Isolating causal relationships is very difficult in natural discussion because discussion is, by nature, a causally complex interactive process. This study provides a novel approach to this problem by using simulated online discussion to experimentally manipulate message characteristics in a timed script that participants believed to be a discussion with other participants. In particular, we manipulated whether simulated online discussion partners expressed disagreement civilly or uncivilly. With this unique design, we examined how individuals differently react to opposing views in an online discussion setting depending on tone of disagreement expression. Our results showed that exposure to uncivil discussion results in increased negative emotions toward the other side, which in turn led to more closed-mindedness and more expression of disagreement with the discussion partner on the other side of the issue, although uncivil discussion and negative emotions are positively related to recall of the other side’s reasons.