  We study the critical spin-polarization energy ($\alpha_{\rm C}$) above which
fractional quantum Hall states in two-dimensional electron systems confined to
symmetric GaAs quantum wells become fully spin-polarized. We find a significant
decrease of $\alpha_{\rm C}$ as we increase the well-width. In systems with
comparable electron layer thickness, $\alpha_{\rm C}$ for fractional states
near Landau level filling $\nu=3/2$ is about twice larger than those near
$\nu=1/2$, suggesting a broken particle-hole symmetry. Theoretical
calculations, which incorporate Landau level mixing through an effective
three-body interaction, and finite layer thickness, capture certain qualitative
features of the experimental results.
