  In the 35 years since the European Muon Collaboration announced the
astonishing result that the valence structure of a nucleus was very different
from that of a free nucleon, many explanations have been suggested. The first
of the two most promising explanations is based upon the different effects of
the strong Lorentz scalar and vector mean fields known to exist in a nucleus on
the internal structure of the nucleon-like clusters which occupy shell model
states. The second links the effect to the modification of the structure of
nucleons involved in short-range correlations, which are far off their mass
shell. We explore some of the methods which have been proposed to give
complementary information on this puzzle, especially the spin-dependent EMC
effect and the isovector EMC effect, both proposed by Cloet, Bentz and Thomas.
It is shown that the predictions for the spin dependent EMC effect, in
particular, differ substantially within the mean-field and short-range
correlation approaches. Hence the measurement of the spin dependent EMC effect
at Jefferson Lab should give us a deeper understanding of the origin of the EMC
effect and, indeed, of the structure of atomic nuclei.
