Daily Mysteries #4: What Causes Left Handedness ? (New Study Findin...

Growing up, I think the first time i learnt about left handedness was seeing my cousin hold the spoon with his left hand.
It was a thought provoking moment for me! Come to think about it, kids are fascinated by almost everything due to their eagerness to learn.

Still, handedness has been a particular topic of interest for science and even social bias for a long time now.

What is Handedness?

Handedness basically defines a person's preference for the use of either hand for more accurate and precise daily actions, whereby the preferred hand is termed as the dominant hand, while the less commonly used as the non-dominant hand.
People with right-hand preference are often termed Right handed, people with left-hand preference and Left handed.
According to statistics, around 10% of the population is left handed.
The handedness is actually not only limited to those two scenarios, there are few others, as follows:

Right handedness: as explained above.
Left handedness: as explained above.
Mixed-handedness: having a preferred hand for different key tasks
Ambidexterity: capability of performing any task equally well with either hand.

Society's Perception & Bias

Lefties (often a term for left-handed people) have been attributed with either being much inferior or much superior, whether in cognitive activities, sports...

Due to cultural, religious, or social bias, lefties are often forced to use right hands, to the extent of being penalized for failing to do so - actions which have been reported across the globe.

Even terms identifying left handed individuals in different languages at times reflect society's 'un-spoken' perception of this difference and bias towards right handedness.
For example, in Italian, the term for Left Handed is "mancino", which is also also translatable as an adjective for dishonest.
In other languages as well, particularly due to the fact that the word "Left" itself is often associated with a bad meaning. For example, in French the word "Gauche" is used for explaining anything left, and is also the term used for left handed people "Gaucher" and a synonym for "maladroit", also dishonest in French.
In our own slang, Arabic - Lebanese, many uses of the term "left handed" relate to the word "failure".

What Causes Left Handedness?

There has been varying theories about left handedness and its root causes across the years.
Several researches had concluded that fetuses' hand preference (through for example suckling on a thumb) is a key indicator for handedness as they grow.

Genetics: While the most prevalent recent theories relied on genetics and heredity, and several earlier studies attributed that if both parents of a child are left-handed, up to 24% of children will be left handed, yet further later research and a multitude of studies concluded that handedness genetic variance cannot be explained by a single genetic locus, with an estimated around 40 loci in play.(Reference 1 below)
Others: other less prevalent theories related prenatal hormonal exposure, prenatal vestibular asymmetry, and/or ultrasound screening as contributing factors to left handedness.
Epigenetics: A recent 2017 study conducted by Ocklenburg, et al, came up with a complete new perception into the cause behind left handedness. While prior theories, including the genetics theory, considered that the brain asymmetry is the cause for the differences between left and right handedness, yet this study's finding indicates that the roots of the difference are in the spinal cord's formations, being the essential cause behind brain asymmetry leading then to left handedness.

Our findings suggest that molecular mechanisms for epigenetic regulation within the spinal cord constitute the starting point for handedness, implying a fundamental shift in our understanding of the ontogenesis of hemispheric asymmetries in humans.

Relating all this to to environmental factors and influences to the genes at spinal cord level and hence to the expression asymmetries:

On the one hand, we could show that DNA methylation of CpG islands shows substantial asymmetries that are related to RNA expression asymmetries.
...
On the other hand, we could also show that the asymmetries in gene expression are modulated by miRNA expression asymmetries. Particularly interesting was our finding that for the miRNAs asymmetrically expressed towards the right spinal cord at week eight, KEGG pathway analysis revealed a substantial effect of the TGF-β signaling pathway. This is an intriguing finding, as both nodal growth differentiation factor (Nodal) and left-right determination factor (Lefty), two of the key proteins for establishing bodily left–right asymmetry during development are part of the TGF-β superfamily

(Reference 2 below)

Are Lefties Better Achievers?

While this is a highly debatable topic and has been subject to several studies, the performance of left handed individuals in cognitive skills, math, music, or even sports is at many times claimed to be better.
Many historical figures such as Leonardo da Vinci, Mark Twain, Mozart, Marie Curie, Nicola Tesla and Aristotle... were left handed.
A recent study with 2,300 students (Reference 5 below) found that left handed people performed better at mathematical intensive tasks, and that basically:

handedness, as an indicator of connectivity between brain hemispheres, does influence cognition to some extent

yet due to the fact that those results only were only seen in "male" adolescents,

This pattern of results was particularly clear in male adolescents. By contrast, when the task was not so demanding, such as when doing simple arithmetic, there was no difference between left- and right-handers.

could drive us to be skeptic of the findings, especially that results from older and other studies did not reach similar conclusions (See Chris McManus. April 14, 2012. Is It True That Left-Handed People Are Smarter Than Right-Handed People? Scientific American. Retrieved: 26 May 2013.)

So in short, left handedness is now believed to be caused due to environmental factors in spinal cord. Lefties are normal people, might just be smarter than us, or not, yet definitely not a cause to label or bias them as evil!

Thank you for reading through!

@mcfarhat

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Daily Mysteries #4: What Causes Left Handedness ? (New Study Findings)