  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 second,
supersymmetry. For gravitation these two elements are present in
MacDowell-Mansouri supergravity. The search for an "effective duality" in this
theory presents technical and conceptual problems that we discuss.
Nevertheless, by means of a field theoretical approach, which in the abelian
case coincides with $S$-duality, we exhibit a dual theory, with inverted
couplings. This results in a supersymmetric non-linear sigma model of the
Freedman-Townsend type.
