What Determines If You Are Right or Left Handed?

Genes

• Results of a 2007 Oxford study showed that a gene called LRRTM1 plays a vital role in determining the functional specializations of different parts of the human brain. In right-handed people, the left hemisphere of the brain controls speech and muscular coordination. Researchers have since observed that the reverse of this is true for around 70 percent of lefties; that is, their right hemispheres have more developed neurological connections. Previous studies indicate that a person's dominant hand is often opposite to the more specialized hemisphere, leading to the conclusion that LRRTM1 has a significant impact on handedness.

  In spite of this, further studies show that even in cases where both parents are left-handed, the chances of their child's also being left-handed are only 26 percent. This means that the gene influences, rather than determines, handedness.

Testosterone

• Another theory is that testosterone levels during fetal development also affect handedness. Suppression of the development of the brain's left hemisphere, believed to be an effect of prenatal testosterone, results in the greater development of the right hemisphere. Some believe that this is the reason why left-handedness is more common in males than in females -- the odds being 1.23 to 1 -- and even more so in male twins.

Brain Damage

• Brain damage occurring from difficult or stressful births and from prenatal nutrient deficiencies has long been considered a cause of left-handedness in some people. This is called "pathological left-handedness." Figures showing that southpaws comprise 20 percent of the mentally retarded and 28 percent of the profoundly mentally retarded populations support this theory. The fact that modern medical technology has reduced the incidence of traumatic births without affecting the proportion of left-handed people indicates that brain damage cannot account for all occurrences of left-handedness.

Other Factors

• Several other factors may contribute to a person's handedness, including a sustained injury to one hand as well as well as "re-education" -- a practice common decades ago, when left-handed children were forced to write with their right hand. Other environmental factors -- such as convenience, social pressure and even hormones -- can also determine a person's hand dominance.