Systems and methods for enhanced detection of fraudulent electronic transactions are disclosed. In one embodiment, a system uses the ongoing stream of transactions to construct and maintain a dynamically evolving merchant relationship graph. When a proposed transaction is submitted to the system, the system computes a predicted likelihood that the given account would make a transaction with these characteristics with the given merchant. The graph is used to compute transitive relatedness between merchants which may be indirectly associated with one another, as well as to compute aggregate relatedness, when there are multiple avenues of relationship between two merchants.