If you guessed that these celebrities are all left-handed, then you guessed correctly.

According to M. K. Holder, an anthropologist at Indiana University, somewhere between 70% and 95% of the world's 6 billion people are right-handed, and about 5% to 30% are left-handed. A certain percentage is probably ambidextrous (can use both hands). In an article in Scientific American, Holder stated that these percentages "appear to be universally true for all human populations anywhere in the world."

• Using the ranges Holder gives, what number of people in the world are left-handed?
• Survey your class to determine what percentage is left-handed. How does the percentage compare with the general percentages given here?

Disagreement reigns in the scientific community about a standard definition of what is left- or right-handed. While we may classify ourselves as left- or right-handed based on which hand we write with, the actual definition is much more complex. Scientists agree that showing a preference for a particular hand goes beyond which hand is used for writing, painting, or pushing the buttons on a telephone.

"Handedness" research focuses on the issue of "sides" of the body and a person's tendency to use one hand over another for a variety of tasks. The research examines other behaviors, such as which eye is used to look through a camera eyepiece, which hand is used to open a door, or which foot is used to kick a ball.

Dr. Amar J. S. Klar, a geneticist at the National Cancer Institute researching handedness, defines individuals as right-handed if they perform the following activities with their right hand only: throw a ball, bowl, shoot marbles, cut with a knife, use a hammer, saw, use a spoon, write, cut with scissors, and sew. Doing any of these things with the left hand or either hand makes one a "non right-hander."

However, after studying handedness for over 160 years, the scientific community still cannot give an absolute reason why humans are biased toward right-handedness. Tests such as the Edinburgh Handedness Inventory can help researchers gain more information. Participants are asked to indicate which hand they prefer to use to perform 10 simple tasks such as writing, using a spoon, using a toothbrush, and opening a box lid. The results range from 20 (completely right-handed) to -20 (completely left-handed).

• Take the Edinburgh Handedness Inventory. The results are immediate.

Dr. Holder is conducting her own research into human handedness. Over five years, thousands of people responded to her online questionnaire, and she is now analyzing the data gathered. You can still read about her study and respond to the questionnaire for future research. The questions focus on hand, eye, ear, or foot preference for a variety of everyday tasks.

• Complete the Hand Preference Questionnaire for yourself.

Klar found that when one or both parents are left-handed, the odds are that 20% of their children will be left-handed. (For example, if a couple has five children, chances are that one will be left-handed.) Two right-handed parents produce left-handed children about 8% of the time.

Based on these statistics, Klar believes that a "single functional gene makes us right-handed." There is more to Klar's theory. Since "nearly 50% of the children of two left-handers are right-handed," he says, then there must be some "non-functional allele" that causes this "fifty-fifty split." (An allele is an alternative form of a gene for a specific trait.) To prove the existence of the gene, Klar and his team are making a genetic map by collecting blood samples from families that have met certain criteria.

In contrast to Klar's theory is one proposed by psychologist Dr. Stanley Coren of the University of British Columbia in Vancouver, Canada. Coren is the author of The Left-Hander Syndrome. He hypothesizes that all people are naturally right-handed, but due to a type of brain trauma that occurs in the womb or at birth, some people become left-handed.

The Geschwind-Behan-Galaburda (GBG) Theory of Left-Handedness, first proposed in 1987, also downplays the role of genetics. The GBG Theory proposes that certain chemical variations and hormonal changes in the womb at the time the brain is developing may lead to left-handedness. The effects of these changes may help explain why dyslexia, immune disorders, and language disorders are most prevalent in left-handers.

Still other researchers believe that handedness is a learned behavior. Because the majority of individuals are right-handed, most children develop the "habit" to conform to the right-handed world at a very early age.

• Come up with your own theory of what causes left-handedness.

Imagine a well-oiled, no-nonsense, efficient machine. That's the left hemisphere. Recognition of words, letters, numbers, logic, and reason reside here. The right hemisphere is more artistic and abstract. Understanding the expression of emotion, appreciating music and art, and seeing the "big" picture are all right hemisphere functions. Very simply stated, the left hemisphere is more rational and logical, while the right one is more intuitive and artistic.

In right-handers, language development is controlled by the brain's left hemisphere, which also controls the more dominant right hand. In about 30% of left-handers, language function is centered in the right hemisphere, which also controls the left hand.

Could this be the reason behind the creative genius of Paul McCartney, Beethoven, Jay Leno, Phil Collins, Julia Roberts, Lisa Kudrow, Keanu Reeves, Annie Lenox, Isaac Newton, Sarah Jessica Parker, Michelangelo, and Bill Gates?

If you're right-handed, try using the object with your left hand. If you're left-handed and don't have any trouble using these objects, then chances are good you've adapted to using the more readily available right-handed one.

• scissors
• camera
• three-ring or spiral-bound binder
• measuring cup
• manual can opener
• computer keyboard
• guitar
• retractable tape measure
• wristwatch
• golf clubs
• car with stick shift
• playing cards

Take a look at numerous products designed for left-handers.