Invariant measures concentrated on countable structures

Understand

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, i.e., the pointwise stabilizer in Aut(M) of an arbitrary finite tuple of M fixes no additional points.
• When M is a Fraisse limit in a relational language, this amounts to requiring that the age of M have strong amalgamation.