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Monday
Sep262016

Is Left the Right Side?

Most people aren’t familiar with the problems left-handed individuals face: ink on your hands, uncomfortable scissors and the computer mouse that wasn’t made for you. Only about 10-15% of humans have a preference for the left hand and for centuries have been demonized, forced to learn using the “right hand” and suffer from higher accident rates when using equipment that was designed for right-handed people. Why? It’s still a mystery why there is such an imbalance favoring the right hand, but archeological evidence suggests that the phenomenon is not new. Neandertals seem to have had an abundance of tools designed to be used with the right hand.

Humans aren’t unique in their preference for one side over the other: Many animals use one side of their extremities more frequently, and like for humans, the asymmetry is evident in the brain, where the “opposite” side hosts the dominant motor control areas. One particularly curious example are elephants, which have their own interpretation of handedness: When they rip out greens from the ground, they use their trunk to wrap it around the grass and pull. They can now either wrap their trunk clockwise or counterclockwise around the food and surprisingly most individuals only ever go in one direction. It is pretty evenly distributed and there doesn’t seem to be such a strong imbalance as with human handedness. Oddly enough, it has been found that elephants without such a side preference (they do exist!) are at a disadvantage, because they feed significantly slower. A possible explanation is that through the lateralization of the behavior, the neuronal circuits governing the movement are more efficient compared to animals that do not show such a side preference, because only one hemisphere is active during the movement if it is lateralized.

It is still unclear what the source of handedness on a population level is. Research with horses and wild chimpanzees suggests a genetic component, but it will take more time to disentangle how and why we observe the right-handedness in human populations. Or wrong-handedness, for the ones who are smudging their ink as they take notes.   

MGS

Sources:

(1)    Neanderthal Lifeways, Subsistence and Technology 2011, Handedness in Neanderthals (pp.139-154)

(2)    J. Comp. Psychol. 2003, 117(4), 371-9

(3)    Laterality 2009, 14(4), 413-22

(4)    J. Comp. Psychol 2015, 129(4), 377-87

(5)    PNAS 2005, 102(35), 12634-38

(6)    Behav. Processes 2008, 79(1), 7-12

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