Spin-dependent forces between quarks in hadrons
  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. 
D.B. Lichtenberg