This article develops the familiar metaphor of teaching as performance towards a definition of "teaching as performative act," where words and actions aim to effect cognitive, affective, and behavioral changes in learners. To what extent, however, are the consequences of pedagogical actions commensurate with their intended effects? Can a science of "effective teaching" effectively delineate, ascertain, and predict the effects of teachers' pedagogical practices? Through the lens of speech act theory, I argue that teaching consists of "pedagogical perlocutions"--speech acts whose observed and unobserved effects on learners exceed authorial intention and scientific prediction. Attempts to subdue this excess of effects lend themselves to definitions of teacher effectiveness scripted by the instruments and institutions of scientifically based research. I conclude by considering the ways in which these definitions of "effects" and "effectiveness" are themselves the performative effects of performance-based teacher assessment regimes. (Contains 9 notes and 3 tables.)