What makes a left-hander?

Science 2017-07-24

Imagine someone is passing you a cup of coffee – which hand will you use to take it? Did you have to think about that? In an everyday situation, this is usually a subconscious decision and you simply reach with your dominant hand, just as you do to write and eat. However, as a lefty I’ve always been curious about this, so I’ve decided to have a look in to what might cause this difference. About 9 in 10 people are right-handed, leaving a small minority of left-handers,1 with famous left-handers including Barack Obama, Leonardo da Vinci and Buzz Aldrin.

Over the years, left-handers have been the subject of many superstitions2, ranging from a bit of bad luck all the way to associations with the devil. If you have a left-handed grandparent, they may have even been forced to use their right hand when learning to write at school3. In a slightly more modern context, any left-hander will know that tools such as scissors and tin openers are often designed for easy use in the right hand, and this can leave us struggling with what should be simple tasks! So why does the world have such a bias towards right-handedness?

Among many other factors, language is a strong contributor to the ongoing negative reputation of left-handedness. For example, in French, “droit” not only means “right”, but also straight, law and right in the legal sense, so has strong positive connotations. Left, “gauche”, is similar to more negative words with meanings such as clumsy, graceless and unhappy. This trend is echoed in many other languages around the world, in fact in Latin, left can be translated as “sinister”. Even in certain religions, the right-hand side is preferred, for example in Christianity the right hand of God is the favoured hand.

So, we know that there is a long-standing negative reputation for use of the left hand, but this isn’t something a baby will take into account when developing a dominant hand. Here follows a quick run-through of some theories behind hand dominance.

Contributing Factors

Little girl drawing with pencil at home. Top view.


Hand preference does in fact begin to develop in the womb, with around 40 genes chipping in to choose a dominant side6, but this isn’t the only influence! In fact, in around 20% of identical twin sets, one twin is right-handed and the other is left-handed. This means there must be more factors at play. One study showed that a significant right hand bias is established by 13 months4, and further insight suggests that handedness develops at different ages for different tasks5. Therefore, it is likely that you have an overall bias but that your choice of hand for less important tasks is a mixture of nature and nurture.

Spinal Cord

Another idea is that the spinal cord indicates a preference for right or left handedness9. During pregnancy, a child will develop a preference for moving one hand. Expanding on this, certain precursors of handedness develop before the motor cortex in the brain has formed a connection to the spine. It is because of this that researchers now think the spinal cord is more significant than the brain in the development of a dominant hand.

Social Cooperation

The final factor I have explored is “social cooperation”. Humans are a social species, placing high value on cooperativity in society, and as a result, the trend of right-handedness has evolved8. This TED-Ed video gives a brilliant explanation of social competition vs cooperation as well as a good overview of some extra factors10.


Man working at desk with pens in both hands!

Although you may use different hands for different tasks, you are likely to have a definite overall tendency to use one hand more. Ambidexterity is the ability to use both left and right hands equally well, and is a rare phenomenon, with less than 1% of the population being classified as truly ambidextrous. There was a time when being ambidextrous was encouraged, it was even claimed that learning to use your weaker hand could give you “cross dominance” and improved brain function, but science has since shown that this is not true11.

It is of course possible to learn to use your non-dominant hand, for example after a stroke or serious injury many people have had to learn to use their weaker hand in order to write, but unless this happens at a very early age it is unlikely to result in complete ambidexterity12. However, being naturally ambidextrous from birth has been associated with disadvantages such as academic difficulties and developmental conditions13.


It’s clear that there are many complicated factors behind hand dominance, and there’s still a lot more to learn before we fully understand it. If you want to find out more, this quick test can tell you just how left/right inclined you are. Your result might surprise you! Tweet me @HelenAtNotch and let me know what you get.


  1. https://www.livescience.com/17009-left-handedness-ambidexterity.html
  2. http://www.anythinglefthanded.co.uk/being-lh/lh-info/myths.html
  3. http://www.nature.com/news/2007/070716/full/news070716-4.html
  4. http://oaktrust.library.tamu.edu/bitstream/handle/1969.1/158722/Bates
  5. https://www.insidescience.org/news/babies-dont-develop-handedness-all-once
  6. https://ghr.nlm.nih.gov/primer/traits/handedness
  7. http://www.rightleftrightwrong.com/brain.html
  8. https://news.northwestern.edu/stories/2012/04/left-handed-minority
  9. https://www.sciencedaily.com/releases/2017/02/170217095904.htm
  10. https://www.youtube.com/watch?v=TGLYcYCm2FM
  11. https://io9.gizmodo.com/why-training-yourself-to-be-ambidextrous-is-a-bad-idea-458673693
  12. http://www.bbc.co.uk/news/magazine-23086540
  13. https://www.scientificamerican.com/article/can-training-to-become-ambidextrous-improve-brain-function/