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New book explores the mysteries of southpaws

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Former U.S. President Bill Clinton is among the nation's famous lefties.

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.

Southpaws rule! The most powerful left-handers

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 these 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.

The truth is we know little about why people prefer to use one hand over another, and it's unique to humans to have a large right-handed majority. In the animal world, there are roughly equal numbers who prefer their left paw to their right.

"Most left-handers are just left-handed, nothing more," Smits concludes.

 Here are some interesting facts from "The Puzzle of Left-handedness":

• Left-handedness is slightly more common in men than women.

• Left-handers appear to have an edge in sports where two opponents face each other, such as baseball, tennis, boxing, and fencing. This is probably because southpaws get more opportunities to hone their skills against righties when practicing.

• Five of the last seven American commanders in chief were left-handed. (Jimmy Carter and George W. Bush were not.)

• In the 1992 US presidential election, the sitting president (Bush the elder), and both his challengers -- Bill Clinton and Ross Perot -- were southpaws.