What we know about the Theoretical Foundation of Duality in Electron   Scattering
  We consider some of the things that we understand about the theoretical underpinnings of duality, including items such as why the resonance peak/background ratio is constant in general, why it falls for the $\Delta(1232)$, what we might expect for scaling and duality with longitudinal or spin-dependent structure functions, and what kind of scaling or duality we might expect for semiexclusive processes. 
Carl E. Carlson (W & M)