 | The common belief seems to be that each person is either left or right handed (with a rare true ambidexter popping up occasionally). However, handedness is not black and white in this way.
There is a term, 'relative handedness' which is, in simple terms, a measure of the ability of the right hand minus the ability of the left. People vary as to whether they are:
extremely left handed
moderately left handed
weakly left handed
ambidextrous
weakly right handed
moderately right handed
extremely right handed
... plus all the points in between these.
The 'extreme' handers have less ability in their non-dominant hand than the majority of people with the same handedness. So, for example, an extreme left hander would have less ability in his/her right hand than most left handers.
The 'weak' handers can do a lot with their non-dominant hand. They are close to, but not quite at the ambidextrous point.
For those of a mathematical inclination, if R and L are right and left handed ability, respectively, then the measure of relative handedness is taken to be:
100 x (R-L)/(R+L)
That is, the difference in the hands' ability divided by the total ability of both hands, multiplied by 100 to make it a percentage. Right handers of all types will have a positive relative handedness. Left handers will have a negative value. It turns out that using this measure and graphing relative handedness (x-axis) with number of people (y-axis) gives what appears to be 2 normal distribution (bell shaped) curves, one for the right handers and a much smaller one for the lefties (as there are less of them), the 2 curves linking at the ambidextrous point (where relative handedness = 0 as there is no difference in the hands' abilities).
There's more info about this at: http://cogprints.ecs.soton.ac.uk/archive/00000153/00/relhand_tim.html. (Though it's quite technical - not for the fainthearted! It's also not very complimentary to ambidexters!)
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