Wednesday, October 27, 2010

Handedness and ability at maths: evidence from Ireland

There is a great deal of interest both popularly and amongst scholars about whether cognitive ability is predicted by handedness. The literature contains many findings which cannot be simply summarized and there are many many myths. Evidence for Ireland has been non-existent, as far as I am aware, until now with the release of the Growing Up in Ireland data.
So what can we say? Below I plot the density of attainment at a maths test that the 8 year olds in GUI sat.
Sadly, if you are a ciotóg, you can see the distribution is shifted to the left - but not by much. The good news is that when you look at the distribution of reading ability, there is no difference at all.
In numerical terms, left-handers are about 8% of a standard deviation lower. By comparison girls are about 11.5% of one standard deviation lower.


Peter Carney said...

Kevin, interesting post.

is there also something interesting happening at the tails or is my eyesight a little off?

Kevin Denny said...

Maybe the left tail but very few there so hard to infer anything. I could try a quantile regression I suppose.
There is some work that says lefties are over-represented amongst the "super clever", say the top .01% but you need huge samples to pick that up.
There is also a bunch of findings that suggest that left-handedness is associated with birth/pregnancy complications ("pathological left-handedness") in effect that they have minor brain damage. Evidence is very mixed on this. This might suggest an excess of lefties in the lower tail.

Mark McG said...

Are these differences statistically significant Kevin?

That's interesting about pregnancy, when I was looking at birthweight in this dataset it seemed like left handers may be born smaller (from what I remember about .05kg less on average).

Kevin Denny said...

Yup statistically significant (p=.002 in a regression) and robust to controlling for birthweight and various family background stuff.
One of the nice things about handedness is that it is orthogonal to lots of socio-economic characteristics, most things in fact, so controlling doesn't make much difference usually.
I have just run some quantile regressions and Peter's hunch is correct: the left-handedness penalty is more pronounced in the left tail. Thats very interesting.