But in counties where a violent protest met these criteria, Democratic vote share was 1 to 1.5 points lower, depending on the exact statistical model. The paper does additional work to determine whether this relationship is plausibly causal. As noted above, this relationship is large enough to cost Humphrey four states (DE, IL, NJ, and OH) in 1968 under the hypothetical that there were no violent protests after the assassination of Martin Luther King Jr. in April of that year.

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Wasow’s paper is not attempting to address contemporary politics. But an obvious question is whether his findings have implications for the potential impact of the protests after the deaths of Michael Brown, Eric Garner, and Freddie Gray.

On the one hand, there are obviously many, many differences between the 1960s protests and those in 2014-2015. Radley Balko discusses this at length here. To pick just one, the 1960s protests were larger and more numerous.

On the other hand, underlying Wasow’s paper is a theory about how nonviolent and violent protests have a different psychological impact. This theory could have some relevance now.

Wasow argues that nonviolent protests trigger an identity that bridges disparate groups — and, in particular, creates more sympathy among majority groups for the minority group that is protesting and therefore the potential for a cross-racial coalition.

But violent protests have the opposite effect — strengthening each group’s own identity and creating a “circle the wagons” mentality motivated by a concern for safety.