A digital advertising campaign run ahead of the 2020 US presidential election affected voters’ choices — but the effect was small and costly to achieve, an analysis finds1.

Minali Aggarwal at Yale University in New Haven, Connecticut, and her colleagues studied a US$8.9-million campaign run across five states that were awash with political advertising and where presidential candidates from both major parties had a decent chance of victory. Run by a left-leaning non-profit organization, the campaign featured 536 paid digital advertisements on websites such as Facebook to raise support for Democratic candidate Joe Biden over Republican candidate Donald Trump. Over an 8-month period, the campaign reached more than 1.7 million people an average of 754 times.

The team found that among people exposed to the campaign, voter turnout was 0.4% higher for those who favoured Biden and 0.3% lower for those who favoured Trump, compared with control-group voters who were not exposed to the campaign. The results suggest that even significant and expensive digital advertising campaigns are likely to have only modest effects on voter behaviour, although those effects could still be important in tight elections.