To bring order in concepts related to individual learner differences, Curry (1983) designed the three-layered onion model. As this model provides an interesting way to distinguish related concepts--such as cognitive styles and approaches to studying--on the basis of their stability in learning situations, ample studies build further on this model. Given that only few studies have been conducted to empirically test the model, we conducted two studies to address this research gap. In the first study (N = 113), analyses do not show a clear causal path from three concepts belonging to different layers in relation to learning outcomes. In the second, longitudinal study (N = 162), no support is found for differences in stability between cognitive styles and approaches to studying. Our research does not provide solid evidence for the onion model, which warns to be cautious with applying the assumptions of theoretical models in educational practice without empirical support. (Contains 3 tables and 1 figure.)