Q. What is the chance of having a left handed child?A. This article was written by Keith Milsom, owner of Anything Left-Handed, in response to lots of questions from Left Handers Club members.
About 10% of the world's population are left-handed and it seems that left-handedness runs in families, with the handedness of the mother being an important factor. So what are the chances of having a left-handed child?
Various studies have shown that around 10% of the world's population are naturally left-handed and that this seems to be a fairly consistent figure over a long period of time and in all countries and cultures. The way left-handedness is measured and the fact that many cultures deliberately suppress left-handedness complicate the picture, but left-handedness seems to stay at about that level anyway. More recent studies among children show a higher level, may be increasing.
It also seems to run in families, although no gene for left-handedness has been isolated and it is not possible to predict left-handedness in the same way as, say, eye colour. What we do know is that the more left-handers you have in the family, the more chance of having left-handed children. The probabilities of various left-right parent combinations having a left-handed child vary quite a lot depending on which research you look at.
Dr Chris McManus reported in his book Left Hand, Right Hand on a study he had done based on a review of scientific literature which showed parent handedness for 70,000 children. On average, the chances of two right-handed parents having a left-handed child were around 9% left-handed children, two left-handed parents around 26% and one left and one right-handed parent around 19%.
In The Left-Handers Handbook, Diane Paul, repeated this and also noted that Left-handed mothers are more likely to have left-handed children than left-handed fathers. This was based on research done by Stanley Coren for his book The Left Hander Syndrome, where he showed the chances of two right-handed parents having a left-handed child were around 10%, a right-handed mother and left-handed father the same, 10%, left-handed mother and right-handed father 20% and two left-handed parents 35%.
A large survey carried out by Anything Left-Handed showed that only 1.4% of left-handers who responded had two left-handed parent, 24% had one left-handed parent and 75% had two right-handed parents.
Are all these numbers consistent with each other?
If those percentages are true, would the level of left-handedness in the population as a whole stay at a consistent level?
I have produced a model that calculates the mix of handedness in parents and then multiples by the assumed probabilities for each mix having left-handed children to calculate the total number of left-handed children and their percentage of the population. For example, if 10% of the population is left-handed, only 1% of all couples will be made up of two left-handers (like me an my wife, but there is a theory that left-handers attract each other which would distort this, but that is for another article!).
Putting Chris McManus's percentages into the model shows an increase from 10% to 11.5% in the level of left-handedness in the population in one generation and if that continued for very long left-handers would no longer be in the minority! Stanley Coren's figures, although quite different, also give an increase in left-handedness to 11.2% and neither of them give a match to the figures from the Anything Left-Handed survey. Most significantly, the number of left-handed children with two left-handed parents is 2.3% for McManus and 3.1% for Coren. To get down to the level of only 1.4% found in the Anything Left-Handed survey, the chances of two left-handed parents having a left-handed child need to be reduced to around 14% - much closer to the other parent mixes.
Juggling the figures to get the best match to all these results, plus also keeping the level of left-handedness stable at around 10%, these probabilities of having a left-handed child seem to fit:
Two right handed parents, 9%
Left handed father, 12%
Left handed mother, 16%
Two left handed parents, 20%
There are many more variables that affect these figures and one study showed that left-handers have less children on average that right handers. This is meant to be a guide rather than a scientific conclusion.
However, whichever set of assumptions you use, some interesting figures come out:
* More than 50% of left-handers do not know of any other left-hander anywhere in their living family.
* Around 75% of left-handers have two right-handed parents and only 2% have two left-handed parents.
* Between 7 and 8 out of 10 children born to two left handed parent will be right handed.
Of course, the chances of having a left-hander in the family increase the more children you have. So there is still hope for right-handed parents - if you have enough children, you may still be lucky enough to have a left-hander!
