Statechart diagrams are a popular modeling tool amongst designers as it provides the capabilities to model the behavior of a state-dependent system or system component. Statechart diagrams are one of the pillar diagram types of the Unified Modeling Language (UML), the de-facto modeling language for object-oriented systems, used to model requirements of a system. Although statechart diagrams have received great attention from the research community, there lacks research that evaluate the cognitive effectiveness of state transition diagrams. This paper presents a scientific evaluation of the cognitive effectiveness of UML statechart diagrams. The results of the evaluation reveal several problems with the UML statechart notation that reduces their cognitive effectiveness and that should be addressed.