Psychology of the Left Hemisphere: The Brain's Interpreter

The study of split-brain patients forces us rethink our own decision-making

Posted Aug 11, 2020

Where do our decisions come from? It's an odd question. Where else could they come from but from our own heads? If pressed to get more specific, we likely think of the "self" — a single decision-maker, residing in our heads, commanding our choices.

This makes intuitive sense, but how does this work when the mind is split in two?

The brain is a hyperconnected organ, and ordinarily, the brain's two hemispheres are in constant conversation, sharing information through the band of fibers known as the corpus callosum. In rare circumstances, however, this band of fibers connecting the hemispheres is surgically severed.

The surgery leaves the person with two distinct sides of the brain that are unable to communicate.

Observations of this clinical population, known as split-brain patients, provide a fascinating glimpse into our psychology. They force us to confront these intuitions about a unified sense of self and a centralized decision-maker. 

 Photo by Robina Weermeijer on Unsplash
Split-Brain patients provide a fascinating window into our decision-making.
Source: Photo by Robina Weermeijer on Unsplash

Unable to communicate, one hemisphere often takes action based on information that the other doesn't have access to. For example, you can selectively instruct the right hemisphere to get up and walk to the kitchen. But when you ask the person, "why did you get up?" only the language dominant left hemisphere has the linguistic capability to respond. 

The left hemisphere has no idea. But what's interesting is that the person never just says, "I don't know." Instead, without hesitation, it makes up a reason on the spot, "Oh, I just felt like stretching up my legs a bit, that's all," or "Oh, I wanted to look out the window." 

The "decision" is made independent of the left hemisphere's knowledge, and once it's acted upon, the left hemisphere interprets and explains away this action, as if it was its own. 

Choice, Decision-Making, and the Interpreter in All of Us

Michael Gazzaniga, perhaps the world's leading expert in split-brain patients, dubs this phenomenon, "The Interpreter." It's as if one entity makes a decision, and the other interprets and explains it. 

It's easy to see this behavior as nonsensical. However, "The Interpreter" isn't limited to split-brain patients. It's often true of us all. 

 Photo by Matias Dubini on Unsplash
The "science" of choosing stockings teaches us a lot about our decisions.
Source: Photo by Matias Dubini on Unsplash

Consider the following scenario. You're shown an array of stockings hanging on a wall, and asked to pick which ones you like the best. You scan the collection and make your selection. After, you're asked why you picked these particular stockings. You respond with whatever comes to mind: the color, the quality of the fabric, the design, or any such combination. 

This was the set-up for the famous experiment by Richard Nisbett and Timothy Wilson. They found that participants are actually much more likely to choose the stockings on the far right of the array. Even when stockings are identical, the ones on the right are chosen with alarming regularity. 

However, when asked why they made this decision, nobody ever says, "I just picked the ones on the right." Like the ignorant left hemisphere, we're blind to this factor. Instead, we make up an explanation to justify it after the fact. 

The Psychology of The Interpreter in Everyday Life

We interpret our behavior in a way that makes it seem like a logical decision all along, when in fact, our true motivations are unknown. As NYU psychologist Jonathan Haidt describes, "The conscious mind thinks it's the Oval Office, when in reality it's the press office." 

 Photo by Jon Tyson via UnSplash
We're faced with thousands of choices every day. Where does deciding end and interpreting begin?
Source: Photo by Jon Tyson via UnSplash

Parents will often enforce kids' bedtimes "for their own good," when a self-serving motive seems just as likely—that parents simply want an hour or two of peace and quiet without the kids. Of course, many parents genuinely believe that bedtimes are good for their children, but that belief is self-serving enough that we should be skeptical that it's the full story. 

How many more of our "choices" have unknown, ulterior motives, which we explain away once they are made? 


We like to think of ourselves as rational actors, making deliberative, well-thought-out decisions. But more often than we think, we don't utilize these rational abilities to make choices. Instead, we use them to rationalize the choice once it's made. 

To the question of where exactly our decisions come from, it may be impossible to answer truthfully. True motivations can be elusive, and our decision-making apparatus is opaque. But one thing is certain: The source of our decisions is a much less unified place than we assume. Even when we're convinced that we're deciding, we may just be interpreting. 

This post originally appeared on the Neuromarketing Blog PopNeuro.


Gazzaniga, M. (1998) The Split Brain Revisited, Scientific American, July 1998, 54.

Haidt, J. (2006). The Happiness Hypothesis: Finding Modern Truth in Ancient Wisdom, New York: Hachette Book Group

Nisbett, R. E., and Wilson, T. (1977). “Telling More Than We Can Know: Verbal Reports on Mental Processes.” Psychological Review 84 (3): 231–59.

Simler, K, and Hanson, R. (2018). The Elephant in the Brain: Hidden Motives in Everyday Life. 1st ed. New York: Oxford University Press.