Form factors in relativistic quantum mechanics: Is there a favored   approach? Why?
  Form factors of a simple system have been calculated in various forms of relativistic quantum mechanics, using a single-particle current. Their comparison has shown large discrepancies. The comparison is extended here to instant- and front-form calculations in unusual momentum configurations as well as to a point-form approach inspired from the Dirac's one (based on a hyperboloid surface). It is found that these new results depend on the momentum transfer, Q, through its ratio to the total mass, Q/M, (closely related to the Breit-frame velocity of the system). They evidence features similar for a part to those shown by an earlier ``point-form'' implementation (based on hyperplanes perpendicular to the velocity of the initial and final states). It thus appears that the standard instant- and front-form calculations, which generally do well compared either to experiment or to predictions of a theoretical model, rather represent exceptional cases. An argument explaining the success of these last approaches is presented and discussed. It is based on transformations of currents under Poincar\'e space-time translations, going beyond the energy-momentum conservation property which results from the Lagrangian invariance under them. Depending on the approach, analytic or approximate numerical methods are proposed to correct form factors for missing constraints then expected. 
B. Desplanques