Since a gyromagnetic factor equal to 2 follows from Diracs equation, it is a frequent misconception to think that a -factor 2 is a consequence of relativity; it is not. The factor 2 can be obtained from the linearization of both the Schrödinger equation and the relativistic Klein–Gordon equation (which leads to Diracs). In both cases a 4-spinor is obtained and for both linearizations the -factor is found to be equal to 2; Therefore, the factor 2 is a consequence of the minimal coupling and of the fact of having the same order of derivatives for space and time.
Physical spin  particles which cannot be described by the linear gauged Dirac equation satisfy the gauged Klein–Gordon equation extended by the  term according to,
Here,  and  stand for the Lorentz group generators in the Dirac space, and the electromagnetic tensor respectively, while  is the electromagnetic four-potential. An example for such a particle, is the spin  companion to spin  in the  representation space of the Lorentz group. This particle has been shown to be characterized by  and consequently to behave as a truly quadratic fermion.