The Mysteries of Handedness

Wikipedia defines handedness as "a human attribute defined by unequal distribution of fine motor skills between the left and right hands". Most people are either right- or left-handed. However, some have cross-dominance, which indicates a degree of versatility in each hand. Usually, a person favors one hand for certain activities and the other hand for others. Cross-dominance can also refer to mixed laterality, which refers to the side of the body favored.

Handedness fascinates me, largely because I'm cross-dominant. I write, throw, and paint my nails with my right hand, and, generally speaking, am right-handed. However, I am left-eye dominant, and use a riding crop and perform juggling tricks with my left hand. I kick equally well with both feet, but prefer being on the left side of the field (or anywhere else).

Types of Handedness

• right-handedness, the most common type (studies suggest 70-90% of the world population are right-handed)
• left-handedness, less common (about 10% of people)
• mixed-handedness/cross-dominance, difficult to say how common. Many people identify handedness by their writing hand, and therefore may not count themselves as mixed-handed; also, many left-handed people become mixed-handed by virtue of necessity in order to function in a right-handed world.
• ambidexterity, naturally exceptionally rare, though it can be learned. However, those who learn ambidexterity still tend to prefer their dominant hand.

Ambidexterity is perhaps the most famous type of cross-dominance. About 1% of people are ambidextrous. It's more common to find ambidextrous people who started off being left-handed, but learned to use their right hand equally well, either deliberately or as a result of societal pressure in schools and workplaces. Since many everyday items (can openers, scissors) are designed for right-handed people, left-handed people learn to use them right-handedly, and thus develop motor skills in their non-dominant hand much more than right-handed people, who are rarely forced to use their non-dominant hand. However, certain activities, such as typing, knitting, juggling, and percussion can help develop ambidexterity, as they require motor skills in both hands.

Theories of Handedness

The most common theory is division of labor, which states that since both motor control of speaking and handiwork require fine motor skills, it'd be more efficient to have the same hemisphere of the brain do both. Since, in most people, the left brain controls speaking, right-handedness would be much more common. (The brain hemispheres control the opposite side of the body). Objections to this theory include that the opposite is not true for left-handed people: in over 50% of left-handed people, the left brain controls speaking, and 25% of left-handed people use both sides equally. Another objection is that there is no explanation for why either side would be more likely to perform a certain function, though that problem is part of a more general inquiry as to why functions and organs display laterality, or preference for one side of the body or other.

There are also theories that imply that left-handedness, or anything not right-handedness, is a result of some sort of problem suffered before or at birth. Some statistics support this theory--difficult or stressful births seem to happen more commonly among babies that grow up to be non-right-handed--but there is no practical explanation for why this should be.

The "twins theory" suggests that all left-handed people were originally part of a set of identical twins, whose right-handed twin failed to develop. Though the "vanishing twin" theory may seem far-fetched, it is true that twins have a high frequency of right-handedness/left-handedness in the pair.

A newer theory looks at handedness differently. It suggests that handedness is not a simple preference, because the hands actually work together to accomplish tasks. For example, with writing, while the dominant hand writes, the other hand orients and grips the paper, and provides the context from which the dominant hand operates. Therefore, both hands are equally important, but one is specialized for fine movements, and the other for broad movements.

Which One's Better?

Probably neither. Most advantages/disadvantages of either hand are largely societal/cultural constructs, and most of popular psychology's notions of a type of handedness being more artsy/logical/etc. is probably false.

How-ev-er, there are certain correlations that have proven difficult to debunk. Left-handed people are often said to be more intelligent, more eloquent, and have better problem-solving skills than right-handed people. This is often attributed to their "wider range of thinking" given by the fact that they have a fuller use of both sides of their brains. A disproportionately high number of Nobel Prize winners are left-handed, and, of the past seven Presidents, four (Obama, HW Bush, Ford, and Clinton) were left-handed, and one (Reagan) was ambidextrous. The last time the US Presidential Election featured no left-handed candidates from a major political party was 1972.

But don't despair, righties. Lefties smudge their writing when they write. (In left-to-right languages, anyway).