Speakers systematically overestimate their communication effectiveness (Keysar & Henly, 2002). We argue that doing so is adaptive, reducing the risk of social anxiety and withdrawal from social situations. This hypothesis was tested by having speakers who scored low and high for fear of negative evaluation (FNE), a hallmark of social phobia, attempt to convey a specific meaning of ambiguous statements to a listener and then estimate their communication effectiveness. Low-FNE speakers consistently overestimated their effectiveness, expecting the listener to understand their intended meaning more often than listeners actually did. In contrast, high-FNE speakers' estimates of communication effectiveness were consistent with the listener's actual understanding. Signal detection analysis revealed that low- and high-FNE speakers were equally able to discriminate communication success from failure, but low-FNE speakers exhibited a stronger positive response bias. In conclusion, overestimating one's communication effectiveness is adaptive, and accurate estimation is associated with dysfunction.