We investigate Hamiltonians with attractive interactions between pairs of fermions coupled to angular momentum
J. We show that pairs with spin
J are reasonable building blocks for the low-lying states. For systems with only a
J=Jmax pairing interaction, eigenvalues are found to be approximately integers for a large array of states, in particular, for those with total angular momenta $I\ensuremath{\leqslant}2j$. For
I=0 eigenstates of four fermions in a single-
j shell we show that there is only one nonzero eigenvalue. We address these observations using the nucleon pair approximation of the shell model and relate our results with a number of currently interesting problems.