One explanation for why people accept ideologically welcome misinformation is that they are insincere. Consistent with the insincerity hypothesis, past experiments have demonstrated that bias in the veracity assessment of publicly reported statistics and debunked news headlines often diminishes considerably when accuracy is incentivized. Many statements encountered online, however, constitute previously unseen claims that are difficult to evaluate the veracity of. We hypothesize that when confronted with unfamiliar content, unsure partisans will form sincere beliefs that are ideologically aligned. Across three experimental studies, 1,344 conservative and liberal US participants assessed the veracity of 20 politically sensitive statements that either confirmed or contradicted social science evidence only known to experts. As hypothesized, analyses show that incentives failed to correct most ideological differences in the perceived veracity of statements. 66 to 78% of partisan differences in accuracy assessment persisted even when monetary stakes were raised beyond levels in prior studies. Participants displayed a surprising degree of confidence in their erroneous beliefs, as bias was not reduced when participants could safely avoid rating statements they were unsure about, without monetary loss. These findings suggest limits to the ability of disciplining interventions to reduce the expression of false statements, because many of the targeted individuals sincerely believe them to be true.