Intellectual humility, which entails openness to other views and a willingness to listen and engage with them, is crucial for facilitating civil dialogue and progress in debate between opposing sides. In the present research, we tested whether intellectual humility can be reliably detected in discourse and experimentally increased by a prior self-affirmation task. Three-hundred and three participants took part in 116 audio and video-recorded group discussions. Blind to condition, linguists coded participants’ discourse to create an intellectual humility score. As expected, the self-affirmation task increased the coded intellectual humility, as well as participants’ self-rated prosocial affect (e.g., empathy). Unexpectedly, the effect on prosocial affect did not mediate the link between experimental condition and intellectual humility in debate. Self-reported intellectual humility and other personality variables were uncorrelated with expert-coded intellectual humility. Implications of these findings for understanding the social psychological mechanisms underpinning intellectual humility are considered.