Fake news (i.e., misinformation) on social media has sharply increased in the past few years. We conducted a behavioral experiment with EEG data from 83 social media users to understand whether they could detect fake news on social media, and whether the presence of a fake news flag affected their cognition and judgment. We found that the presence of a fake news flag triggered increased cognitive activity and users spent more time considering the headline. However, the flag had no effect on judgments about truth; flagging headlines as false did not influence users’ beliefs. A post hoc analysis shows that confirmation bias is pervasive, with users more likely to believe news headlines that align with their political opinions. Headlines that challenge their opinions receive little cognitive attention (i.e., they are ignored) and users are less likely to believe them.