# What is the chance of having a left handed child?

*This article was written by Keith Milsom, then 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 Right Hand, Left 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 parents 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!

## Chances of having multiple left-handed children

To get the chance of having a number of left- habded cgildren in a rown, just multiply the appropriate probability by itself. This question we recived gives and example:

Q – I have a question about the dynamics of my family. I am left-handed, and my husband is right-handed. We have four children, 3 of whom are left-handed. The jury’s still out on our 4th, as she is only 4 months old. She does have a counter-clockwise hair whorl, which I hear may be correlated with handedness. Anyway, I was curious what the odds are of having 3 children in a row who are left-handed? I can’t seem to find the statistics anywhere, but I know it must not be common.

A – Well, according to my article, the chance of a left handed woman and right handed man having a left handed child is 16% (.16) or about 1 in 6. To get the probablity of having three left handed children in a row you just multiply that by itself, i.e. .16 x .16 x .16 = .004 (0.4% or about 1 in 250). If the fourth child is also left handed that would be a probability of 1 in 1,700.

## Keith’s model of left-handedness

This is my Excel sheet used for calculating left-handed 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 LEFT-HANDED CHILD? |
|||||||||

Starting number of individuals | 1,000,000 | ||||||||

Starting number of couples | 500,000 | ||||||||

Assumed starting rate of left-handedness in the population | 10% | ||||||||

Percentage chances of having a left-handed 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 left-handed children | 10.0% | ||||||||

Percentage of all children born to two right-handed parents who are left-handed | 9.0% |