hemisphere superiority is misguided, as the right hemisphere is responsible for several of the brain’s most essential functions.

Difference #1: Implicit vs. Explicit Meaning

Despite the popular caricature of the right hemisphere as “silent,” McGilchrist contends that it plays a crucial role in understanding language. By examining the effects of injuries to the right and left hemispheres, he argues that, though the left hemisphere grasps formal linguistic rules, only the right hemisphere grasps the implicit meaning that language conveys.

He notes that, when people suffer from right hemisphere damage (and therefore rely on their left hemisphere), they often speak sentences that are syntactically and grammatically flawless but nonsensical. In a similar vein, children that suffer right hemisphere injuries struggle to understand entire sentences, even when they know each of the individual words.

McGilchrist concedes that the left hemisphere can understand denotative meaning. For this reason, the left hemisphere has a much larger vocabulary than the right hemisphere. But he reiterates that only the right hemisphere is able to understand the meaning of those terms in context.

Likewise, McGilchrist asserts that only the right hemisphere can grasp metaphors because metaphors don’t depend on the denotative meaning of words, but rather on their connotative meaning. The upshot, according to McGilchrist, is that the right hemisphere is crucial for understanding the world because we can’t understand the world without metaphors. After all, many aspects of life—such as beauty, love, and pain—can’t be described through denotative language alone. To understand and describe such phenomena, McGilchrist suggests we need to use metaphor.

Difference #2: Abstract vs. Contextual Perception

The right hemisphere’s ability to understand meaning in context hints at another difference between the hemispheres: Only the right hemisphere perceives objects in context. Specifically, McGilchrist argues that the right hemisphere sees objects within broader surroundings, while the left hemisphere sees objects abstracted from those surroundings.

Individual Objects vs. Categorizations

Because the right hemisphere prefers to examine objects in context, while the left hemisphere prefers abstractions of concrete objects, a related difference arises: The right hemisphere thinks in terms of individual objects, while the left hemisphere thinks in terms of broader categories.

Difference #3: Wholes vs. Individual Parts

In addition to perceiving objects in context as opposed to abstracting them, another perceptual difference arises with respect to parts and wholes. According to McGilchrist, because of its preference for abstraction, the left hemisphere breaks objects into their constituent parts while the right hemisphere, with its preference for context, focuses on the whole picture that these parts compose. 

To show as much, McGilchrist cites drawings from patients with hemisphere damage. Those with right hemisphere damage were unable to draw coherent wholes; when asked to draw a person, for instance, they couldn’t accurately place the body parts to create a unified human figure. Those with left hemisphere damage, by contrast, could draw coherent wholes, but with a lack of detail in individual parts; when asked to draw a tree, for instance, such individuals often drew an outline of a tree that lacked details on individual branches or leaves. 

Similarly, patients with damage to either hemisphere had differing abilities to recognize parts and wholes. For instance, McGilchrist cites a patient with right hemisphere damage who could only recognize a house in a picture by recognizing its chimney (a part) and then inferring that it must be a house. 

Hemisphere Differences in Philosophical Debates on Wholes vs. Parts

It’s possible that the hemisphere differences McGilchrist describes have influenced philosophical debates, such as those at the heart of mereologythe study of the relationship between parts and the wholes they compose.

On the one hand, some philosophers endorse mereological nihilism, the position that individual parts never make up a composite whole. Though arguments for mereological nihilism are many, one common argument contends that we should reject the existence of wholes because they would be causally redundant. For example, because we can (in theory) explain how a baseball shatters a window by appealing to the atoms that make up the baseball, we don’t need to posit that a baseball (the whole) exists; we can claim that only the atoms (the parts) exist. This radical position, which only recognizes parts and not wholes, might stem from an excess of the left hemisphere.

By contrast, others endorse mereological universalism, the position that any set of individual parts always makes up a composite whole. One common line of reasoning claims that any distinction between parts that do compose a whole and parts that don’t would be arbitrary; in turn, parts must always compose a whole. So, for example, there exists a composite whole made up of (say) your couch, television, and toilet. This view, it would seem, could stem from an excess of the right hemisphere, which focuses on wholes rather than parts.

Global Attention vs. Focused Attention

According to McGilchrist, the reason why the right hemisphere perceives wholes while the left hemisphere perceives parts has to do with hemisphere differences relating to attention. He argues that the right hemisphere is crucial for broader, global attention that grounds the left hemisphere’s narrower, selective attention. 

For example, McGilchrist notes that patients with left hemisphere injuries lose much of their capacity for focused attention—our ability to hone our attention in on a specific stimulus, like a blade of grass. Conversely, those with right hemisphere injuries struggle with vigilance—our capacity for awareness of our entire surroundings that lets us detect possible threats.

(Shortform note: Plausibly, the neurological basis of Attention Deficit Hyperactivity Disorder (ADHD) is evidence against McGilchrist’s claim that the left hemisphere grounds our capacity for selective attention. Indeed, individuals with ADHD score consistently lower on tests measuring selective attention, even though ADHD is traditionally associated with a right-hemisphere deficit. Still, other neuroscientists have argued that the link between ADHD and right-hemisphere deficits is tenuous, weakening the case against McGilchrist’s claim.)

Difference #4: Emotional vs. Dispassionate

Having seen how the brain’s hemispheres differ in their perceptual abilities, we’ll now discuss their differences in expressing and perceiving emotions. According to McGilchrist, the right hemisphere is primarily responsible for processing and expressing emotions. To illustrate the right hemisphere’s role in processing emotions, McGilchrist examines the brain areas that are activated when we view art. For example, he notes that the left hemisphere is more activated when looking at abstract, emotionless paintings, while the right hemisphere is more activated by deeply emotive art.

Similarly, McGilchrist points out that feelings of depression and melancholy are associated with excess right hemisphere activation, suggesting the right hemisphere plays a crucial role in processing those emotions.

In addition to processing emotions, McGilchrist claims that the right hemisphere also helps us express emotions. Patients with right hemisphere damage, therefore, lose the ability to convey their emotions through facial expressions.

The Right Hemisphere’s Capacity for Empathy

Because of its superior ability to perceive emotions, McGilchrist asserts that only the right hemisphere has the capacity for empathy. Moreover, McGilchrist points out that patients who suffer from right hemisphere damage often lose the ability to empathize, while those with similar left hemisphere damage retain this ability.

Difference #5: Intuitive vs. Non-Intuitive Thinking

In addition to processing our emotions, McGilchrist suggests that the right hemisphere plays a key role in reasoning—contrary to the stereotype that reasoning belongs to the left hemisphere. In particular, he argues that the right hemisphere specializes in more implicit forms of reasoning, while the left hemisphere specializes in more explicit forms of reasoning. 

To show as much, McGilchrist first examines so-called “aha!” moments, in which we solve some problem while not concentrating on it. These “aha!” moments, he notes, are associated with a sharp uptick in right hemisphere activity, suggesting it helps generate them. By contrast, McGilchrist asserts that the left hemisphere is better suited for explicit reasoning that involves formal application of logical rules.

5 Left and Right Brain Differences That Might Surprise You

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Like what you just read? Read the rest of the world's best book summary and analysis of Iain McGilchrist's "The Master and His Emissary" at Shortform.

Here's what you'll find in our full The Master and His Emissary summary:

  • How pop psychology has given us the wrong impressions of the brain's hemispheres
  • Why the right hemisphere is actually more important than the left
  • What would happen if left-hemisphere thinking took over the world

Elizabeth Whitworth

Elizabeth has a lifelong love of books. She devours nonfiction, especially in the areas of history, theology, and philosophy. A switch to audiobooks has kindled her enjoyment of well-narrated fiction, particularly Victorian and early 20th-century works. She appreciates idea-driven books—and a classic murder mystery now and then. Elizabeth has a blog and is writing a book about the beginning and the end of suffering.

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