To be clear, since the paper is a bit messy, here’s how they calculated a few variables.
Handedness index, HI: pick an individual. Check how many of the tasks they completed with the right hand (R) vs. the left hand (L). Then plop it into the formula (R-L) / (R+L).
So for example, if Alice used her right hand 60% of the time for any given task, R=0.6, L=0.4, HI(Alice) = (0.6-0.4)/(0.6+0.4) = +0.2.
Now let’s say Bob used his right hand 20% of the time. HI(Bob) = (0.2-0.8)/(0.2+0.8) = -0.6. Note the sign matters.
Mean handedness index, MHI: it’s mean, just like me. *ba dum tss* Just sum this stuff up and divide by the number of individuals. e.g. the MHI for the whole population of my example above would be (+0.2 -0.6)/2 = -0.2. So righties increase the score, lefties decrease it.
Mean absolute handedness index, MABSHI: disregard signal, then mean. The MABSHI for the population above would be (|+0.2| + |-0.6|)/2 = (0.2+0.6)/2 = 0.4. So stronger preference towards one hand (whichever it is) raises the score.
My personal take:
They found correlation between brain size, arm:leg ratio, and handedness… and that’s it. The title implies a cause (“why”), and that it has to do with right handedness, but both things are AFAIR (as far as I read) absent.
I think this is all a big red herring, mind you. We humans coördinate the usage of both our hands for a lot of tasks, where each hand performs a different movement:
- swing hammer with one hand, guide the nail with the other
- hold bow with one hand, pull the string and guide arrow with the other
- hold the mayo jar with one hand, twist lid with the other
- etc.
you get the idea, right? I think handedness encourages this sort of coördination, and it’s essential for more complex tasks other primates don’t typically perform. As such I don’t think it’s necessarily correlated to every instance of tool usage, as in the TOOL variable, but to specific tasks.
I don’t understand how this explains anything. Given that humans are the only large-brained, long-legged primate, how are they demonstrating handed-ness is connected to those traits specific over any other uniquely human traits?
