Left Handed Vs. Right Handed in Twins

There has been a long debate about whether handedness is a genetic or environment-induced trait. Scientific research has supported both sides of the argument. The studies conducted on hand preferences of identical and fraternal twins are important in formulating the major theories regarding handedness. Twins are ideal test subjects to determine the genetic versus environmental contributions in specified traits, such as handedness.

The Genetic Argument

• Many argue that handedness is genetically determined because a child's hand preference correlates to parents' hand preference. A study by D. C. Rife published by Ohio State University showed 92 percent of children are right-handed if both parents are right-handed. Forty to 50 percent of children are left-handed if both parents are left-handed, according to Dr. Robert Needleman, in an article on DrSpock.com. When there is one right-handed parent and one left-handed parent, approximately 80 percent of their children are right-handed. These data indicate a genetic bias towards right-handedness, and that right-handedness may be controlled by a specific genotype--those without the right-handed genotype appear to have a more random likelihood for either preference. So the studies make a strong argument for handedness being a genetic trait. However, this does not rule out the possibility of handedness being learned.

Twins and the General Population

• Twins are more likely to be left-handed than the general population, according to a study published in "Behavior Genetics." This would suggest a genetic link for hand preference, since twins share all or half of the same genes. So research on twins may provide answers on the genetic link to handedness. If handedness were entirely genetically determined you would be able to make certain predictions about twins: You would expect identical twins to always have the same hand preference. And fraternal twins would show the same distribution of same handedness as other siblings. However, statistics from scientific research have not confirmed these predictions.

Identical and Fraternal Twins

• Statistics regarding handedness of identical and fraternal twins suggest there is no genetic contribution to handedness. About 21 percent of identical twins do not have the same hand preference, according to the Ohio State University study. Just 3.4 percent of identical twins were both left-handed, while 76 percent were both right-handed, according to Rife's study. That leaves over 20 percent of the identical twins having different handedness. Fraternal twins showed results similar to identical twins. This challenges the genetic argument for hand preference.

Cerebral Asymmetry

• Cerebral asymmetry refers to there being differences between the left and right brain hemispheres. The dominant hemisphere in a person is the larger and more active side of his brain. A dominant left hemisphere is believed to induce right-handedness. Left-handed twin pairs may lack a genotype that causes a dominant left hemisphere and a right-handed preference, according to a study published by the Proceedings of the National Academy of Sciences. Therefore, cerebral asymmetry may provide a link to genetic causes of handedness.

Testosterone Theory

• Scientists have suggested that prenatal exposure to testosterone may increase chances of left-handedness. If this were the case, then females with a male twin would have a higher chance of left-handedness, because testosterone is thought to pass between twins within the uterus. Also, males with a male twin would be left-handed more frequently than males with a female twin. However, a study published by "NeuroReport" did not confirm these predictions, therefore challenging the testosterone theory.