An associative Boolean tree is a plane rooted tree whose internal nodes are labelled by and or or and whose leaves are labelled by literals taken from a fixed set of variables and their negations. We study the distribution induced on the set of Boolean functions by the uniform distribution on the set of associative trees of a large fixed size, where the size of a tree is defined as the number of its nodes. Using analytic combinatorics, we prove a relation between the probability of a given function and its tree size complexity.