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.
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:
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.