In Charlottesville this summer, white supremacist chants of “Sieg heil!” and “Jews will not replace us”, were followed by a driver who drove his car into counter-protesters. Last year, Donald Trump encouraged his followers “knock the crap out of” protesters, adding that he would cover their legal fees. And amid an upsurge of violence speech against Muslims, hate incidents against mosques and Islamic centers in the US were markedly up in the last year.



This isn’t normal political discord any more. It’s not even “just” hate speech. Political speech has now veered into an even more dangerous terrain: incitement.

The first amendment protects freedom of expression, but inciting another person to commit assault or murder is in itself a crime, and has been so since the founding of our republic. Standing incitement law requires that a speaker intentionally advocate the commission of a crime, where such advocacy is directed at producing “imminent lawless action” and is likely to incite the crime being encouraged. This is a high bar and was consciously designed to allow nearly all political speech, including hate speech.



Yet as political speech has pushed right up against this bar, there has been a proliferation of incitement challenges in courts. Two sisters who suffered injuries after the Dodge Challenger driven by James Alex Fields Jr slammed into their car have filed a $3m lawsuit against neo-Nazi and white supremacist groups for organizing and inciting violence on social media in the run-up to the march in Charlottesville.



A case is still pending against the president in US district court after a judge upheld three protesters’ right to claim damages against Trump and his campaign for inciting audience members at a rally. Judge David Hale found that Trump’s words “Get ’em out of here” were not protected by the first amendment and could constitute incitement to riot insofar as they were “an order, an instruction, a command” to his supporters to commit assault and battery.

The stakes in these cases are high – balancing free speech with public safety – yet incitement law is frustratingly murky. The US supreme court has determined that courts must conduct a review of “the content, form and context of a given statement”, but it provides no guidance in determining what types of inciting speech and contexts are the most likely to trigger a violent response.



Neuroscience experiments show how denigrating speech against an out-group can facilitate inhumane acts

Judges and juries are left to rely on personal hunches and to search for guidance from previous landmark cases, which may have a manifestly different pattern of facts. Drawing a bright line matters not only for the courts, but also private sector entities, like internet providers, whose terms of service typically prohibit incitement – the reason given by Google and GoDaddy when they stopped hosting the American Nazi website Daily Stormer.

But new social science research can help fill the gap left by the courts at this crucial time. For instance, neuroscience experiments by psychologists Lasana Harris and Susan Fiske demonstrate how denigrating speech against an out-group can dehumanize and facilitate inhumane acts.



In a recent psychological study conducted for my book Incitement on Trial, we used different types of speech to test participants’ willingness to empathize with members of another group or to justify violence against them. Dehumanizing language (eg, calling a person an “ape” or “cockroach”) had significant effects, but speeches that referenced past crimes and called for revenge had the greatest effect on participants’ willingness to countenance violence.



Intriguingly, derogatory speech couched in religious or far-right nationalist language was not especially influential. Further, stereotyping of other groups, while offensive, may not inculcate a greater tolerance of violence. Not all types of derogatory or inciting speech are the same in their concrete effects on attitudes towards violence, and courts might take this into account when reviewing the content of speeches.

Previous studies have repeatedly confirmed that listeners are more swayed by charismatic speakers who are authority figures and who use graphic, intense language that draws upon existing beliefs and prejudices. Repetition often works: a statement repeated across a variety of mass communication formats, from radio to television to social media, is more likely to be believed, even if it is patently false.



The emotional state of the audience matters and a population is more receptive to inciting speakers during times of political instability, insecurity and economic austerity. Finally, an incitement to violence is more likely to be acted on if the speaker declares that there is a direct existential threat to the audience and identifies a distinct course of action to remove the threat.

Many of the factors identified above apply to the stream of coded and explicit rhetoric from the current administration and substantiate the claim that President Trump has repeatedly incited imminent lawless action.

Assembled into a checklist, these factors could provide guidance to judges and juries as they assess the gravity of a speech act and its likelihood of triggering violence. For incitement law to function at all, courts must define the boundary between protected speech and unprotected speech that incites a crime.



Social research can make a valuable contribution in identifying the content, form, and circumstances most likely to result in imminent lawless action, and to ensure that the limits of permissible speech are based on systematic research, rather than unproven assumptions.