The decision to protest is strategic: an individual's participation is a function of her beliefs about others' turnout. Models of protest often assume strategic complementarity; however, the challenge of collective action suggests strategic substitutability. We conduct the first field experiment directly manipulating individuals' beliefs about others' protest participation, in the context of Hong Kong's pro-democracy movement. We elicit university students' planned participation in an upcoming protest and their prior beliefs about others' participation, in an incentivized manner. One day before the protest, we randomly provide a subset of subjects with truthful information about others' protest plans, and elicit posterior beliefs about protest turnout, again in an incentivized manner. This allows us to identify the causal effects of positively and negatively updated beliefs about others' protest participation on subjects' turnout. We consistently find evidence of strategic substitutes. Analysis of control group subjects and survey evidence reinforce our experimental findings.
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