Why is left-handedness so rare?
Answer by Amy Gorin:
There is a substantial advantage to being right-handed – A right-handed mother will hold her infant with its ear against her aortic arch (where the heartbeat is loudest, calms the kid down), a right handed warrior will have their vital organs further away from their opponent, a right-handed artisan will have a steadier hand, since the pulse surge is slightly weaker in the right hand.
Once something is ubiquitous, however, game theory predicts that using the alternate strategy confers an advantage simply because it’s unexpected – the left-handed warrior will be fighting opponents who are unused to fighting lefties, the left-handed gatherer can dexterously reach fruits and eggs and other things that the righty might not be able to, etc. (Interestingly, lefty mothers very often still hold infants as if they were righties).
If the alternate strategy confers an advantage because it is unexpected, there must be a balance point at which the alternate strategy has maximum return. That would be at about ten percent of the population. Voila.
(And, ten percent is also the rough percentage that Kinsey found for the alternate strategy of same-sex attraction).