The quantitative validity of asymptotic particlendash;hole symmetry in a fluid at its liquidndash;vapor critical point is determined by means of the exact mapping of the fluid Hamiltonian onto that of an effective Landaundash;Ginzburgndash;Wilson model studied first by Hubbard and Schofield. A particular threehyphen;particle correlation of a reference fluid is identified as that which controls the breaking of liquidndash;vapor symmetry, as manifested in a linear mixing of the pure Isinghyphen;like scaling fields and a singularity in the coexistence curve diameter. The inherent smallness of the mixing coefficient in a pairhyphen;potential fluid is shown to reflect the weak density dependence of the second moment of the twohyphen;particle direct correlation function of the reference system. It is further demonstrated that threehyphen;body interactions of the Axilrodndash;Tellerhyphen;type enhance the broken particlendash;hole symmetry found in a purely pairwisehyphen;additive Hamiltonian, and detailed calculations give diameter anomaly amplitudes which vary linearly with the fluid polarizability, in quantitative agreement with recent experiments.
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