Positivity constraints on initial spin observables in inclusive reactions.

For any inclusive reaction of the type A1(spin 1/2)+A2(spin 1/2)-->B+X, we derive new positivity constraints on spin observables and study their implications for theoretical models in view, in particular, of accounting for future data from the polarized pp collider at Brookhaven RHIC. We find that the single transverse spin asymmetry A(N), in the central region for several processes, for example, jet production, direct photon production, and lepton-pair production, is expected to be such that |A(N)| < or approximately 1/2, rather than the usual bound |A(N)|< or =1.