Robert Nola (2003) has argued that anti-rationalist interpretations of science fail to adequately explain the process of science, since objective reasons can be causal factors in belief formation. While I agree with Nola that objective reasons can be a cause of belief, in this paper I present a version of the strong programme in the sociology of knowledge, the Interests Thesis, and argue that the Interests Thesis provides a plausible explanation of an episode in the history of ape-language research. Specifically, I examine Terrace, Petitto, Sandess, & Bever (1979, 1980) illegitimate comparison of the signing of their chimpanzee, Nim, with data from human early childhood language development, and argue that Terrace et al.'s interests played a causal role in determining their sceptical beliefs concerning ape linguistic abilities. However, I go on to argue that Terrace et al.'s interests are not the only causal factors in determining their beliefs: objective reasons, associated with the institution of new methodologies, were also causally determinative of Terrace et al.'s sceptical beliefs. Consequently, I argue that belief formation in science is a multi-factorial affair wherein both interests and objective reasons have causal roles. I finish the paper with two conjectures concerning the proper locus of scientific rationality.