Building on previous research on the tendency in students of diverse ages to overrely on proportionality in different domains of mathematics (e.g., geometry, probability), this study shows that--when confronted with missing-value word problems--Flemish primary school pupils strongly tend to apply proportional solution strategies, also in cases where they are not applicable. The evolution of this tendency is also investigated. It appeared that the overreliance on proportionality already emerges in the 2nd grade, but it increases considerably up to the 5th grade. [For complete proceedings, see ED489597.]