Left and right handedness and identical twins? - Twin Life Online | Life With Twins
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Left and right handedness and identical twins

Left and right handedness and identical twins?

I have noticed lately than one of my twins, C, appears to have a definite preference for using her left hand, while my other twin, J, appears to prefer her right hand. As they are identical twins I had not thought they would have different hand preferences, so I have been trying to research this a little.

Here are some basic facts:

  • Less than 10% of the population is left-handed
  • Males are 50% more likely to be left-handed than females
  • Around 20% of all identical twins have 1 which is left-handed and 1 which is right-handed

Many people have tried to argue that handedness is genetic, but due to lots of studies with twins, it has been discovered that handedness is not genetic.

Position in the womb

One theory is that handedness is determined by the positioning in the womb. The ear that faces out of the womb tends to receive the most stimulation and input, therefore stimulates the development to that side of the brain. However, this is not the case for all babies, so this theory has large flaws.

Increased testosterone

Another theory is about prenatal experience and increased levels of testosterone exposure inside the womb.This may decrease the development of the left hemisphere which would in turn explain the increased incidence of left-handedness in males as well as with multiples.

Mirror twins theory

Mirror twins are twins that have developed when the egg splits quite late (around days 9-12). They have genetically identical DNA, but have small mirror image differences, for example, left-handed and right-handed, mirror image dental problems, cowlicks on different sides.

All these theories have their place, but most have been challenged, so we still are not able to explain why people have a preference for left or right-handedness. However, it does explain why identical twins may not have the same hand preference (as it is NOT genetic). I can also tell that my twins are not mirror twins. Their egg must have split very early (in the first 3 days), as they each had their own placenta and sac, also the hair and cowlicks are on the same sides etc.

This should explain why my twins appear to each have a different hand preference.

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