  Large transverse single-spin asymmetries (TSSA) in hadron production at
forward rapidity have been observed in polarized $p$+$p$ interactions for many
decades, over a large range of center-of-mass energies, and have led to the
investigation of spin-momentum correlations such as the Sivers and Collins
effects. In the last few years, it has been discovered at RHIC that these
single-spin asymmetries may be enhanced or suppressed in $p$+$A$ collisions,
and the nuclear-size and centrality dependence have been studied. A variety of
phenomena have been observed and likely they do not have all a single
explanation; we see an apparent quenching of the TSSA in forward charged hadron
production with increasing nuclear size, while we see an enhancement in the
asymmetry in $J/\psi$ production, and in very forward neutron production even a
sign change in the asymmetry is seen. Other systems, such as $\pi^0$ production
at central rapidity, do not display a nuclear-size dependence. These
observations provide a bridge between the study of the initial state in
heavy-ion collisions and that of the nucleon spin puzzle, and open up a new
method for the investigation of cold nuclear matter.
