Deliberation theory posits that users’ willingness to participate in online comment sections should increase if the discussions are more evidence-based. However, extant empirical research does not clearly support this assumption. The current study argues that social comparison processes and the metacognitive perception of knowledge mediate the relationship between evidence in comments and participation intention in different ways. Findings from two online experiments (N_Study1 = 368; N_Study2 = 854) support this assumption: For three different topics, the results show that providing evidence in comments, as opposed to merely opinions, increases participants’ perceived knowledge by increasing their factual knowledge. At the same time, evidence in comments decreases participants’ perceived knowledge through social comparison with other commenters. Higher perceived knowledge is related to increased participation intention. In summary, the studies reveal psychological mechanisms that explain why high deliberative quality of online discussions does not necessarily stimulate further user participation.