In the context of field theory two elements seem to be necessary to search
for strong-weak coupling duality: first, a gauge theory formulation and, se
cond, supersymmetry. For gravitation these two elements are present in MacD
owell-Mansouri supergravity. The search for an "effective duality" in this
theory presents technical and conceptual problems that we discuss. Neverthe
less, by means of a field-theoretical approach, which in the Abelian case c
oincides with S duality, we exhibit a dual theory, with inverted couplings.
This results in a supersymmetric nonlinear sigma model of the Freedman-Tow
nsend type. [S0556-2821(99)07710-3].