The idea that the human brain is divided into a logical left half and a creative right half has become one of the most enduring metaphors in psychology and popular culture. While this simplification obscures the complexity of neural processing, it points to a genuine biological reality. The brain is structurally and functionally divided into two hemispheres, each specialized in subtle but important ways. Understanding why this split exists and why each hemisphere controls the opposite side of the body reveals much about human evolution, perception, and behavior (Corballis, 2014).

A large group of nerve fibers, known as the corpus callosum, runs across the two halves of the brain. The corpus callosum provides for ongoing communication between the two sides of the brain to allow for the unification between the two sides of perception, movement, emotion, and thought into one single experience. However, despite this linkage, it has been shown that each hemisphere does have distinct abilities. The ability to process language appears to be more localised predominantly to the left hemisphere of the human brain in the majority of individuals. At the same time, spatial awareness and specific aspects of emotion and music perception are more right-lateralized. These tendencies are statistical rather than absolute, but they raise a central question: why did the brain evolve this way at all? (Hugdahl, 2011)

Why the Brain Is Split Into Two Hemispheres

Humans are not the only vertebrates that have a division between their left and right hemispheres. Many vertebrates exhibit varying degrees of asymmetry or lateralization, indicating that lateralization provides an evolutionary advantage. One of the major advantages of lateralized brains seems to be increased efficiency. The brain is able to process information simultaneously and at the same time across both sides, which decreases the amount of time it takes to perform a task (reducing duplication) and allows for greater flexibility in responding to changes in the environment (Corballis, 2014).

Another benefit of specialization is the reduction of conflict. When two parts of the brain are trying to control the same behaviour at the same time (as in the case with bilateral brain integration), there may be interference. Specialization reduces or eliminates this interference by determining which hemisphere has a dominant function. For example, if one hemisphere becomes better at producing a fine motor sequence than the other, while also being responsible for monitoring location, the organism will have greater certainty in its decisions and actions due to the lack of internal competition. Therefore, over many generations of evolution, efficiencies gained through specialization have likely promoted increased separation in the types of functions performed by each hemisphere.

In addition to the evolutionary perspective, developing a brain creates an environment that furthers specialization. As the brain matures, many neural circuits intersect with each other frequently, which makes stronger connections. When a certain set of skills is predominantly organized in one hemisphere during developmental stages of the brain, a feedback loop is created. The many neurological, hormonal, and genetic factors that influence the functional development of the left hemisphere during infancy and childhood further strengthen the level of dominance. One specific skill that appears to be strongly associated with the left hemisphere of the brain is language acquisition (Hugdahl, 2011).

Why the Left Brain Controls the Right Side of the Body

One of the most striking features of brain organization is contralateral control. The left hemisphere controls the right side of the body, and the right hemisphere controls the left. This arrangement arises from the way nerve fibers cross over as they descend from the brain to the spinal cord. The majority of motor pathways cross at the level of the brainstem, resulting in opposite-side control.

The evolutionary roots of this crossover are not completely understood, although there are multiple theories to explain its occurrence. Some suggest that simple cross-wiring in the early nervous systems improved the sensorimotor coordination of an organism. An example of this is that visual input obtained from an area of the environment can be quickly connected to motor output on the opposite side of the body, allowing for the organism to make quick and adaptable movement. Other theories argue that the contralateral regions evolved as a limitation during development and would have remained if they were functionally adequate to be continued through natural selection.

From a functional perspective, contralateral control allows each hemisphere to integrate sensory input and motor output from corresponding halves of the perceptual world. Information from the right visual field is processed primarily by the left hemisphere, which then controls the right hand that may act on objects in that field. This tight coupling between perception and action likely enhances precision and speed (Sperry, 1968).

What Modern Neuroscience Really Says About Left and Right Brains

People create popular ideas about hemispheric differences by creating the myth of "left-brain or right-brain" personality types. Modern neuroscience essentially eliminates this belief by showing that nearly all cognitive functions rely on multiple networks that are located across the two hemispheres (which includes many to most of the tasks). In addition, these cognitive networks are communicating continuously across the corpus callosum (the area connecting the two hemispheres) (Corballis, 2014).

Neuroscience Essential Reads

Research on split-brain patients has provided many of the best examples of evidence for the types of specialization on each side of the brain (hemispheric specialization). Studies of split-brain patients show distinct processing styles when their two hemispheres cannot communicate. The left hemisphere is known for creating and explaining ideas in words, and the right hemisphere is generally better at creating visual and holistic connections. On the other hand, in normally functioning brains (intact brains), both hemispheres work together to create one coherent thought stream that is difficult to separate. Therefore, it is often difficult to tell that the two hemispheres play different roles in the processing of information within the brain (Gazzaniga, 2005).

Neuroscientists currently view lateralization of function as a spectrum from less to more lateralized, as well as context- and other variable-dependent. Lateralization is not simply an either-or function; instead, each hemisphere is viewed as a specialized partner, rather than a competitor. Communication between the two hemispheres increases the efficiency and flexibility of thought processing while maintaining the overall integrity of awareness through numerous neural connections (Sperry, 1968; Gazzaniga, 2005).