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Is there a cognitive biasing effect that makes people believe that long statements / expressions / articles / books are more truthful than short ones (assuming that they are equal semantically)?

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up vote 4 down vote accepted

Message length is a peripheral cue in the elaboration likelihood model. This means that a message's length affects the likelihood that its recipient will be persuaded when the recipient is not scrutinizing the message's content attentively. When a message is evaluated through peripheral attention instead of central focus, simple heuristics that are easily fooled determine the extent of persuasion that results.

Petty and Cacioppo (1986, part 1) cite their earlier study (Petty & Cacioppo, 1984) as demonstrating that more arguments in favor of a message increases attitude positivity toward the message when the audience is not personally invested in understanding it even if the arguments are weak. This doesn't work when the audience is highly involved – then a greater number of weak arguments only makes attitudes worse.

Petty and Cacioppo (1986, part 2) also cite Wood, Kallgren, and Preisler's (1985) study, which persuaded its relatively inattentive participants more strongly with long messages than short messages. Again, this effect did not vary with message quality, but disappeared among participants who paid closer attention.

@Eoin recommended Heit and Rotello (2012), who replicated the results of Wood and colleagues. In fact, they even found the effect of message length among participants who were told not to judge arguments by length. This also occurred when listening to verbal arguments. Good stuff...

- Heit, E., & Rotello, C. M. (2012). The pervasive effects of argument length on inductive reasoning. Thinking & Reasoning, 18(3), 244-277. Retrieved from
- Petty, R. E., & Cacioppo, J. T. (1984). The effects of involvement on responses to argument quantity and quality: Central and peripheral routes to persuasion. Journal of Personality and Social Psychology, 46(1), 69–81. Retrieved from
- Petty, R. E., & Cacioppo, J. T. (1986). The elaboration likelihood model of persuasion. Advances in Experimental Social Psychology, 19, 123–205. Retrieved from (part 1), (part 2), and (part 3).
- Wood, W., Kallgren, C. A., & Preisler, R. M. (1985). Access to attitude-relevant information in memory as a determinant of persuasion: The role of message attributes. Journal of Experimental Social Psychology, 21(1), 73–85. Retrieved from

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This effect is also well documented in the reasoning literature; see Heit and Rotello, 2012, for example. – Eoin Apr 28 '14 at 15:05
@Eoin, why don't you add this as an answer? – user626528 Apr 28 '14 at 20:46
@user626528 - I'm really only adding a useful reference to Nick's answer, rather than a standalone perspective. Both Petty and Cacioppo, and Heit and Rotello describe largely the some phenomenon, but the former are interested in social interactions (persuasion), while the latter come from the reasoning tradition (and are particularly interested in analysing these phenomena from a signal detection perspective). – Eoin Apr 29 '14 at 10:10

It seems like a casual version of the illusion of validity, but the illusion of validity is a more general bias that additional data generates additional validity. It's often used more in a lab setting then a debate setting, where an additional experiment may be included to lend support to a hypothesis, but the experimental outcome isn't actually surprising given the results of other experiments.

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