Zilch currents, supersymmetry and Kaluza-Klein consistency

We give a simple explanation for the fact that one can always perform a consistent truncation of a normal second-order Kaluza-Klein supergravity theory to the irreducible massless graviton supermultiplet. The basic reason for consistency is the highly restrictive requirement that massive fields of spin s≥1 couple at lowest order to conserved currents constructed from field strengths of the massless fields. In particular, one understands in this way the consistency of the truncation of D=11 supergravity compatified on the seven-sphere to d=4, N=8 gauged supergravity. (orig.)