A syllogism is a kind of logical argument in which one proposition (the conclusion) is inferred from two or more others (the premises) of a specific form. The classical example of a valid syllogism is:
- All humans are mortal. (major premise)
- Socrates is human. (minor premise)
- Therefore, Socrates is mortal. (conclusion)
An example of an invalid syllogism is:
- All teenage girls are ambitious.
- Teenage girls study hard.
- Therefore, girls study hard because they are ambitious.
Typically a majority of test subjects in studies incorrectly identify this syllogism as one in which the conclusion follows from the premises. It might be true that in the real world that a) girls study and b) this is because they are ambitious. However, this argument is a fallacy, because the conclusion is not supported by its premises. The validity of an argument is different from the truth of its conclusion: there are valid arguments for false conclusions and invalid arguments for true conclusions. Hence it is an error to judge the validity of an argument from the plausibility of its conclusion. This is the reasoning error known as belief bias.
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In a series of experiments by Evans, Barston and Pollard (1983) participants were presented with evaluation task paradigms, containing two premises and a conclusion . In other words, the participants were asked to make an evaluation of logical validity. The subjects, however, exhibited a belief bias, evidenced by their tendency to reject valid arguments with unbelievable conclusions, and endorse invalid arguments with believable conclusions. It seems that instead of following directions and assessing logical validity, the subjects based their assessments on personal beliefs.
Consequently, these results demonstrated a greater acceptance of more believable (80%), than unbelievable (33%) conclusions. Participants also illustrated evidence of logical competences and the results determined an increase in acceptance of valid (73%) than invalid (41%). Additionally, there’s a small difference between believable and valid (89%) in comparison to unbelievable and invalid (56%) (Evans, Barston & Pollard, 1983; Morley, Evans & Handley, 2004).
It has been argued that using more realistic content in syllogisms can facilitate more normative performance from participants. It has been suggested that the use of more abstract, artificial content will also have a biasing effect on performance. Therefore, more research is required to understand fully how and why belief bias occurs and if there are certain mechanisms that are responsible for such things.. There is also evidence of clear individual differences in normative responding that are predicted by the response times of participants 
- Robert J. Sternberg; Jacqueline P. Leighton (2004). The Nature of Reasoning. Cambridge University Press. p. 300. ISBN 978-0-521-00928-7. Retrieved 3 September 2013.
- 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 (3): 285–306. doi:10.3758/BF03196976.
- 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.
- Morely, N. J.; Evans, J. St. B. T., & Handley, S. J. (2004). "Belief bias & figural bias in syllogistic reasoning". The Quartely Journal of Experimental Psychology 57A (4): 666–692. doi:10.1080/02724980343000440.
- Stupple, E.J.N.; L. J. Ball, J. St. B. T. Evans & E. Kamal-Smith (2011). "When logic and belief collide: Individual differences in reasoning times support a selective processing model". Journal of Cognitive Psychology 23 (8): 931–941. doi:10.1080/20445911.2011.589381.
- Markovits, H.; G. Nantel (1989). "The belief-bias effect in the production and evaluation of logical conclusions". Memory and Cognition 17 (1): 11–17. doi:10.3758/BF03199552.
- Klauer, K.C.; J. Musch, B. Naumer (2000). "On belief bias in syllogistic reasoning". Psychological Review 107 (4): 852–884. doi:10.1037/0033-295X.107.4.852. PMID 11089409.
- Dube, C.; C. M. Rotello, E. Heit (2010). "Assessing the belief bias effect with ROCs: It’s a response bias effect". Psychological Review 117 (3): 831–863. doi:10.1037/a0019634. PMID 20658855.