We study effective SU(2) gauge field actions that include bound-state contributions from the fermion determinant. Drawing on an analogy with polymers, we construct polaron-extrema--instanton--anti-instanton structures bound by localized fermion-antifermion pairs. We argue that they are the relevant ones when massless fermions are present. As a check on our methods, we redo 't Hooft's calculation by computing instanton zero modes and determinants from the scattering data of exactly solvable one-dimensional problems with Morse-Rosen potentials.