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THINKING & REASONING, 2005, 11 (4), 382 389 Rapid responding increases belief bias: Evidence for the dual-process theory of reasoning Jonathan St. B. T. Evans and Jodie Curtis-Holmes University of Plymouth, UK In this study, we examine the belief bias effect in syllogistic reasoning under both standard presentation and in a condition where participants are required to respond within 10 seconds. As predicted, the requirement for rapid responding increased the amount of belief bias observed on the task and reduced the number of logically correct decisions, both effects being substantial and statistically significant. These findings were predicted by the dual-process account of reasoning, which posits that fast heuristic processes, responsible for belief bias, compete with slower analytic processes that can lead to correct logical decisions. Requiring rapid responding thus differentially inhibits the operation of analytic reasoning processes, leading to the results observed. Dual-process theories of reasoning have been gaining in popularity in recent years (Evans, 2003; Evans & Over, 1996; Kahneman & Frederick, 2002; Sloman, 1996; Stanovich, 1999, 2004). These theories posit two distinct processes of reasoning that compete for control of the response that participants make in reasoning tasks. Heuristic or System1 processes are characterised as rapid, implicit, associative, and heavily contextualised, whereas analytic or System2 processes are described as slow and sequential but capable of abstraction and generalisation. Note that a key difference is the speed of processing. The reason for this is that analytic processing is a sequential process requiring use of central working memory and is constrained by its limited capacity. By contrast, heuristic processes operate through massively parallel implicit systems that exert an unconscious influence on responding. In support of this distinction is evidence of substantial correlations between general intelligence and working memory Correspondence should be addressed to Jonathan St. B. T. Evans, Centre for Thinking and Language, School of Psychology, University of Plymouth, Plymouth PL4 8AA, UK. Email: j.evans@plymouth.ac.uk # 2005 Psychology Press Ltd http://www.tandf.co.uk/journals/pp/13546783.html DOI: 10.1080/13546780542000005

RAPID RESPONDING AND BELIEF BIAS 383 capacity with abstract deductive reasoning but not pragmatic, belief-based reasoning (Stanovich, 2004). While a mass of evidence has been recorded supporting the idea that there are two different processes of reasoning, more research is needed to confirm the characteristics that theorists have attributed to the cognitive systems that may underlie these observations. A relevant methodological innovation reported by Roberts and Newton (2002) is the rapid-response reasoning task. The idea is that by constraining participants to respond within a short period of time, the slower analytic process of reasoning will be differentially inhibited. For example, on the Wason selection task, there is a wellestablished non-logical tendency, known as matching bias (Evans, 1998), to select cards that match the explicit content of conditional statements, regardless of the presence of negations. Roberts and Newton (2002) showed that measures of matching bias were significantly increased when a rapidresponse version of the selection task was compared with a free-time version. This is as dual-process theory would predict, since any influence of analytic reasoning to inhibit the bias would be suppressed by the requirement to respond quickly. A central phenomenon in dual-process accounts of reasoning is that of belief bias the tendency to evaluate the validity of an argument on the basis of whether or not one agrees with the conclusion, rather than on whether or not it follows logically from the premises. The phenomenon is one of the earliest reported in the psychology of reasoning (Wilkins, 1928) but the modern study of the effect dates from the paper of Evans, Barston, and Pollard (1983), which established the phenomenon with all relevant experimental controls. Evans et al. showed that participants evaluations of conclusions were substantially affected both by the logical validity of the arguments and by the believability of the conclusions. With the help of protocol analyses Evans et al. characterised the effect as involving a within-participant conflict between logic-based (analytic) processes and belief-based (heuristic) processes. Such a conflict is to be expected in view of contemporary dual-process theories. In support of this, it has been shown that participants high in general intelligence are more able to resist belief biases (Stanovich & West, 1997) and that logical and belief-based responding are neurologically differentiated (Goel & Dolan, 2003). In this study, we extend the rapid-response methodology of Roberts and Newton (2002) to the study of belief bias in syllogistic reasoning. Using a conclusion-evaluation paradigm with problems similar to those of Evans et al. (1983), we compare performance under rapid-response and free-time tasks. We predict that the participants required to respond rapidly will show (a) an increased level of belief bias and (b) a reduced level of logical responding.

384 EVANS AND CURTIS-HOLMES Participants and design METHOD A total of 50 undergraduate students of the University of Plymouth participated, with 25 randomly assigned to each of two experimental groups. The groups differed in whether a rapid-response or free-time version of the task was presented, but each attempted to solve the same 12 syllogisms. Within-participant factors were the logic of the arguments (valid or invalid) and the believability of the conclusions presented (believable or unbelievable). Materials and procedure The syllogisms presented fell into four categories: Valid-Believable (VB), Valid-Unbelievable (VU), Invalid-Believable (IB), and Invalid-Unbelievable (IU). As in the study of Evans et al. (1983), two logical forms were used throughout: 1 No A are B; Some C are B, therefore Some C are not A (valid) 2 No A are B; Some C are B, therefore Some A are not C (invalid) Four actual syllogisms, one of each type, were taken from the materials used by Evans et al. (1983) and, in addition, eight new syllogisms were devised for this study, so that there were three repetitions of each type. An example of a Valid-Unbelievable syllogism is, No healthy people are unhappy; some astronauts are unhappy, therefore some astronauts are not healthy people. Note that if the conclusion were to be reversed to some healthy people are not astronauts, the syllogism would become Invalid-Believable. However, different problem materials were used for all 12 syllogisms, which were presented in an independently randomised order to each participant. Syllogisms were presented on a computer screen, one at a time, and participants instructed to click on a yes or no box to indicate whether or not the conclusions necessarily followed from the premises. In both groups the premises were presented alone for 5 seconds, and then the conclusion and yes/no boxes appeared underneath. In the free-time group, participants then had as long as they wished to make a response. In the rapid-response group, a timer bar appeared 3 seconds after the conclusion and scrolled down in a further 2 seconds, followed by the message make a decision now. By this means participants were constrained to respond in 5 seconds after appearance of the conclusion and within 10 seconds of presentation of the premises. We believe this should tightly constrain reasoning time, as Thompson, Streimer, Reikoff, Gunter, and Campbell (2003) showed that under free-time conditions participants will average

RAPID RESPONDING AND BELIEF BIAS 385 more than 20 seconds to evaluate a very similar set of syllogisms to those used in the present study. RESULTS The percentage of conclusions accepted in each condition is shown in Figure 1. As predicted, the belief bias effect appears stronger for both valid and invalid syllogisms in the rapid-response group, but the preference for valid over invalid syllogisms appears stronger in the free-time group. In order to provide statistical analysis of these trends, in accordance with standard practice, we computed three indices for each participant. The logic index (VB + VU IB IU) measures the difference between acceptance of valid and invalid conclusions: the bigger the index, the more logical the responding. The belief index (VB + IB VU IU) similarly measures the difference in acceptance of believable and unbelievable conclusions: the bigger this index, the more belief bias is observed. The interaction index (VU + IB VB IU) measures the extent to which belief bias is greater on invalid than valid syllogisms, a trend observed by Evans et al. (1983) and in most replication studies (see, for example, Klauer, Musch, & Naumer, 2000; Newstead, Pollard, Evans, & Allen, 1992). The mean scores on each index are shown in Table 1, together with the results of one-sample t-tests (one-tailed) computed to show whether each index was significantly above zero. In the free-time (control) group, the usual trends were observed. All three indices are positive and significantly above zero. In the rapid-response group, all three indices are also positive Figure 1. Mean percentage acceptance rates for all conditions.

386 EVANS AND CURTIS-HOLMES TABLE 1 Computed indices for each condition plus reults of one sample t tests (one-tailed) Logic index Belief index Interaction index Rapid response group (n=25) Mean 0.76 3.32 0.20 SD 1.44 1.46 1.38 t(24) 2.67 11.33 0.72 p 5.01 5.001 n.s. Free time group (n=25) Mean 1.96 1.88 0.60 SD 1.94 2.06 1.26 t(24) 5.03 4.54 2.38 p 5.001 5.001 5.025 and those for logic and belief are significantly above zero, although the interaction effect is non-significant. In order to test our two principal hypotheses, we conducted two-sample t- tests to compare logic and belief indices between the two groups. As predicted, the logic index scores were significantly lower in the rapidresponse group, t(48) = 2.49, p 5.01, one-tailed, while the belief index was significantly higher in the rapid-response group, t(48) = 2.84, p 5.01, onetailed. DISCUSSION The results of this study strongly confirm the predictions of the dual-process theory. It is clear that the requirement to respond rapidly (within a total of 10 seconds from presentation of the premises) has not inhibited all analytic processing of the syllogisms, since the logic index is still significant for this group. However, the logic index is substantially and significantly lower than for the control group. Conversely, the belief index is substantially and significantly higher for the group constrained to rapid responding. In order to understand more precisely what is going on, it is instructive to study the percentage acceptance rates for all conditionals as shown in Figure 1. First, examine the black bars that show the acceptance rates when rapid responding is required. This is what happens largely due to heuristic responding with analytic processing minimised. Clearly, the dominant tendency is to accept believable conclusions and reject those that are unbelievable. Now compare these with the white bars to see what changes occur when time permits further analytic processing to occur. In two of the four conditions, Valid-Believable and Invalid-Unbelievable, there is essentially no change. This makes sense theoretically, because these

RAPID RESPONDING AND BELIEF BIAS 387 conditions produce no conflict between heuristic and analytic responding. There is no reason for analytic responding to change a decision to accept a valid conclusion or to reject an invalid one. It is in the belief logic conflict cases, Valid-Unbelievable and Invalid- Believable, that the experimental manipulation has its substantial influence. In the case of valid arguments, a substantial belief bias effect is reduced (but not eliminated) by provision of extra time for the response. This is entirely due to accepting some valid but unbelievable arguments that would otherwise have been rejected. In the case of invalid arguments, the effect of instructions is to reduce to some extent acceptance of believable conclusions that would otherwise have been endorsed. Even after the influence of analytic processing, however, there is still a strong tendency (60% + ) to accept believable fallacies. The findings are also of interest with regard to some long-standing theoretical arguments as to the cause of the belief bias effect and the belief by logic interaction (Evans, Newstead, & Byrne, 1993, Chapter 8). The mental model theory of belief bias (Oakhill & Johnson-Laird, 1985; Oakhill, Johnson-Laird, & Garnham, 1989) accounts for the belief bias effect on invalid arguments on the grounds that search for counterexamples may be terminated when conclusions are believable. The drop in acceptance of Invalid-Believable conclusions in this study in the free-time condition could be due to the occurrence of more such searching when time permits. However, the model theory has always had difficulty accounting for belief bias on valid syllogisms, with a suggestion that this may reflect simply a response bias (Oakhill et al., 1989; see also Evans, Handley, & Harper, 2001; Klauer et al., 2000), a concept external to the theory. Obviously, there are no counterexamples to a valid argument, so it is not clear what participants would be doing in the free-time condition to partially inhibit this bias when time for reasoning is permitted. An older debate concerns the Selective Scrutiny and Misinterpreted Necessity models (Evans et al., 1983; see also Evans et al., 1993). The former describes the effect as a belief-first phenomenon. That is, people tend to accept conclusions that are believable and are more likely to engage in analytic reasoning when conclusions are unbelievable. The present study provides good support for the Selective Scrutiny model since (a) belief bias appears to be the default response when time is limited and (b) allowing more time for analytic reasoning results in more correction to biased responses to unbelievable (and valid) arguments than to believable (and invalid) ones. The Misinterpreted Necessity model describes analytic processing as preceding belief bias, which evidently fits the present results less well. However, it is also proposed in this model that people misunderstand the concept of logical necessity so that a conclusion that may follow as is the case with all invalid syllogisms in this study can be

388 EVANS AND CURTIS-HOLMES decided by belief. This could explain why the provision of additional reasoning time appears to debias invalid arguments less than valid ones in this study. In conclusion, we have provided strong support for the dual-process theory of reasoning in general, and cast light on some of the specific theoretical debate about the causes of belief bias in the process. We have also shown that the methodology of the rapid-response reasoning task, introduced by Roberts and Newton (2002) for study of the Wason selection task, is a powerful technique that may be capable of shedding new theoretical light on a range of different reasoning tasks. Manuscript received 7 December 2004 Revised manuscript received 14 February 2005 REFERENCES Evans, J. St. B. T. (1998). Matching bias in conditional reasoning: Do we understand it after 25 years? Thinking and Reasoning, 4, 45 82. Evans, J. St. B. T. (2003). In two minds: Dual-process accounts of reasoning. Trends in Cognitive Sciences, 7, 454 459. Evans, J. St. B. T., Barston, J. L., & Pollard, P. (1983). On the conflict between logic and belief in syllogistic reasoning. Memory and Cognition, 11, 295 306. Evans, J. St. B. T., Handley, S. H., & Harper, C. (2001). Necessity, possibility and belief: A study of syllogistic reasoning. Quarterly Journal of Experimental Psychology, 54A, 935 958. Evans, J. St. B. T., Newstead, S. E., & Byrne, R. M. J. (1993). Human reasoning: The psychology of deduction. Hove, UK: Lawrence Erlbaum Associates Ltd. Evans, J. St. B. T., & Over, D. E. (1996). Rationality and reasoning. Hove, UK: Psychology Press. Goel, V., & Dolan, R. J. (2003). Explaining modulation of reasoning by belief. Cognition, 87, B11 B22. Kahneman, D., & Frederick, S. (2002). Representativeness revisited: Attribute substitution in intuitive judgement. In T. Gilovich, D. Griffin, & D. Kahneman (Eds.), Heuristics and biases: The psychology of intuitive judgement (pp. 49 81). Cambridge: Cambridge University Press. Klauer, K. C., Musch, J., & Naumer, B. (2000). On belief bias in syllogistic reasoning. Psychological Review, 107, 852 884. Newstead, S. E., Pollard, P., Evans, J. St. B. T., & Allen, J. L. (1992). The source of belief bias effects in syllogistic reasoning. Cognition, 45, 257 284. Oakhill, J., & Johnson-Laird, P. N. (1985). The effects of belief on the spontaneous production of syllogistic conclusions. Quarterly Journal of Experimental Psychology, 37A, 553 569. Oakhill, J., Johnson-Laird, P. N., & Garnham, A. (1989). Believability and syllogistic reasoning. Cognition, 31, 117 140. Roberts, M. J., & Newton, E. J. (2002). Inspection times, the change task, and the rapidresponse selection task. Quarterly Journal of Experimental Psychology, 54A, 1031 1048. Sloman, S. A. (1996). The empirical case for two systems of reasoning. Psychological Bulletin, 119, 3 22. Stanovich, K. E. (1999). Who is rational? Studies of individual differences in reasoning. Mahwah, NJ: Lawrence Erlbaum Associates Inc.

RAPID RESPONDING AND BELIEF BIAS 389 Stanovich, K. E. (2004). The robot s rebellion: Finding meaning the age of Darwin. Chicago: Chicago University Press. Stanovich, K. E., & West, R. F. (1997). Reasoning independently of prior belief and individual differences in actively open-minded thinking. Journal of Educational Psychology, 89, 342 357. Thompson, V. A., Streimer, C. L., Reikoff, R., Gunter, R. W., & Campbell, J. I. D. (2003). Syllogistic reasoning time: Disconfirmation disconfirmed. Psychonomic Bulletin and Review, 10, 184 189. Wilkins, M. C. (1928). The effect of changed material on the ability to do formal syllogistic reasoning. Archives of Psychology, 16, no. 102.

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