ELI5 Left Handed - why aren't 50% of people lefties?

Why aren't 50% of people lefties? Why are right handed scissors not usable by left handed folks? Why isn't everybody ambidextrous? Why are a disproportionate number of US presidents left handed?   ... We explain like i'm five!     Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: happyweeze, zodde, schorhr, deviousterran, llittle_llama, fox-mcleod, samzeman, mathem_ & howmanydads   To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]