Left Handed‏

New book explores the mysteries of southpaws

Article written by Cari Nierenberg

How do we explain that through the centuries, all over the world, there has always been a consistent left-handed minority of people of around 10 percent? Author Rik Smits attempts to answer this question in his new book "The Puzzle of Left-handedness."

There’s no definitive reason why one person is a rightie and another is a leftie, argues Smits in his book. There are several theories, though, and plenty of left-handed lore.

"We know for certain that genetics is involved in left-handedness, since it runs in families" says Smits, who is left-handed and a science writer in the Netherlands. A left-handed parent is twice as likely to have a left-handed child, and two left-handed parents are three to four times as likely to have a southpaw son or daughter.

Still genetics alone can't fully account for the constancy of the 1-in-10 distribution of lefties among the population. Another theory is that left-handedness can sometimes be a result of disturbed development in the womb or of brain damage (no matter how slight) before, during, or after birth. A third possibility is hormonal -- that lefties might be exposed to higher concentrations of testosterone while the brain is developing.

Smits presents an interesting theory of his own: Left-handedness is a side effect of identical twinning.

He explains his ideas this way: When the embryos split at an early stage in the pregnancy -- around the first week -- this division would result in identical twins. And twinning may give rise to minor mirror-imaging effects, including left-handedness. But Smits suggests that most embryo splits don't always result in two viable fetuses, and the process often goes wrong. He proposes that perhaps a left-handed fetus survives and is born while the "clandestine" twin, the rightie, is lost early in the pregnancy, before would-be parents know of its existence.

There are other intriguing links between twins and left-handedness. Left-handedness occurs roughly twice as often in twins -- both identical and fraternal sets. And in the majority of cases, left-handedness affects only one identical twin. Smits ideas have not been scientifically tested.

While the right-handed majority may consider lefties intriguing or peculiar, Smits argues that from an early age left-handed people always have to do something extra to figure out how to reverse the processes demonstrated to them -- whether it's handwriting, tying their shoelaces or a necktie, or slicing bread.

He also contends it's a myth that southpaws die nine years earlier than their right-handed counterparts, an idea first proposed in the early 1990s by psychologist Stanley Coren. Other researchers have since said that conclusions were based on flawed analysis and arguments.

Although left-handedness has been linked with everything from hay fever and alcoholism to criminality and mental retardation, Smits claims there's no good evidence to support these associations either.