  General Relativity can be reformulated as a diffeomorphism invariant gauge
theory of the Lorentz group, with Lagrangian of the type $f(F\wedge F)$, where
$F$ is the curvature 2-form of the spin connection. A theory from this class
with a generic $f$ is known to propagate eight degrees of freedom: a massless
graviton, a massive graviton and a scalar. General Relativity in this formalism
avoids extra degrees of freedom because the function $f$ is special and leads
to the appearance of six extra primary constraints on the phase space
variables. Our main new result is that there are other theories of the type
$f(F\wedge F)$ that lead to six extra primary constraints. However, only in the
case of GR the dynamics is such that these six primary constraints get
supplemented by six secondary constraints, which gives the end result of two
propagating degrees of freedom. This is how uniqueness of GR manifests itself
in this ``pure spin connection" formalism. The other theories we discover are
shown to give examples of irregular dynamical systems. At the linear level
around (anti-)de Sitter space they have two degrees of freedom, as General
Relativity, with the extra ones manifesting themselves only non-linearly.
