  We study the single transverse spin asymmetries in the single inclusive
particle production within the framework of the generalized parton model (GPM).
By carefully analyzing the initial- and final-state interactions, we include
the process-dependence of the Sivers functions into the GPM formalism. The
modified GPM formalism has a close connection with the collinear twist-3
approach. Within the new formalism, we make predictions for inclusive $\pi^0$
and direct photon productions at RHIC energies. We find the predictions are
opposite to those in the conventional GPM approach.
