Contrast Ratio: Shifting Burden of Proof in Informal Arguments

Previous research demonstrated an antiprimacy effect in conversational arguments (Bailenson & Rips, 1996; Rips, 1998). In those studies, participants perceived the speaker presenting the 1st proposition in the dialogue as having more burden of proof than the 2nd speaker. In this article, I demonstrate that antiprimacy occurs because the 1st speaker tends to proportionally have fewer contrasts than does the 2nd speaker. A claim only provides contrast if it attacks another speaker’s previous claim. First speakers tend to have low contrast ratios because the initial claim in the argument does not clash with an existing claim. In these experiments, antiprimacy shifts as a function of the ratio of contrasts between the 2 speakers. In Experiment 1, antiprimacy disappeared when the 1st speaker offered 1 more claim than did the 2nd speaker. In Experiment 2, antiprimacy decreased as the number of total claims in the argument increased. In Experiment 3, the 2nd speaker accrued an extra contrast and shifted the burden of proof by querying the 1st speaker. In all 3 studies, contrast ratio can explain burden of proof decisions. These results offer insight toward the manner in which people process conversational arguments and assess burden of proof.