Most studies of so-called conjunction errors in probabilistic thinking have asked people to estimate the probabilities of hypothetical, or fictional events. In a previous study, we found conjunction errors to occur, although less frequently, in a real-life prediction task (concerning Norway's chances in three upcoming World Cup soccer games). In the present study, Norwegian voters were asked to evaluate the chances of a final YES or NO to the European Union in the 1994 referendums in Finland, Sweden, and Norway, separately and in combination. Despite the familiarity of the issue, a large majority committed conjunction errors, e.g. by rating YES in both Sweden and Norway equally or more likely than YES in Norway. The results indicate that conjunction errors are not reduced by realism and familiarity, at least not for events that are seen as causally determined. They are, however, affected by outcome valence. We found as predicted fewer conjunction errors with negative than with positive events. Pro-EU subjects committed fewest errors with NO-conjunctions, whereas anti-EU subjects produced fewer errors with YES-conjunctions.