Evidence suggests that political polarization in the United States may be due, in part, to liberal and conservative partisans living in different factual realities, as a consequence of being exposed to information streams that rarely challenge their background beliefs and cherished narratives. In this project, we approached the issue of biased access to political information at the level of information emission, by focusing on ordinary citizens’ decisions to share political news headlines on simulated social media touching on four controversial issues: gun control, abortion, sex equality and racial equality. Across 8 studies, we found robust evidence that participants have a sharing preference for politically congruent news items over incongruent ones, and that this sharing bias increases with the moral importance of the issue. Those effects were observed on true and false headlines alike. Perceived accuracy and coalitional motivations to share headlines to advance political goals were among the main motivations to share. The transmission preference for congruent content and its interaction with issue importance held in spite of manipulations of the anonymousness of sharing (from an anonymous vs. a personal social media account), and audience’s political congeniality (agree vs. disagree). Intervention messages reminding participants of their susceptibility to the myside bias had little moderating influence. While biased communication on politics may be rational for the individual, it likely contributes to reinforce partisan differences in perceptions of society, and may reinforce affective polarization.