  The Slutsky equation, central in consumer choice theory, is derived from the
usual hypotheses underlying most standard models in Economics, such as full
rationality, homogeneity, and absence of interactions. We present a statistical
physics framework that allows us to relax such assumptions. We first derive a
general fluctuation-response formula that relates the Slutsky matrix to
spontaneous fluctuations of consumption rather than to response to changing
prices and budget. We then show that, within our hypotheses, the symmetry of
the Slutsky matrix remains valid even when agents are only boundedly rational
but non-interacting. We then propose a model where agents are influenced by the
choice of others, leading to a phase transition beyond which consumption is
dominated by herding (or `"fashion") effects. In this case, the individual
Slutsky matrix is no longer symmetric, even for fully rational agents. The
vicinity of the transition features a peak in asymmetry.
