Background: The purported reciprocity between motivation and academic achievement in education has largely been supported by correlational data. Aims: Our first aim was to determine experimentally whether motivation and achievement are reciprocally related. The second objective was to investigate a potential behavioural mediation pathway between motivation and achievement by measuring the objective effort expended on learning. Finally, we studied the causality of these relations by analysing the dynamics between motivation and achievement (rather than examining them as individual constructs) when perceived achievement was experimentally manipulated. Sample(s): The study employed a short-timeframe experiment in which 309 Dutch undergraduate students (M[subscript age] = 19.89, SD = 2.08) learned new English vocabulary. Methods: Their motivation, effort, and achievement were measured at multiple time points within one hour. Midway through the experiment, participants received manipulated feedback indicating an achievement decline, which was expected to influence their subsequent motivation, effort, and actual achievement. A random-intercept cross-lagged panel framework was employed to model how one construct influenced another over time. Results: We found a unilateral effect of achievement on motivation (i.e., no reciprocity), which remained stable across the time points. Our experimental manipulation partially supported a causal interpretation of the unilateral achievement[right arrow]motivation pathway. Additionally, no mediation effect of effort was identified: motivation was not associated with effort, nor was effort linked to achievement. Conclusions: Our findings underscore the importance of further exploration of behavioural mediation pathways, a broad operationalization of motivation, and the application of appropriate modelling strategies to investigate the motivation-achievement reciprocity.