Ambidexterity is a rare human trait where an individual possesses the ability to use both the right and left hands with equal skill for complex tasks, such as writing or drawing. This is distinct from mixed-handedness, or cross-dominance, which describes a preference for using different hands for different activities, like writing with the right hand but throwing a ball with the left. While roughly 10% of the population is left-handed and 90% is right-handed, true ambidexterity is estimated to occur in only about 1% of people. The underlying causes of this unusual trait are sought in the complex interplay between brain structure, genetic programming, and early development.
The Role of Cerebral Lateralization
The neurological basis for ambidexterity lies in the concept of cerebral lateralization, which is the specialization of the brain’s hemispheres for different functions. In most people, the left hemisphere controls speech and language, as well as the motor control for the right side of the body, making them right-handed. Ambidextrous individuals often show a less defined or more bilateral distribution of these functions across both hemispheres compared to strongly right- or left-handed people.
This reduced specialization means that language and motor control are not as strongly dominated by one side of the brain. The corpus callosum, a thick bundle of nerve fibers connecting the two hemispheres, plays a role in this balance. Studies suggest that individuals with reduced functional lateralization, such as ambidextrous people, may exhibit a larger corpus callosum, which could facilitate communication between the hemispheres. Alternatively, conditions like agenesis of the corpus callosum, where the structure is partially or completely absent, have been linked to a greater predisposition for ambidexterity, suggesting that less inter-hemispheric communication may also result in a lack of hand dominance.
The motor cortex in each hemisphere controls the opposite side of the body, so having equal dexterity in both hands suggests that the motor control areas in both the left and right hemispheres are equally developed for fine motor skills. This symmetrical control is the functional hallmark of natural ambidexterity.
Genetic and Developmental Influences
Handedness, including ambidexterity, exhibits a complex pattern of inheritance, with heritability estimates suggesting a genetic influence of approximately 24%. No single gene determines handedness, but researchers have identified certain gene candidates that contribute to the variability in brain asymmetry. The gene LRRTM1 (Leucine-rich repeat transmembrane neuronal 1) was the first gene associated with human handedness, specifically with mixed- or left-handedness.
The LRRTM1 gene is maternally suppressed, meaning its influence stems only from the paternally inherited copy. This gene is expressed in specific forebrain structures during development and may affect neuronal differentiation and connectivity, potentially influencing the establishment of brain asymmetry. The genetic predisposition for a less-lateralized brain state is what sets the stage for ambidexterity to develop.
Early developmental factors also play a part in determining handedness. Prenatal influences, such as hormonal exposure, specifically higher levels of testosterone, have been hypothesized to affect brain development and lateralization. Additionally, the position of the fetus in the womb has been proposed as a possible environmental influence that could affect hand preference. These factors interact with the genetic blueprint to determine the final pattern of brain organization, resulting in the symmetrical motor control seen in ambidexterity.
Distinguishing Natural Versus Trained Ambidexterity
A significant distinction exists between true, natural ambidexterity and learned, or trained, mixed handedness. True ambidexterity is a rare neurological state where an individual has been equally skilled with both hands since childhood, reflecting a lack of inherent brain dominance for motor control. This type of ambidexterity is characterized by equal performance ability with both hands across virtually all skilled tasks.
In contrast, many individuals who identify as ambidextrous have developed their non-dominant hand through training or necessity, which is more accurately classified as mixed-handedness. For example, many left-handed people learn to use their right hand for tasks like using scissors or a can opener because these tools are designed for right-handed users. This learned skill is a behavioral adaptation and does not necessarily reflect the symmetrical brain organization of a naturally ambidextrous person.
Trained ambidexterity is often seen in athletes who practice switch-hitting or throwing with both arms. The defining feature remains the level of skill: a truly ambidextrous person can write or perform other complex motor skills equally well with either hand without extensive practice. The ability to use both hands proficiently is a valuable skill, but it does not equate to the innate, symmetrical motor control that defines the naturally ambidextrous brain.