Understanding proportions is a time intensive process that does not come cheap during late childhood. Children have to undergo many informal learning opportunities before they can gain from direct instruction on proportional reasoning. In this study, we want to find out whether physics curricula dealing with the concept of density will prepare for learning from a curriculum on proportional reasoning. A 2x2 design with the factors “Physics curricula” (with, without)and “Concept used to introduce proportional reasoning” (speed, density) was applied to altogether 253 children coming from 12 classrooms at the beginning of grade 5. We expected the group “density, with physics curriculum” to outperform the other three groups. It turned out, however, that only the students who scored on the highest quartile of an intelligence measure gained from the prior knowledge acquired in the physics curricula. More capable students can boost their proportional reasoning by prior physics learning.