Manipulating search engine results can sway undecided voters by 20 percent or more, according to a study from Robert Epstein and Ronald E. Roberston at the American Institute for Behavioral Research and Technology.
Their report shares the findings of five double-blind, randomized controlled experiments involving more than 4,500 voters in America and India:
The results of these experiments demonstrate that (i) biased search rankings can shift the voting preferences of undecided voters by 20 percent or more, (ii) the shift can be much higher in some demographic groups, and (iii) search ranking bias can be masked so that people show no awareness of the manipulation... Given that many elections are won by small margins, our results suggest that a search engine company has the power to influence the results of a substantial number of elections with impunity. [PNAS]
The researchers also found that search engines' influence on voter decisions would be greatest in "countries dominated by a single search engine company" — like, say, the United States, where two out of three searches are performed via Google.