  We measure the effective mass (m*) of interacting two-dimensional electrons
confined to an AlAs quantum well while we change the conduction-band valley
occupation and the spin polarization via the application of strain and magnetic
field, respectively. Compared to its band value, m* is enhanced unless the
electrons are fully valley and spin polarized. Incidentally, in the fully spin-
and valley-polarized regime, the electron system exhibits an insulating
behavior.
