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

*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 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% |

It’s a pity that so much research has gone into determining whether or not you will be left-handed without taking the age of the mother into consideration. You will find (as I have in my studies of thousands left-handed people over the last 30 years) that a mother’s chances of having a left-handed child when she gives birth at 18 is 5%, increasing to 95% when she is 28 years old. It has very little to do with genes or whether the parents were left- or right-handed.

This means that there are more left-handed people being born now than a few decades ago, because women tend to start a family later due to careers being priority when they leave school.

The way to determine whether someone is left-handed is to make them clap their hands. If the left hand is clapped into the right, they are a lefty, even if they were taught / forced to write with the right hand.

Me and my husband are both right handed as are our parents. All five of our children are left handed. I have been told that this is very rare. Someone also told us that if you are left handed you may of started off in the womb as a twin. Im not sure if there is any truth in that though.

My mother was left-handed, my father was right-handed. They had 8 children.

Four girls and four boys. The four boys and one sister were left-handed. Three sisters were right-handed. Quite unusual.

There are simply too many comments below for me to ensure I’m not repeating another’s opinion on this subject but I must voice my own – I’m not sure I agree with your formula to produce a statistic regarding same-handedness in successive children of a family, solely based on those qualities of the parents, or that any such correlation can be derived from the perspective of this article. I personally don’t understand how other factors are not considered; a child’s tendency to mimic, or the possibility siblings may sit next to each other during tasks demonstrating a particular handed preference, as opposed to across where the mimic would be mirrored. Even the minute but relevant chance of an injury to one or more children to influence handedness, I would have to deduce a skew of results. The fact any of these situations have a possibility of happening to any random number of the tallied subjects, or the absence of any record these were observed and accounted for, should invalidate the ability to formulate any quantifiable outcome, even an approximate one. I understand this is not a scientific conclusion as you stated, but I personally do not believe it to even qualify as a guide any more than in the insistence of variable odds in any session of coin tosses. While I am not qualified in this area, there is documented evidence to support the existence of tribes, communities, and cultures that influence the handedness of children from a very young age. Unless I missed a section outlining the ability to accurately test the left-handedness of a child that has already been influenced to be right-handed, I can’t agree with the integrity of this data for any cross-section of the populace. I’ll also disclose the opinion, not based on any scientific fact but personal experience and observation, that handedness in and of itself is a spectrum and not of binary quality. I can attest to numerous examples of mothers and fathers (myself included), preferring one-handed or the other, or both, to in fact use different hands for different functions (obviously assumed to be determined mostly by the ease of the task being performed). Conversely, to highlight the opinion that even this assumption can’t be calculated, examples of the opposite being true have been observed. (One would be of a ‘right-handed’ male, a professional hairdresser, yet is only comfortable using his left hand and left-handed scissors to cut hair; another being a ‘right-handed’ female, yet opens every jar, jug, bottle, installs a lightbulb, or even uses a screwdriver with her left hand, regardless of the angle or position that would otherwise account for this unintuitive behavior). One would even have to consider the entire field of psychology, an admittedly inexact science in and of itself, must be accounted for to even attempt a quantification of this phenomenon that claims to produce any accurate result relative to even a casino.

Hi Brad and thank you for your detailed comments. I agree entirely that there are many factors to consider, though we do feel that handedness is an inate genetic characteristic and note developed by “nurture” (you can force someone to change their writing hand but that doers not change theior brain structure, just confuses it!). We just wanted to do some calculations that gave an overview of how the numbers work but would be very interested in any other models that may throw light on this.

Keith

There is no one in my family who is left handed. I have searched through all of my family trees and everything, yet still seem to find no left handers. My culture is one of which who does not mind the idea of left handedness, and in some areas of the country seem to encourage it, as it is said that being ambidextrous is said to increase brain activity.

I am the only left hander in my family. How is this possible, if the right handed gene is dominant?

Just curious, the part about having 3 left-handed children in a row. Is that the correct math? I would think that math would be the chances of having 3 left-handed children just in general? Just curious. I may be wrong though, haven’t had a math class in quite a while :P.

In my immediate family of five, only one brother was right-handed. How rare is it for only 20% of a family to be right-handed?

Both my parents and my sister are right handed, and I am (very) left handed. My four grandparents were right handed as well, but my uncle from my father’s side is left handed. We cannot trace it further back, unfortunately.

I married a right handed and my two children are right handed as well. However, both are left kickers.

In my family, the female grandchildren are either left-handed or ambi-dexterous The males are all right-handed.

For what it’s worth, I’m a leftie, with a grandfather who was also lefthanded, and both my parents were/are righthanded. I just learned one of my cousins is also lefthanded (yay!). My three boys–and oldest kids–are all lefties, too, but their sister, finally decided to keep her dad company and be righthanded like him. But…the kids’ dad and their uncle were/are lefthanded, also. So I guess my boys were the fortunate many, although they were very kind to the righthanded family members, and the oldest even learned to be ambidextrous. I guess you could say he was an ally :-).

Father and mother both right handed – brother right handed.

I am ambidextrous – can write with either hand, and do a variety of things comfortably with either hand – and some things, although few, with only r or with only l.

Is there a club / website for me?

I am not convinced at all by the percentages! I seem to see more and more left handed people these days, especially waiters and waitresses. My neighbours are both right handed but have three left handed children. I am left handed-ish (normally write with my left hand but do most other things with either hand) but my parents were both right handed and we know of no other member of the family who was left handed. My son is left handed (and once worked in a left handed office (all four of them were left handed). His wife is right handed but one of their two children is left handed. I could go on ….!

I am the only left handed child of two right handed parents. I know of no grandparent of mine being left handed. I am married to a right handed man but our first two children are left handed. Our third child is mainly right handed but says she is a bit ‘mixed up’ and could be either.