  A striking prediction of several extensions of the constituent quark model,
including the unquenched quark model, the pion cloud model and the chiral quark
model, is a proportionality relationship between the quark sea asymmetry and
the orbital angular momentum of the proton. We investigate to which extent a
relationship of this kind is corroborated by the experiment, through a
systematic comparison between expectations based on models and predictions
obtained from a global analysis of hard-scattering data in perturbative Quantum
Chromodynamics. We find that the data allows the angular momentum of the proton
to be proportional to its sea asymmetry, though with a rather large range of
the optimal values of the proportionality coefficient. Typical values do not
enable us to discriminate among expectations based on different models. In
order to make our comparison conclusive, the extrapolation uncertainties on the
proportionality coefficient should be reduced, hopefully by means of accurate
measurements in the region of small proton momentum fractions, where the data
is currently lacking. Nevertheless, the unquenched quark model predicts that
quarks account for a proton spin fraction much larger than that accepted by the
conventional wisdom. We explicitly demonstrate that such a discrepancy can be
reabsorbed in the unknown extrapolation region, without affecting the
description of current data, by imposing the unquenched quark model expectation
as a boundary condition in the analysis of the data itself. We delineate how
the experimental programs at current and future facilities may shed light on
the region of small momentum fractions.
