  We investigate a possible unified theory of all interactions which is based
only on fundamental spinor fields. The vielbein and metric arise as composite
objects. The effective quantum gravitational theory can lead to a modification
of Einstein's equations due to the lack of local Lorentz-symmetry. We explore
the generalized gravity with global instead of local Lorentz symmetry in first
order of a systematic derivative expansion. At this level diffeomorphisms and
global Lorentz symmetry allow for two new invariants in the gravitational
effective action. The one which arises in the one loop approximation to spinor
gravity is consistent with all present tests of general relativity and
cosmology. This shows that local Lorentz symmetry is tested only very partially
by present observations. In contrast, the second possible new coupling is
severely restricted by present solar system observations.
