Handedness

One of my daughters is left-handed. This may not sound particularly unusual, but actually there are many more male left-handers than female. And nobody seems to know why there is this disparity. I guess this is the root of my fascination with the topic of handedness: it seems to pop up everywhere. Often it cannot be explained. And its effects can be more far reaching than might otherwise be expected.

Today I am going to confine my discussion to human handedness. Maybe I will look at other aspects of asymmetry another day. I like to think in terms of symmetry, or lack thereof, as handedness is just the obvious manifestation that …

Handedness is thought of as being a preference for using one hand over the other, where strength or precision is required. It goes beyond hands. I, for example, am right handed, right footed and left eyed. Testing which eye is dominant is interesting. Here is how to do it: With both eyes open, point with an index finger up to a corner of the room at ceiling level. Then, while keeping your hand still, close each eye in turn. With one eye, the finger will not appear to point to the right place. With the other, your dominant eye, it is still lined up.

I am not sure what use this information can be put to. I have noticed that, if I am working from a paper document at my computer, I prefer it to be on my left, which is handy as, obviously, my mouse is on the right.

Handedness seems to be related to the “wiring” in the brain, as there is no discernible difference between right and left arms that cannot be accounted for by the preferential use of one of them. The human body is an odd mix of symmetry and asymmetry. On the inside, some organs come in symmetric, mirror image pairs [e.g. kidneys, lungs]; others are on just one side [e.g. liver, appendix]; and others are central, but are structurally asymmetrical [e.g. heart, stomach]. The brain looks symmetrical, but each lobe performs different functions.

I used to wonder whether left handed people’s brains were programmed the other way around, but this is obviously not the case – otherwise I would be right eyed, for example. There is a very rare condition where an individual is completely “back to front” – their entire body is a mirror image of the “normal” arrangement. Sadly, they tend to be prone to a number of health issues and have low life expectancy.

So much for the inside of the human body – what about the outside? Here symmetry is the order of the day. We are almost exactly mirrored from left to right, with only very minor universal exceptions [I can only think of one and that only occurs in males]. Research has shown that we tend to find such symmetry attractive. Subjects have been asked to rank pictures of faces in order of attractiveness and they consistently put the most symmetric at the top. The theory is that the symmetry is a sign of “healthy” genes and, hence, this is a tool we use to select a genetically attractive mate.

There is a saying: “Beauty is in the eye of the beholder.” It seems like it is actually in the brain and could even be quantified and measured, which I, for one, find rather disconcerting. But it does lead to the geek’s perfect chat up line: “Darling, you are so symmetrical.”

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Posted September 2nd, 2010, by

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This blog is a discussion of embedded software matters - news, comment, technical issues and ideas, along with other passing thoughts about anything that happens to be on my mind. The Colin Walls Blog

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Commented on 2 September 2010 at 23:50
By George

One could surmise that the difference in left-handed females and males is a result of a male dominated world in the past. That is I have heard that some people who should be left-handed are right-handed because of conditioning. the norm is to be righ-handed so they were forced/trained to be right-handed. one could take this philosophy and apply it to females who in past history were not considered as “valuable” to a family let’s say in an agricultural based society. Being born left-handed or more so the need to adjust for such a opposition to the norm was not granted to females historically. Illiteracy amongst the female population versus males being the extreme prejiduce and injustice in this case. Now we have a genetic predisposition that occurs.

    Commented on 6 September 2010 at 15:01
    By Chris Edwards

    There is a notorious piece of research on life expectancy of lefthanders that was tripped up by this issue – because there were fewer lefthanders older than 40-50 in the sample set they concluded lefthanders don’t live as long.

    One thing worth bearing in mind about lefthanders is that the brain has a greater degree of symmetry than with righthanders – rather than there being a gene for lefthandedness it seems more likely that we lack one or more of the genes that help the development process laterally segregate parts of the brain. It’s possible that this genetic cause is X-linked so that it appears more predominant in males rather than females.

      Commented on 6 September 2010 at 15:10
      By Colin Walls

      Hi Chris.

      Yes, I’d heard of that discredited research.

      The brain symmetry thing is interesting. It seems to me that, since we [in the West anyway] are educated in a very left-brain oriented fashion, it is logical that this hemisphere would develop more in most people. As it also controls the right hand side of the body, I guess it would develop more for that reason too in right-handers. Hence the asymmetry in that group and more symmetry in left-handers. I guess that, in some other society, with right brain oriented education, the right handers would have the great symmetry.

Commented on 3 September 2010 at 07:30
By anne

this is of great interest to me: I am righthanded but have two children who are lefties and several grandchildren and now gtgrdchildren who are also lefties. Very hard to teach the girls to knit, crochet, etc. Very hard to remember to set the dinner utensils the right way for them [Ellen would probably have some rather pithy comments about my performance there] Terry was the one who learnt how to place his paper so that he could write in a coherent manner and he taught Ellen. I can’t remember what hand Jason favoured but in fact his whole body was assymetrical – you could see in his facial features that there was a real difference. Not surprising that he eventually developed a sideways scoliocis. Also internally one kidney developed properly in the womb but not the other one. Various other internal development injuries, especially spine; he never grew taller than 5’2″ which meant he always paid half price on the buses – so there are some advantages!!
You have here a rather full explanation of why I am interested.

Commented on 3 September 2010 at 09:32
By Colin Walls

George: Very interesting theory. I have not heard that angle before. It makes complete sense to me.

Anne: Glad you found this topic interesting. It sounds like you are pointing to a genetic cause for left-handedness. Are/were either of your parents or any grandparents left-handed? Maybe you can ask your Mum next week. Your family provides quite a lot of generations that one could draw a map of handedness, which would be interesting.

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