In the Klein-Gordon equation (spin 0), the mass dependence is (only) through [itex]m^2[/itex], whereas in the Dirac equation (spin 1/2) it's through [itex]m[/itex]. Does this mean that for spin 0 particles, we can just as well describe them as having negative mass without changing any of the physics (whereas for the spin 1/2 particle there would be a difference)? Perhaps this is simply another way of saying that spin 0 particles are their own antiparticles (?), whereas spin 1/2 particles are not (in the Dirac sea picture)? However, that can't really be the case, since we also have [itex]m^2[/itex] in the Proca equation, which would mean that all spin 1 particles are their own antiparticles, which is not the case. So what does these different types of mass-dependence signify?