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Humans display bilateral symmetry, meaning that one side of the body or brain tends to dominate, whether it's the hand or foot, usually left or right. This phenomenon is referred to as laterality, which also encompasses lateralization—the specialization of function in either the right or left hemispheres of the brain. There are numerous examples of laterality in human biology, and this list aims to highlight some of them (and more).
10. Handedness
Handedness is one of the most recognized forms of laterality, so we'll begin with it. It refers to the hand that an individual prefers to use, regardless of its efficiency, or the hand that performs faster or more accurately in manual tasks. Approximately 88.2% of people are right-handed, while 8-15% are left-handed. The remainder may be ambilevous (showing no dominant hand) or ambidextrous. Research also suggests that hand preference can be observed in fetuses, with the hand most commonly held close to the mouth being the dominant one. There are also findings indicating that left-handed people may sometimes draw figures oriented to the right.
In 2007, researchers found that certain alleles of at least one of three single-nucleotide polymorphisms located upstream of the previously identified LRRTM1 gene were associated with left-handedness.
9. Footedness
Footedness, commonly observed in board sports, refers to the preference for positioning one's left or right foot forward on the board. It also correlates with the foot most likely to be used when taking the first step. This suggests a dominant foot, with the majority of individuals having a dominant right foot. If you've ever played football (or soccer, depending on where you're from), you're likely familiar with the challenge of using a less skilled foot or leg, highlighting the importance of becoming proficient with both feet.
Surveys conducted in Brazil have shown a decline in the prevalence of left-footedness as people age, which might be related to the right hemisphere of the brain aging faster than the left hemisphere. (Note: Some argue that the survey on which this hypothesis is based is flawed.)
8. Ocular Dominance
People usually have one dominant eye from which they prefer to receive visual input, a phenomenon called ocular dominance. This is somewhat similar to the laterality observed with the right or left hand, though the dominant eye and dominant hand do not always match. This happens because both brain hemispheres control both eyes, each overseeing a different half of the visual field, thus each eye corresponds to a different half of both retinas. Approximately two-thirds of people are right-eye dominant, with one-third being left-eye dominant, although in some cases, neither eye shows dominance. In normal binocular vision, parallax comes into play, so the dominant eye is relied upon more for accurate positional information. However, this can vary depending on where the person's gaze is directed. You can determine your dominant eye by using a test such as this one.
7. Facial Asymmetry
Look at your reflection in the mirror. Focus on the way your lips curve into a smile or smirk, and see if you can identify which side seems more expressive. Facedness, or Facial Asymmetry, refers to the degree to which emotions or information are conveyed by the right or left side of the face, known as hemifaces. Much like the eyes, each hemisphere of the brain controls a specific part of the hemiface. Research suggests that the left side of the face is more likely to express emotion, is less restrained, and shows culture-specific signals, such as winking or raising an eyebrow. These are all signs of Facial Asymmetry. The right side of the face, on the other hand, is primarily associated with conveying verbal information rather than emotions.
6. Aural Asymmetry
Although it won't tell you which ear you hold your phone with, aural asymmetry reveals which hemisphere of the brain is more responsive to different types of stimuli. Studies have shown that when speech-like sounds are produced, the right ear tends to respond more strongly, while the left ear is more sensitive to musical tones. To clarify, the study involved speech-like clicks and continuous tones. Additionally, ear preference is heavily influenced by environmental factors, such as the direction from which the sound originates, making 'earedness' a less pronounced form of laterality.
5. Ambidexterity
Ambidexterity refers to the ability to skillfully use both left and right limbs, such as hands, with equal proficiency. This condition is one of the most well-known forms of cross-dominance. True natural ambidexterity is rare; instead, the level of skill with each hand is typically what determines a person's degree of ambidexterity. Among left-handed individuals, it is becoming more common to find people who have developed ambidextrous abilities, having learned to use their non-dominant hand for tools designed primarily for the right-handed.
4. Dextrocardia
Dextrocardia is a congenital condition where the heart is located on the right side of the body, instead of the usual left side. There are two forms: dextrocardia of embryonic arrest (DoEA) and dextrocardia situs inversus (DSI). In DoEA, the heart is simply positioned farther to the right in the thorax than normal, typically accompanied by severe heart defects. In DSI, the heart is a mirror image of its normal position on the right side of the body. While individuals with DSI generally do not experience additional medical issues from the condition, they may be more susceptible to problems with the bowel, esophagus, bronchial system, and heart.
3. Limb Dominance
Laterality, often referred to as limb dominance, is especially evident in animals. Many examples exist of limb dominance among animals, such as parrots preferring one foot when holding an object, or frogs and rabbits showing limb preference in their forelimbs and hind limbs, observable through bone weight and stress patterns. Even fossilized remains of mastodons reveal varying tusk lengths, which point to the presence of laterality in these ancient creatures.
2. Viscera
The viscera refer to the collective internal organs of the human body, but they are still influenced by laterality. Organs such as the lungs, kidneys, and adrenal glands typically mirror each other, while others, like the liver, are located in specific areas, with the liver being on the right side, beneath the diaphragm. The normal anatomical arrangement of internal organs is called Situs Solitus.
Situs Ambiguus, also known as heterotaxy, is a rare congenital condition where the major internal organs are abnormally distributed within the chest and abdomen.
Situs Inversus, previously mentioned on Mytour in Another 10 Bizarre Medical Tales, is a congenital condition in which the body's major organs are either mirrored or reversed from their usual positions.
1. Brain Lateralization
The left hemisphere of the brain is responsible for analytical thinking, structure, rules, time management, logic, mathematics, planning, language, speech, and physical activities. It processes information for the right side of the body. In contrast, the right hemisphere is more focused on aesthetics, emotions, intuition, color perception, sensory input, relationships, awareness, rhythm, humor, and motor skills, particularly those related to the left side of the body. The left hemisphere tends to mature more quickly than the right. Note: For left-handed individuals, the functions of the hemispheres are reversed.
