Several studies reported systematic and operation-specific errors in adults’ mental arithmetic, such as overestimation of addition and underestimation of subtraction outcomes (reviews in Mioni et al., 2021; Shaki et al., 2018). We argue that this pattern reverses true calculation signatures because encoding- and response-related errors distorted proper assessment. Here we use symbolic multi-digit stimuli and responses to avoid such peripheral error induction. We control result sizes and, for the first time, manipulate operand position through calculation instructions (e.g., “29+19” vs. “Add 19 to 29”). Across two experiments with either auditory operands (N=30) or visual operands (N=30), we identify arithmetic anchoring bias in both addition and subtraction: The larger of two operands induces calculation errors independent of its position, generating overestimation of subtraction and underestimation of addition. Our finding with task-relevant anchors extends the well-known anchoring bias into the domain of mental arithmetic and eliminates several competing theories about the origin of mental calculation biases.

References:
Mioni, G., Fischer, M. H., & Shaki, S. (2021). Heuristics and biases in the mental manipulation of magnitudes: Evidence from length and time production. Quarterly Journal of Experimental Psychology, http://dx.doi.org/10.1177/1747021820967663
Shaki, S., Pinhas, M., & Fischer, M. H. (2018). Heuristics and biases in mental arithmetic: Revisiting and reversing operational momentum. Thinking & Reasoning, 24(2), 138-156. http://dx.doi.org/10.1080/13546783.2017.1348987