Since the proposal of the Tsallis generalized entropy, the general explanat
ion of the role played by the parameter q that defines which specific entro
py to pick among a whole family, remained somewhat partial and tentative, a
lthough some particular examples were taken into account specifically. The
purpose of the present paper is to present a rigorous formal derivation of
a mathematical expression giving the parameter q in terms of the momentum f
luctuations of a stochastic process, thus furnishing at least one of its po
ssible physical origins.