Laterality refers to the functional asymmetry found in the human body and brain. It describes the consistent preference for using one side of the body or for processing certain information in one hemisphere of the brain over the other. This preference is a foundational characteristic of biological organization, resulting in a predictable division of labor between the two halves of the body and the brain.
Manifestations of Laterality
The most recognized expression of laterality is handedness, the consistent preference for using the right or left hand for tasks like writing or throwing. Handedness is the least symmetric motor preference, with approximately 87% of the population preferring the right hand. This asymmetry extends to other paired motor and sensory organs.
Footedness describes the preference for a foot when performing actions such as kicking a ball or stepping up a stair. About 20% of people prefer the left foot, making it less right-sided than handedness. Eyedness, or visual laterality, is determined by the dominant eye used for sighting or aiming, with about 31% of individuals preferring the left eye.
Earedness refers to the preference for one ear when listening to a faint sound or holding a phone to the head. This is the most evenly distributed preference, with nearly 44% of people showing a left-ear preference. These behavioral manifestations serve as external evidence of a deeper, underlying neurological organization.
Hemispheric Specialization
The neurological basis for laterality is hemispheric specialization, which dictates that the two halves of the cerebrum have distinct roles. The two hemispheres are separated by the median longitudinal fissure but are connected by the corpus callosum, a massive bundle of nerve fibers. This structure acts as the primary communication pathway, integrating and transferring sensory and motor information between the sides of the brain.
Control of the body follows the principle of contralateral organization: the left hemisphere controls movement and sensation on the right side of the body, and vice versa. For most people, the left hemisphere is the site for language function, including specialized areas for speech production and comprehension. This dominance also extends to analytical thought, logic, and sequential processing.
In contrast, the right hemisphere tends to specialize in nonverbal and spatial tasks. These functions include spatial awareness, the processing of emotional context in language, and the recognition of faces. However, the corpus callosum ensures that the hemispheres constantly communicate to coordinate complex actions and decisions.
The lateralization of function is not absolute, but rather a tendency that enhances cognitive efficiency through a division of labor. For instance, while the left hemisphere handles the grammar and lexicon of speech, the right hemisphere contributes to the interpretation of intonation and prosody. This functional distinction is a hallmark of human neuroanatomy.
Factors Influencing Development
The development of lateral preference is influenced by both genetic and environmental factors. Genetic influence is evident in family and twin studies, though it is not determined by a single gene. Research indicates that genetic components account for about 25% of the variance in handedness.
Molecular genetics studies suggest that laterality is influenced by numerous genes—around 48 for handedness—that are primarily involved in brain development, rather than limb structure. These genes likely establish the fundamental, basic lateralized organization during embryonic development.
Developmental timing also plays a role, with evidence suggesting laterality begins before birth. For example, fetal motor activity in the womb correlates with eventual handedness. After birth, environmental factors fine-tune the preference.
Cultural pressures and learning are important environmental influences, as repeated use of one hand for tasks like writing reinforces the preference. Furthermore, the process of functional specialization continues throughout childhood; for example, the dominance of the left hemisphere for language strengthens as a child ages, with the right hemisphere’s involvement decreasing over time. This interplay between inherited predispositions and external experience shapes the final pattern of laterality.
The Spectrum of Laterality
Laterality is not a binary choice but exists along a continuous spectrum. Most people have a strong right-sided preference across most tasks, but significant variations exist. One variation is mixed laterality, where an individual lacks a consistent preference across different body parts.
A person with mixed laterality might be right-handed but left-footed or left-eyed, using different sides for different tasks. This highlights that lateral preferences are not always aligned. Another variation is ambidexterity, which describes a person having equal skill and performance ability with both hands for a specific task.
True ambidexterity, where a person is equally fast and accurate with both hands, is extremely rare, estimated to occur in only about 0.1% of the population. Mixed-handedness, however, is more common, referring to an ambiguity in preference where different tasks are preferred by different hands. Research suggests that reduced cortical lateralization is associated with mixed-handedness, demonstrating that these variations reflect subtle differences in brain organization.