Bots sustain and inflate striking opposition in online social systems.

Societies are complex systems which tend to polarize into sub-groups ofindividuals with dramatically opposite perspectives. This phenomenon isreflected -- and often amplified -- in online social networks where, however,humans are no more the only players, and co-exist alongside with social bots,i.e. software-controlled accounts. Analyzing large-scale social data collectedduring the Catalan referendum for independence on October 1 2017, consisting ofnearly 4 millions Twitter posts generated by almost 1 million users, weidentify the two polarized groups of Independentists and Constitutionalists andquantify the structural and emotional roles played by social bots. We show thatbots act from peripheral areas of the social system to target influentialhumans of both groups, mostly bombarding Independentists with negative andviolent contents, sustaining and inflating instability in this online society.These results quantify the potential dangerous influence of political botsduring voting processes.