Twins and Hand Dominance and Preference

Twins Can Be Right-Handed or Left-Handed or Perhaps One of Each

left-handed and right-handed twins
Why are some twins left-handed and some right-handed?. KidStock / Blend Images / Getty Images

Are your twins right-handed or left-handed? Or perhaps they are one of each? Or even ambidextrous?  Twins have played an important role in the scientific research about handedness, although in many ways they muddle the mystery more than resolve it.

Handy Facts on Handedness and Twins

  • Less than 10% of the population is left-handed.
  • There are slightly more left-handed males than females.
  • About 20% of all identical twin pairs have one right-handed twin and one left-handed twin.

Strange customs and beliefs are associated with left-handedness in cultures around the world and throughout history. Unfortunately, in most cases, left-handedness is linked with sinister or dubious characteristics. For example, an ancient Iroquois legend describes the creation of the world by a set of twins. The right-handed twin created landscapes, plants, and natural creatures. The left-handed twin created snakes, thorns, and storms!

How Handedness Happens

Many people assume that hand preference is a genetic trait. However, that is not completely the case. A quick survey of identical twins with different handedness will confirm the discrepancy. Researchers have worked to identify a genetic link. One study found that even when both parents are left-handed, their offspring are more likely to be right-handed.

There are many theories as to why people display a preference for one hand over the other. It is believed that a combination of genetic and environmental factors come into play. One theory projects that position in the womb determines handedness, which would make sense for twin pairs who are right and left-handed since they're likely to lie in opposing directions in the womb. Another theory suggests that left-handedness occurs due to stress or trauma during pregnancy or birth, nothing that a twin pregnancy is inherently stressful due to multiple fetuses.

On the Other Hand

While all of these theories are interesting, none fully satisfies the issue, because they don't hold true for all babies. Another theory also postulates that prenatal experience influences handedness, explaining that increased levels of testosterone exposure in the womb decrease development of the left hemisphere of the brain. That would explain the higher incidences of left-handedness in males, but also among multiples since hormone levels are increased during pregnancy with multiples.

Finally, one specialist offers an explanation for the handedness discordance among identical twins. In the July/August 2003 edition of Twins Magazine, Dr. Geoffrey Machin explains the phenomenon of mirror image twins, who most often display opposite hand preferences. He explains it in reference to how monozygotic twins split after conception.​

"It is likely that the split...happen[s] so that the twins form side-by-side. This means that the twin-on-the-right has to hurry up and make a new left side because the twin-on-the-left has taken it, and the other twin has to make a new right side. This is probably why one twin often has a dominant right brain, and the other has a dominant left brain."

It Works For Me

That would seem to explain my twins' situation. One of my twin daughters is left handed, and one is right-handed.

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Article Sources

  • Johnston, D.W., et. al. “Nature’s Experiment? Handedness and Early Childhood Development.” Demography, May 2009. Pg. 281.
  • Ocklenburg, S., et al. “Handedness: A neurogenetic shift of perspective.” Neuroscience and Biobehavioral Reviews, December 2013. Pg. 2788.
  • Powledge, Tabitha M. “Left-handedness: Genes and a matter of chance.” Genetic Literacy Project, December 2014.