Some materials, notably graphene and some other 2D nanomaterials, possess spin-polarized edge states, e.g., where one edge may allow the conduction of spins oriented "up" while the opposite edge only allows conduction of "down" spins. My question is not why these spin-polarized states exist, per se, but rather what determines their polarity? What is breaking the symmetry and polarizing/magnetizing a particular edge one way or the other - what distinguishes "up" from "down"? Is there a magnetic field assumed somewhere? Seems like the polarization could just as easily be flipped and everything would work the same, absent something that biases each edge differently.