Momentum in the polls raises electoral expectations

Voters rely on opinion polls to help them predict who is going to win elections. But they are regularly exposed to different polling results over time. How do changes in the polls affect their expectations? I show that when the polls indicate that a party’s support has increased, voters’ expectations for that party’s performance will be higher than they would be at the same vote share but without such evidence of growth, because the party appears to have momentum. Across six survey experiments in Britain (total N > 14,000), I find that this effect persists even when changes in vote share are well within the margin of error, when comparing a small change in vote share to consistently polling at the larger vote share, when the change makes little difference to a party’s objective probability of victory, and when voters have strong preferences that might colour their interpretation of the polls. In short, the appearance of momentum in the polls robustly raises voters’ expectations that a party will win an election. This finding has major implications for any area of research in political science where expectations feature, for theoretical understandings of how people perceive the future, and for salient policy debates about the regulation of opinion polls.