What is the chance of having a left handed child?
This article was written by Keith Milsom, owner of Anything LeftHanded, in response to lots of questions from Left Handers Club members.
About 10% of the world's population are lefthanded and it seems that lefthandedness runs in families, with the handedness of the mother being an important factor. So what are the chances of having a lefthanded child?
Various studies have shown that around 10% of the world's population are naturally lefthanded and that this seems to be a fairly consistent figure over a long period of time and in all countries and cultures. The way lefthandedness is measured and the fact that many cultures deliberately suppress lefthandedness complicate the picture, but lefthandedness 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 lefthandedness has been isolated and it is not possible to predict lefthandedness in the same way as, say, eye colour. What we do know is that the more lefthanders you have in the family, the more chance of having lefthanded children. The probabilities of various leftright parent combinations having a lefthanded 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 righthanded parents having a lefthanded child were around 9% lefthanded children, two lefthanded parents around 26% and one left and one righthanded parent around 19%.
In The LeftHanders Handbook, Diane Paul, repeated this and also noted that Lefthanded mothers are more likely to have lefthanded children than lefthanded fathers. This was based on research done by Stanley Coren for his book The Left Hander Syndrome, where he showed the chances of two righthanded parents having a lefthanded child were around 10%, a righthanded mother and lefthanded father the same, 10%, lefthanded mother and righthanded father 20% and two lefthanded parents 35%.
A large survey carried out by Anything LeftHanded showed that only 1.4% of lefthanders who responded had two lefthanded parent, 24% had one lefthanded parent and 75% had two righthanded parents.
Are all these numbers consistent with each other?
If those percentages are true, would the level of lefthandedness 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 lefthanded children to calculate the total number of lefthanded children and their percentage of the population. For example, if 10% of the population is lefthanded, only 1% of all couples will be made up of two lefthanders (like me an my wife, but there is a theory that lefthanders 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 lefthandedness in the population in one generation and if that continued for very long lefthanders would no longer be in the minority! Stanley Coren's figures, although quite different, also give an increase in lefthandedness to 11.2% and neither of them give a match to the figures from the Anything LeftHanded survey. Most significantly, the number of lefthanded children with two lefthanded 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 LeftHanded survey, the chances of two lefthanded parents having a lefthanded 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 lefthandedness stable at around 10%, these probabilities of having a lefthanded 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 lefthanders 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 lefthanders do not know of any other lefthander anywhere in their living family.
* Around 75% of lefthanders have two righthanded parents and only 2% have two lefthanded 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 lefthander in the family increase the more children you have. So there is still hope for righthanded parents  if you have enough children, you may still be lucky enough to have a lefthander!
Keith's model of lefthandedness
This is my Excel sheet used for calculating lefthanded probabilities. I must state again that this is only a layman's analysis  if there are any statisticians out there who can improve on this please let me know.
Click here to download the actual Excel workbook if you would like to play with the numbers yourself
WHAT IS THE CHANCE OF HAVING A LEFTHANDED CHILD? 












Starting number of individuals 


1,000,000 




Starting number of couples 


500,000 




Assumed starting rate of lefthandedness in the population 
10% 














Percentage chances of having a lefthanded child for each possible parent combination 





Percent of 

% chance 

% of LH 

% of RH 



all couples 
Number of 
of LH 
Number of 
children 
Number of 
children 
Total 
Father 
Mother 
with mix 
Couples 
child 
LH children 
with mix 
RH children 
with mix 
Children 










Left 
Left 
1% 
5,000 
20.0% 
1,000 
2.0% 
4,000 
0.9% 
5,000 
Left 
Right 
9% 
45,000 
12.0% 
5,400 
10.8% 
39,600 
8.8% 
45,000 
Right 
Left 
9% 
45,000 
16.0% 
7,200 
14.4% 
37,800 
8.4% 
45,000 
Right 
Right 
81% 
405,000 
9.0% 
36,450 
72.8% 
368,550 
81.9% 
405,000 










Total 

100% 
500,000 

50,050 
100.0% 
449,950 
100.0% 
500,000 










Resulting percentage of lefthanded children 

10.0% 














Percentage of all children born to two righthanded parents who are lefthanded 
9.0% 



