The objective of this study was to examine how individual interest and knowledge acquisition are causally related. Three hypotheses were tested using a cross-lagged panel analysis (N = 186) and two quasi-experimental studies (N = 68 and N = 108) involving students from schools in Singapore. The first hypothesis is the broadly shared standard assumption on the relation between individual interest and knowledge: the more an individual is interested in a topic, the more (s)he is willing to engage in learning. An alternative hypothesis assumes that individual interest is not the cause but the consequence of the process of learning: individual interest as an affective by-product of learning. Finally, a third possibility is that interest and knowledge influence each other reciprocally. The results supported the affective-by-product hypothesis. Our findings seem at variance with commonly held conceptions that being interested guides knowledge attainment. The implications of these findings for interest research are discussed.