On the problem-size effect in small additions: Can we really discard any counting-based account?

• We collected RTs on small additions (operands from 1 to 4) on a large sample (n = 91).
• We shed light on the puzzling size effect classically observed in mental arithmetic.
• Results challenge received retrieval-based accounts of this effect.
• We propose a new counting-based explanation of the problem size effect.
• Individual differences in the manifestation of the effect reinforce our conclusions.

The problem-size effect in simple additions, that is the increase in response times (RTs) and error rates with the size of the operands, is one of the most robust effects in cognitive arithmetic. Current accounts focus on factors that could affect speed of retrieval of the answers from long-term memory such as the occurrence of interference in a memory network or the strength of memory traces that would differ from problem to problem. The present study analyses chronometric data from a sample of 91 adults solving very small additions (operands from 1 to 4) that are generally considered as being solved by retrieval. The results reveal a monotonic linear increase in RTs with the magnitude of both operands. This pattern is at odds with the retrieval-based accounts of the problem-size effect and challenges the well-established view that small additions are solved through retrieval of the answer from long-term memory. Our results are more compatible with the hypothesis that even very small additions are solved using compacted fast procedures that scroll an ordered representation such as a number line or a verbal number sequence. This interpretation is corroborated by the analysis of individual differences.