A robust finding across research on early childhood educational interventions is that the treatment effect diminishes over time, with children not receiving the intervention eventually catching up to children who did. One popular explanation for fadeout of early mathematics interventions is that elementary school teachers may not teach the kind of advanced content that children are prepared for after receiving the intervention, so lower-achieving children in the control groups of early mathematics interventions catch up to the higher-achieving children in the treatment groups. An alternative explanation is that persistent individual differences in children's long-term mathematical development result more from relatively stable pre-existing differences in their skills and environments than from the direct effects of previous knowledge on later knowledge. We tested these two hypotheses using data from an effective preschool mathematics intervention previously known to show a diminishing treatment effect over time. We compared the intervention group to a matched subset of the control group with a similar mean and variance of scores at the end of treatment. We then tested the relative contributions of factors that similarly constrain learning in children from treatment and control groups with the same level of post-treatment achievement and pre-existing differences between these two groups to the fadeout of the treatment effect over time. We found approximately 72% of the fadeout effect to be attributable to pre-existing differences between children in treatment and control groups with the same level of achievement at post-test. These differences were fully statistically attenuated by children's prior academic achievement. [This article was published in "Developmental Psychology" v52 n9 p1457-1469 Sep 2016 (EJ1112297).]