Why Are We Left Or Right-Handed?

Thumb sucking
Scientists at the Queen’s University in Belfast, Northern Ireland, who studied ultrasound scans of 1,000 fetuses and traced the progress of some of these babies after birth, found that if a fetus preferred to suck its right thumb more than its left at around 10 to 12 weeks within the womb, the child will likely be right-handed after birth. Their report was published in the New Scientist magazine.

Moreover, they also discovered that the preference to move one hand over the other in the womb was associated with the handedness after birth.

The nervous system links to the body from the brain will only begin to develop around 20 weeks of gestation, it does not appear that the brain controls the choice of handedness at this early fetal age. Instead, it could be a local reflex arc involving the spinal cord.
Source: The New Scientist Print Edition (22 Jul 2004)

Hair
Check a baby’s hair. If it swirls clockwise then 95%, the child would become right-handed. Researchers from the National Cancer Institute in Frederick, Maryland have determined that a single gene might control both hair swirl direction and handedness and therefore, that could explain why our brains are asymmetrical.

Those who have one or two copies of the “right” version of the gene would be right-handed and have hair that swirls clockwise. Those who have two “random” versions of the gene could be either right- or left-handed and have hair that swirls in either direction.

However, other scientists are unsure if a single, many genes or “random” genes determine right or left handedness because two left-handed parents can have a right-handed offspring or identical twins do not have the same handedness, one could be right-handed while the other is left-handed.
Source: Klar AJ “Human handedness and scalp hair-whorl direction develop from a common genetic mechanism.” Genetics (Sept 2003) 165(1):269-276

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