Managing disagreement in conversation requires subtlelinguistic and pragmatics skills. One key dimension is thedegree of ‘knowingness’ with which people present theirstance on an issue. It has been hypothesised that framingstances as ‘knowing’, i.e. with higher implied levels ofspeaker certainty limits the potential for challenge by others.We present the first experimental test of this hypothesis. Usinga text based chat-tool paradigm and a debating task we areable to systematically manipulate how ‘knowing’ people’sturns appear to one-another. The results show that ‘knowing’stances tend to close off discussion leading to less carefullyformulated, truncated turns, but do not reliably affect the rangeof solutions considered. Unknowing stances, by contrast,do not affect turn length or formulation but do encouragemore deliberation and include more signals of certainty in themessage contents.