Why Are People Left Handed?

Why are some people left handed? What causes left handedness?

A ton of research papers on the subject are available online, but reading those is like wading through a legal document – it’s hard going! So in this article, I’ve tried to take the data from these studies and compile them into an easy-to-read format that us non-scientists can make sense of.

Why Are People Left Handed?

Scientists are still not entirely sure why some people are left handed, and as a result, it is currently difficult to try and anticipate if a child will be left or right-handed when born.

What we do know is that hand preference starts to manifest while the baby is still developing in the womb. It becomes progressively apparent in a child’s early years and is likely to remain consistent throughout life.

Ultimately hand preference comes down to a variety of factors, partly genetic and partly environmental, but there’s no real conclusive evidence with which to give a definitive answer.

A Lack of Left Handed Study Data

The lack of information to help determine handedness can be partly attributed to the fact that lefties are often excluded from scientific studies in order to reduce variances in the resulting data.

However, recent investigations have shown that left handed people form a substantial segment of the human population. Therefore, left-handedness comes under the normal range of diversity and should not be excluded from studies.

Related Article: How Many People are Left Handed? (More Than You Think!)

Until scientists finally crack this little conundrum we can only hypothesize at best. Here are a few of the theories I found that attempt to explain why people are left handed. Some are a little more far-fetched than others…

What Causes Left Handedness? 5 Theories!

1. Genes

It was previously believed that just a single gene was responsible for determining hand preference. However, newer scientific research has discovered that at least 40 genes may contribute to this complicated recipe!

It is thought that although a single gene will be unlikely to cause a significant effect on handedness, a large number of this specific group of genes will contribute in a meaningful way. Studies have shown that some of these genes will affect the left-right asymmetry of the human body in the very earliest stages of development.

The first genetic influence on handedness to be identified is gene LRRTM1, which studies have suggested causes an increased probability of being left handed amongst those suffering from schizophrenia.

A further example from the same study above is gene PCSK6, which research has shown results in an increased chance of being right handed in people with dyslexia.

However! It is currently not known whether or not these genes affect hand preference in people who do not have these medical conditions.

2. Prenatal Trauma/Hormonal Imbalances

Some scientists believe that babies who are exposed to traumas within the womb are more likely to exhibit left-handed tendencies.   This theory can be backed up by the fact that left handed people are more likely to develop mental illnesses, perhaps as a result of the possible brain damage incurred during the developmental period.  In more modern times it has been suggested that ultrasound scanning of the uterus can affect the development of the brain, perhaps leading to an increased chance of left handedness.

Also, studies have shown that whilst in the womb, exposure to incorrect hormonal levels can lead to an increased frequency of left handedness.  For example a recent study has shown that exposure to higher than normal levels of estrogen can increase the chances of a baby being a lefty.

Other theories suggest that exposure to high levels of testosterone can impede the development of the left side of the brain, which results in the right side becoming dominant.  As you probably know already, left handed people are generally right-brain dominant.  This theory supports the idea that there are twice as many left handed males, due to naturally higher levels of testosterone.

Research also shows that older mothers are more likely to produce left handed babies.

3. The Vanishing Twin Theory

The vanishing twins theory suggests that left handed people were originally one half of identical twin foetuses in the mother’s womb.  However the right handed foetus fails to develop very early on during the pregnancy and is reabsorbed by the mother’s body.  Although twins do have a high occurrence of left/right-handedness in the pair, this theory has been discredited by Australian scientists.

4. Physical Conditioning

People who injure or lose function of their right hand for an extended period of time can switch to a left-handed predisposition – even after the hand heals.  This usually only occurs after a period of eight months or more when the initial change to using the left handed has become the norm for the individual.

5. Copying Left Handed Friends or Family

Some younger children may try to imitate older family members or people they look up to, and end up becoming left handed themselves.  I wonder just how many 11 year old girls have taken to learning left handed guitar because of Justin Bieber?  

Eric Gales, a well known guitarist who plays left handed isn’t actually a lefty! He learned this way early on after imitating his brother.

Do you buy any of these theories?  I kinda like the twin theory myself, that’s some freaky stuff right there!

Further Reading – Why Are Some People Left Handed?

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