  Different mechanisms have been proposed to account for the spin-dependent
interaction between quarks in ground-state hadrons. The first mechanism is the
chromomagnetic interaction arising from one-gluon exchange, a second mechanism
is an interaction arising from meson exchange, and a third from instanton
effects. Some lattice calculations favor the first mechanism for heavy quarks
and the third for light quarks. An argument is presented in favor of the
one-gluon exchange mechanism. However, thus far, nobody has performed a crucial
experiment that can definitively distinguish between these mechanisms.
