Identifying distinct sources of whole number interference in children's decimal comparison: the role of numerical magnitude and inhibitory control

Whole number knowledge can be a detriment to learning decimals, leading children to incorrectly report, for example, that 0.26 > 0.8. Two sources of whole number interference could lead to such errors: Digit length interference (thinking more digits = larger number), and whole referent magnitude interference (26 > 8). It is yet unknown how each interference type affects decimal learning, or how inhibitory control, linked to rational number achievement, supports overcoming either interference. We also examined the predictions of two accounts of learning counterintuitive knowledge. The conceptual change account predicts an abrupt shift in decimal understanding in students with high inhibitory control, while dual process account predicts gradual decimal improvement supported by inhibitory control. Students in two U.S. school districts (grades 6-8, n = 178) completed computerized decimal comparison and inhibitory control tasks. Students’ decimal performance fell into groups, confirmed with cluster analysis. The two most prominent groups, which we named Whole Number Bias and High Performing, differed in their extent of digit length interference, rational magnitude processing, and inhibitory control, aligning with the conceptual change account. However, in High Performing only, inhibitory control correlated with less digit length interference, aligning with the dual process account. In contrast to digit length, both groups were comparably susceptible to whole referent magnitude interference, and inhibitory control did not correlate with such susceptibility. Our findings suggest learning decimals involves first an abrupt shift in conceptual understanding followed by further gradual improvement, where inhibitory control supports overcoming digit length but not whole referent interference.