  I investigate the behavior of spin-dependent parton distribution functions in
the regions of small and large momentum fractions $x$. I present a systematic
comparison between predictions for relevant observables obtained with various
models of nucleon spin structure and a recent global analysis of spin-dependent
distributions, NNPDFpol1.1. Together with its unpolarized counterpart,
NNPDF2.3, they form a mutually consistent set of parton distributions. Because
they include most of the available experimental information, and are determined
with a minimally biased methodology, these are especially suited for such a
study. I show how NNPDFpol1.1 can discriminate between different theoretical
models, even though NNPDF uncertainties remain large near the endpoints $x\to
0$ and $x\to 1$, due to the lack of experimental information. I discuss how our
knowledge of nucleon spin structure may be improved at small-$x$ by future
measurements at an Electron-Ion Collider, and at large-$x$ by recent
measurements at Jefferson Lab, also in view of its $12$ GeV upgrade.
