Fa'afafine in samoa/thailand? How can some cultures just make up a 3rd ?

You have to ask yourself what is the biological function behind the adaption. My guess, and this is purely speculation is that this practice originated to control population since we are talking about islands which are limited in resouces. In Thailand this is just personal choice just like in America, but in Samoa it seems to have a necesary function and actually looks like it is forced on people. My other guess is that since I know some Samoans and these are some people that maybe a long time ago they gave the Fa'fifafine role to sons who did not cut the mustard genetically. Kind of like weeding out the ugly weak little duckling so they do not reproduce thus preserving the large physique that is prevalent in Samoan society.