A recent set of studies (Muenks, Miele, & Wigfield, 2016) introduced the concept of perceived effort source to better explain how students reason about the relation between effort and ability when evaluating the academic abilities of other students. These studies showed that participants who were induced to perceive effort as task-elicited (i.e., as being primarily due to the subjective difficulty of the task) were more likely to view effort and ability as inversely related than participants who were induced to perceive effort as self-initiated (i.e., as being due to students’ motivation to go beyond the basic demands of the task). The current studies expanded on this research by demonstrating that, in the absence of an effort source manipulation, college students spontaneously invoked beliefs about the source of effort when evaluating their own (Study 2) and other students’ (Studies 1–3) abilities. The three studies also showed that our novel measure of individual differences in effort source beliefs was a better predictor of participants’ judgments of math ability (Studies 1 and 2) and verbal ability (Study 3) than a standard measure of their ability mindsets (i.e., beliefs about the extent to which intelligence is malleable). Specifically, participants who naturally tended to perceive effort as task-elicited generally rated students who expended relatively little effort as having more ability than did participants who tended to perceive effort as self-initiated. Implications for research on student motivation and for education practice are discussed.