Invariant Measures Concentrated on Countable Structures

Let L be a countable language. We say that a countable infinite L -structure M admits an invariant measure when there is a probability measure on the space of L -structures with the same underlying set as M that is invariant under permutations of that set, and that assigns measure one to the isomorphism class of M . We show that M admits an invariant measure if and only if it has trivial definable closure, that is, the pointwise stabilizer in Aut(M) of an arbitrary finite tuple of M fixes no additional points. When M is a Fraïssé limit in a relational language, this amounts to requiring that the age of M have strong amalgamation. Our results give rise to new instances of structures that admit invariant measures and structures that do not.