Introduction

Is 14 − 2 = 12 always obvious? Most third graders know the basics of addition and subtraction (Carpenter & Moser, 1984), and solving elementary arithmetic operations is no big deal from this point onwards. We learn from an early age that operations such as 14 – 2 = 12 are always valid, no matter whether one is subtracting apples, cars, or smurfs. However, our claim is that adults whose mathematical knowledge is unquestionable, even outstanding, sometimes fail to solve arithmetic problems involving a single-step solution such as 14 – 2 = 12 when their knowledge about the entities subtracted interferes with the mathematical structure of the problem.

This prediction arises from a growing body of literature suggesting that the daily-life, non-mathematical world knowledge one has about the objects an arithmetic word problem refers to might influence their mathematical representation of the problem and their subsequent choice of a solving strategy. For example, Bassok, Wu, and Olseth (1995) showed that being trained to solve a permutation problem was not always helpful to solve analogous problems. The authors demonstrated that slight, mathematically-irrelevant changes in the semantic relations linking the objects mentioned in the cover stories (e.g., computers assigned to secretaries vs. secretaries assigned to computers) led to significant performance differences. Subsequent research has shown that non-mathematical semantic information related to the entities described in a problem influences lay solvers’ performance (Bassok, Chase, & Martin, 1998; Gros, Sander, & Thibaut, 2016; Thevenot & Barrouillet, 2015; Verschaffel, De Corte, & Vierstraete, 1999; Vicente, Orrantia, & Verschaffel, 2007) as well as strategy choice (Gamo, Sander, & Richard, 2010; Gros, Thibaut, & Sander, 2017) and transfer (Gros, Thibaut, & Sander, 2015) on arithmetic word problems. Most of the available evidence regarding this issue has been collected with children and non-expert adults on problems that were not straightforward (e.g., complex permutation problems). Building on this literature, we propose to go further and show that irrelevant aspects of what we call world semantics (the non-mathematical knowledge about the world that is evoked by the entities described in a specific problem statement) can also mislead experts in mathematics on problems involving basic arithmetic notions, despite them being considered experts in abstract, context-independent reasoning (Dehaene, 2011). We call this proposal the “world semantics view.”

Despite stemming from the aforementioned literature, the claim that world semantics could exert such a pervasive influence and threaten even the highest levels of mathematical expertise is rather innovative, as it challenges the commonly held view in the expertise literature regarding experts’ proficiencies. This expertise view notably considers that experts identify what has been described as the “deep structure” of the problem (Chi, Feltovich, & Glaser, 1981), its “principle” (Ross, 1987), its “objective mathematical structure” (Bassok, 2001), or its “problem space” (Newell & Simon, 1972). This deep structure is independent of the semantics imbued in the problem statement, and as such it is the foundation of experts’ abstract, context-independent reasoning about the problem. Indeed, since by definition mathematics is not empirical and manipulates abstract symbols rather than real-life objects (Davis, Hersh, & Marchisotto, 2011; Russell, 1903), mathematical experts should ignore irrelevant information associated with the entities on which numbers and algorithms operate. They should perceive the deep structure of arithmetic problems that can be solved by simple subtractions (i.e., involving small quantities such as 14 − 2), no matter whether they calculate the price of an apple, the height of a smurf, or the speed of a car. Furthermore, experts are known to show exceptional performance in domain-related tasks (Chi, 2006), they stand out in their ability to generate problem solutions (De Groot, 1965), to detect relevant problem features (Lesgold et al., 1988), to monitor their own comprehension (Chi, 1978), and to qualitatively analyze the task at hand (Voss, Greene, Post, & Penner, 1983) (see Chi, 2006 for a review of experts’ proficiencies). These former studies do not predict that the semantics conveyed by the problem statement could interfere with the experts’ understanding of the problems’ mathematical structure.

We performed two experiments to show that, contrary to this expertise view – but in accordance with the world semantics view – arithmetic problems admitting a single-step solution might pose a challenge to mathematical experts. We presented participants with a series of isomorphic problems involving two numerical values. Crucially, for each problem, a solution was provided (a single subtraction between the problem’s two numerical values), and participants’ task was to evaluate its validity. By varying the semantic, non-mathematical information evoked by the problem statements (e.g., use of an elevator vs. a weighing scale, reference to marbles being won vs. years passing by, mention of hamburger prices vs. statues’ heights, etc.), we intended to show that even math experts are exposed to a deleterious influence of the non-mathematical knowledge evoked by the problem statement.

Our world semantics view predicts that university students (Study 1) – and math experts (Study 2) – will more often fail to recognize the proposed solution when it conflicts with the non-mathematical knowledge about the world evoked by the entities featured in the problem statement than when the solution is consistent with it. Furthermore, it predicts that a recoding process, akin to re-representation (Davidson & Sternberg, 2003; Vicente et al., 2007) is necessary when a problem’s initial encoding leads to a dead end. Indeed, when the semantic content of a problem statement leads participants to interpret the situation in a way that is not compatible with the problem’s solution, then it becomes necessary to build a new representation of the situation congruent with the solution. When successfully performed, such a recoding process should result in longer response times for correct answers conflicting with the problems’ world semantics.

Study 1

Methods

Participants

We recruited 85 adults (50 women, mean age = 23.35 years, SD = 7.82) in the Paris region. All had attended university (mean length of university curriculum = 2.85 years, SD = 1.18), but none majored in mathematics. Considering the low complexity of the math problems involved, participants’ curriculum was a clear indicator that they possessed the mathematical expertise required to solve the problems. Sample size was determined using uncertainty and publication bias correction on results from a previous study (Gros et al., 2016), following Anderson, Kelley, and Maxwell’s recommendations (2017).

Materials

Our materials were inspired by Gamo et al. (2010), who showed that problems with the same formal mathematical structure are nevertheless preferentially solved with one of two available solving strategies, depending on the semantic content of the problem. Consider the weight problem in Table 1: this problem can be solved through two strategies. One is a three-step algorithm consisting of calculating the weight of each individual dictionary to compute the weight of the stack of dictionaries Lola is carrying: 14 – 5 = 9; 5 – 2 = 3; 9 + 3 = 12. The other one is a one-step algorithm that requires understanding that since Lola and Joe carry the same Spanish dictionary, calculating the weight of each book is unnecessary. Since the German dictionary is 2 kg lighter than the Russian dictionary, the weight difference between Joe’s and Lola’s books is of 2 kg as well: 14 – 2 = 12.

Table 1 Two isomorphic problems sharing the same mathematical structure but evoking different aspects of our knowledge about the world
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The duration problem in Table 1 has the same mathematical structure and can be solved using the same solving procedures. However, Gamo et al. (2010) showed that the two solving procedures are not randomly distributed across the two types of problems. Participants favor the three-step algorithm on problems like the dictionary problem (called cardinal problems) and the one-step algorithm on the second type of problems (called ordinal problems). This strategy using imbalance was our starting point. Gamo et al. (2010) and Gros et al. (2017) showed that the differences in the world semantics evoked by the problems resulted in different spontaneous encodings of the situations, from which this imbalance originatedFootnote 1 (see Fig. 1 for a description of this effect). Since cardinal and ordinal problems shared the same structure featuring the same parts and wholes presented in the same order with the same numerical values, the imbalance in strategy use could only be attributed to the variations of the semantic content of the problem statements. Additionally, when considering the correct answers on either algorithm there was no significant difference in adults’ performance between cardinal and ordinal problems, which indicates that the strategy imbalance was not a matter of problem difficulty (Gros et al., 2017).

Fig. 1
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Implementation of the mathematical structure with ordinal versus cardinal quantities, leading to different problem statements, representations, and strategy use

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Gros et al. (2017) have shown that most adults encode collection, price, and weight problems as cardinal representations, whereas they encode duration, distance, and floor problems as ordinal representations. We modified their problems and removed the value of Part 1 so that the three-step strategy could not be used (see Table 2). Consequently, the only solution left was the one-step strategy, which required using the values of Whole 1 and of the Difference (see Fig. 1). The constructed materials are available online (https://osf.io/fxgqh/?view_only=ed1374ef4d204c90a0cb03a30cb0a099). Ordinal problems were 333.5 characters long on average (SD = 38.37) and cardinal problems were 304 characters long on average (SD = 44.94). This length difference was not statistically significant (t(10) = 1.18, p = .26, paired t-test). Crucially, for each problem, participants were presented with the correct one-step solution (e.g. “14 – 2 = 12; Jolene has 12 marbles”). Participants’ task was to decide whether the provided solution worked, or whether there was no solution to the problem. Due to the already established imbalance in strategy use between problems evoking a cardinal encoding and problems evoking an ordinal encoding (Gamo et al., 2010; Gros et al., 2017), we assumed that the measure of participants’ ability to use the only remaining strategy on problems evoking different aspects of world semantics would be an effective assessment of the robustness of these effects.

Table 2 Example of target problems used in the study. Changes introduced from Gros el al.’s (2017) problem statements are italicized in the table for the sake of clarity, but they were not made apparent in the experiment. Translated from French
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The world semantics hypothesis predicts lower performances on cardinal than on ordinal problems, even among experts, because cardinal problems would require a re-representation of the situation when the only solution available is the one-step algorithm. By contrast, ordinal problems should be easier to solve because participants’ spontaneous encoding facilitates the use of the one-step algorithm. Since university-educated adults can be considered experts in solving subtractions such as 14 – 2 = 12, and since the deep structure of a problem is identical regardless of the objects involved, this prediction could not be made without the world semantics view, especially when participants only need to check the validity of the proposed solution. Additionally, we predict that recoding a situation initially encoded as a combination of subsets (such as a cardinal encoding) into a representation in terms of states and transitions between states (such as an ordinal encoding) is a costly process, requiring a longer response time. Although our hypotheses only regard solvable problems, we also included unsolvable distractors in the materials, so that the correct answer would not always be “This problem can be solved.” Among those distractors the value of Whole 1 was removed instead of the value of Part 1, which rendered the problems unsolvable with either algorithm.

Procedure

Participants answered the questions using three keyboard keys on a 17-in. laptop. Instructions stated that “Some of the problems can be solved using the values provided, while other problems cannot be solved with the available information. Your task is to tell apart problems that can be solved from problems that cannot. Answer as quickly as you can, although being correct is more important than being fast.”

Participants were presented with six target problems that were only solvable with the one-step algorithm: three cardinal and three ordinal problems. An equal number of distractors was introduced to fulfill subjects’ expectations regarding the uniform distribution of yes/no answers. Problem order, cover stories, and numerical values were randomized between participants. The value of Whole 1 was between 11 and 15, Whole 2 between 5 and 9, and the Difference was either 2 or 3.

We used a segmented self-presentation procedure displaying the text line by line on the screen when participants pressed the spacebar. Below, a question appeared: “Given the data provided, is it possible to find the solution?” followed by two possible choices: “(A) No, there is not enough information to find the solution.” “(B) Yes, and the following solution is correct:” (followed by, in the case of the marble problem: “14 – 2 = 12. Lucy has 12 marbles in total”). A solution was proposed for each problem, and it was up to the participants to assess whether it was valid or whether the problem was unsolvable.

Results

Data collected for both studies are available online (https://osf.io/fxgqh/?view_only=ed1374ef4d204c90a0cb03a30cb0a099). The dependent variable was the proportion of correct answers for solvable problems (see Fig. 2). Because multiple binary data points were recorded in a repeated design (each participant provided a binary answer to three ordinal and three cardinal solvable problems), the use of repeated measures ANOVA was deemed inappropriate and replaced by a mixed model (Hector, 2015). We used a generalized linear mixed model with a binary distribution, with the cardinal versus ordinal semantic nature of the problems as a fixed factor, and participants as a random effect. In line with our hypothesis, lay adults performed significantly better on ordinal (81.18%) than on cardinal problems (46.67%); z = 7.84, p < .001, R2GLMM(c) = .29.Footnote 2 Additionally, looking at individuals’ response patterns showed us that 65.9% of the participants made fewer mistakes on ordinal than on cardinal problems, 11.8% made no mistakes at all, 15.3% made the same number of mistakes in cardinal and in ordinal problems, and only 7.1% made more mistakes on ordinal than on cardinal problems.

Fig. 2
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Distribution of adults’ answers. *** p < .001

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Further analyses were conducted on participants’ response times (RTs) on solvable problems that had been successfully identified as such by the participants (see Fig. 3). Because the number of correct answers could vary from 0 to 6 for each participant, the number of RT data points varied accordingly, and the use of repeated-measures ANOVA was again deemed inappropriate (Hector, 2015). A linear mixed model with subjects as a random effect and semantic nature of the problems as a fixed factor showed that participants took more time to correctly solve cardinal (M = 34.05, SD = 18.78) than ordinal problems (M = 26.85, SD = 12.49), χ2 (1) = 29.14, p < .001, R2LMM(c) = .44. Additionally, we studied the participants’ individual response patterns to identify whether different participant profiles existed. For each participant, we computed the difference between their mean RTs on correctly solved cardinal and ordinal problems (see Fig. 4) and we performed Hartigan’s dip test for unimodality versus multimodality on the resulting distribution (Hartigan & Hartigan, 1985). The analysis failed to reject the null hypothesis that participants’ responses came from a unimodal distribution (D = .028, p = .94), thus providing no empirical ground to assume that the distribution of response times was multimodal.

Fig. 3
figure 3

Violin plot of adults’ response times (RTs) on correctly identified solvable problems. Middle bars indicate mean RTs; upper and lower bars indicate margins of .95 confidence intervals. *** p < .001

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Fig. 4
figure 4

Distribution of individual differences between cardinal response time (RT) and ordinal RT on correctly solved problems. Bins below the zero value indicate participants whose ordinal RT were higher than their cardinal RT on average, whereas bins above zero indicate participants whose ordinal RT were lower than their cardinal RT on average

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Discussion

The difference in performance between cardinal and ordinal problems indicates that despite their expertise regarding basic subtractions, the adults’ answers were significantly influenced by the semantic content of the problem statements. This confirms previous results obtained with the “complete” version of the problems that could be solved either with the three-step algorithm or with the one-step algorithm (Gamo et al., 2010; Gros et al., 2017). Here, we showed that the strategy imbalance observed in these previous studies was not an effect of mere preference for one strategy over another, but an actual impossibility to identify the relevance of the one-step algorithm on cardinal problems, as attested by the fact that on these problems more than half of the participants rejected a perfectly valid solution, despite only needing to check its validity. Regarding RTs, the fact that correct answers took more time on cardinal problems suggests that recognizing the solution to a problem evoking aspects of world semantics seemingly incompatible with the solution required an extra processing step. This is also supported by the fact that there was no significant difference in length between cardinal and ordinal problems. This is in line with the recoding process we predicted. These results show that the semantic content of a problem can prevent university-educated adults from recognizing a simple subtraction as the solution to a problem whose mathematical structure is undoubtedly within their level of expertise. We designed a second study to identify whether such effects would remain with expert mathematicians, known to be especially accustomed to abstract reasoning.

Study 2

Methods

Participants

We recruited 25 experts (two women, mean age = 23.59 years, SD = 2.81) who had successfully passed the entrance exam of the Science section at the École Normale Supérieure (ENS Ulm) in Paris. This exam is considered as the most demanding one in France, with an entrance rate of 2.02% among university-educated participants (“SCEI Statistics”, 2017). The ENS ranked second in Times Higher Education’s World University Rankings 2016–2017 for Best Small University (Bhardwa, 2017). Although the population sample was smaller than in the first study due to the number of graduates from École Normale Supérieure being limited, sample size was deemed sufficient using uncertainty and publication bias correction on results from a previous study (Gros et al., 2016), following Anderson et al.’s recommendations (2017).

Materials and procedure

Materials and procedure were identical to that of Study 1.

Results

As in Study 1, we analyzed the proportion of correct answers on solvable problems (see Fig. 5) with a generalized linear mixed model. Experts had a higher success rate on ordinal (94.67%) than on cardinal problems (76.00%); z = 2.99, p = .0028, R2GLMM(c) = .25. Additionally, a comparison with Study 1 showed that Study 2 experts’ performance (85.33%) was significantly higher than Study 1 adults’ performance (63.92%), which was another confirmation of their outstanding expertise in mathematics; z = 4.49, p < .001, R2GLMM(c) = .33. Looking at individuals’ response patterns also indicated that 52.0% of the participants made fewer mistakes on ordinal than on cardinal problems, 36.0% made no mistakes at all, 4.0% (one participant) made the same number of mistakes in cardinal and in ordinal problems, and only 8.0% made more mistakes on ordinal than on cardinal problems.

Fig. 5
figure 5

Distribution of experts’ answers. ** p < .01

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Analyses were conducted on participants’ RTs for correctly identified solvable problems (see Fig. 6). As in Study 1, we used a linear mixed model that showed that experts took significantly more time to correctly solve cardinal problems (M = 26.58, SD = 14.03) than ordinal problems (M = 19.45, SD = 8.18), as predicted by our world semantics hypothesis; χ2 (1) = 18.65, p < .001, R2LMM(c) = .37. Unsurprisingly, experts’ RTs on correct answers were significantly shorter (M = 22.63, SD = 11.68) than in Study 1 (M = 29.50, SD = 15.48); χ2 (1) = 7.68, p = .0056, R2LMM(c) = .46. As in Study 1, the computation of individual differences in RTs between cardinal and ordinal problems showed no sign of multimodality (see Fig. 7), and Hartigan’s dip test for unimodality versus multimodality failed to reject the null hypothesis of unimodality (D = .048, p = .96).

Fig. 6
figure 6

Violin plot of experts’ response times (RTs) on correctly identified solvable problems. Middle bars indicate mean RTs; upper and lower bars indicate margins of .95 confidence intervals. *** p < .001

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Fig. 7
figure 7

Distribution of individual differences between cardinal response time (RT) and ordinal RT on correctly solved problems. Bins below the zero value indicate participants whose ordinal RT were higher than their cardinal RT on average, whereas bins above zero indicate participants whose ordinal RT were lower than their cardinal RT on average

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Discussion

Despite their superior performances, high-level mathematicians were still significantly influenced by world semantics. Their performance dropped significantly on cardinal problems, and correct answers required more time on average on cardinal than on ordinal problems. Therefore, despite their proficiency in abstract mathematical reasoning, expert mathematicians failed to disregard irrelevant non-mathematical information when solving the problems, as hypothesized.

General discussion

In this paper, we sought to demonstrate that irrelevant aspects of our non-mathematical knowledge evoked by the semantic content of a problem statement can lead both adults and mathematics experts to encode the problem in such a way that they would erroneously consider valid solutions as incorrect. Indeed, participants failed to identify the solvability of subtraction problems admitting a single-step solution significantly more often when the world semantics they evoked conflicted with the relevant mathematical information, than when the two were congruent. Additionally, correct answers took more time in the conflicting than in the congruent case for both populations, suggesting that the initial spontaneous representation triggered by the semantic content of the problem statement had to be recoded. Although they achieved higher performances overall, high-level experts still rejected several perfectly valid solutions: they fell prey to robust effects of world semantics that current theories of expertise do not account for.

There is a much larger body of literature describing in what terms experts excel in their field than there are studies revealing experts’ shortcomings. However, as Chi (2006, p. 23) stressed, “it is equally important to understand how experts fail,” which was one of the goals of this paper. A few limitations have already been shown to occasionally affect experts’ excellence (see Chi, 2006, for a review). For instance, experts’ proficiencies are limited to their domain of expertise (Ericsson & Lehmann, 1996) and they lack adaptability to irregular situations whose structures differ from what they expect (Sternberg & Frensch, 1992). They have even been shown to gloss over details (Voss, Vesonder, & Spilich, 1980), which paradoxically suggests that they should be good at ignoring surface properties unrelated to the formal structure of the problems. More recent works have even hinted at biases slowing down experts within their own domain of expertise (Goldberg & Thompson-Schill, 2009; Obersteiner, Van Dooren, Van Hoof, & Verschaffel, 2013). However, we believe none of these accounts would have predicted our results, since they do not explain how mathematically irrelevant contextual information may significantly hinder experts’ abstract reasoning on problems within their very field of expertise, to the extent that they would not identify the validity of the solution handed out to them. Here, mathematical experts failed to do what they are good at: engaging in abstract reasoning on concrete entities to find a single-step solution. Our results suggest that when mathematical knowledge and world semantics conflict with one another, masters of abstraction can run into a concrete wall.

This effect is understandable since world semantics and mathematical knowledge often (although not always) naturally align with each other, which explains how some superficial cues are highly correlated with deeper principles (Bassok, Pedigo, & Oskarsson, 2008; Blessing & Ross, 1996). It follows that solvers rely on those cues at all levels and tend to make mistakes when world and mathematical semantics do not align. Overall, it seems that these effects of semantic (in)congruence between world semantics and mathematical knowledge have been greatly undermined on the account of mathematics being an inherently abstract domain in which rules and concepts are valid independently from the objects they are applied to. Our results show how prevalent the influence of world knowledge is on arithmetic reasoning, even among the individuals who should be the least subject to it. This suggests that experts will never be completely freed from the influence of world knowledge; having an outstanding level in mathematics is not enough to systematically perceive that 14 – 2 = 12.