Why are some people left-handed?

Some things really are a mystery, and that's surely the case with left-handedness. Numerous theories abound but there are no definite answers.

Psychologist Stanley Coren from the University of British Columbia reports some research shows a link between a mother's age when she gives birth and left-handedness in her child. A mother over 40 is 128% more likely to have a left-handed baby than is a woman in her twenties. Other research shows a link to brain trauma either in utero or during birth. According to Coren, this is supported by the fact that among people who are mentally retarded, left-handedness is 2 ½ times more common.

Genetics might be a factor, as indicated by research analyzing parents' and children's hand preferences. There's also the anecdotal evidence of the famous Kerr Clan of Scotland, who adapted their castles to accommodate the extremely high number of lefties in the family. Nevertheless, more recent scientific research among people with the surname didn't report an increased incidence of left-handedness.

Then there's the study connecting brain lateralization and hand preference. Researchers have theorized that hand preference is the opposite to whichever side of the brain ones' speech and communication specialization is based in. For most people, communication is centered in the left brain, and the majority of people are right-handed. Certainly there are exceptions to this theory, but it continues to be studied, including by a researcher named Martha K. Holder at Indiana University.

As for sword fighting -- a man named Thomas Carlyle theorized that early combatants during the Bronze Age who held their swords in the right hand and shields in the left hand (to cover their heart) were more likely to survive. But sheer brawn or luck was the more likely culprit, especially since ancient artwork and archaeology shows that a dominance of right-handedness existed prior to the Bronze Age.

So for now, the ultimate reason why some of us are left-handed remains a mystery. (Lefties probably wonder what all the hubbub is about anyway.)

Resources:

Lefty enthusiast Lorin Elias collects wonderful links to lefty Web sites. Find things like shops that cater to lefties and famous left-handers.

Read excerpts from Stanley Coren's book The Left-Hander Syndrome.

Indiana University researcher Martha K. Holder's Web site explores the brain lateralization theory. Her explanation appears in the Scientific American "Ask the Expert" column.