A candlelight vigil on the Las Vegas Strip following the mass shooting at a country music festival. (Chris Wattie/Reuters)

On Oct. 1, 2017, a man named Stephen Paddock opened fire on attendees at an outdoor concert festival in Las Vegas, killing at least 58 and wounding 500 more, in the deadliest shooting in modern American history.

As the news filtered in, policymakers, pundits and members of the public debated whether the incident should be classified as terrorism. As with other recent violent incidents — including the shootings at Fort Hood and Charleston — opinions diverged widely.

Sheriff Joseph Lombardo of the Las Vegas Metropolitan Police Department announced that the Las Vegas attack was not terrorism, because it was perpetrated by “a solo actor. A lone wolf.”

The FBI made a similar declaration, noting the absence of ties to foreign organizations — although the Islamic State quickly took credit for the attack regardless. But the Democratic National Committee and Democratic lawmakers like Rep. Jim Cooper (D-Tenn.) characterized the attack as an “act of terror.”

Stephen Paddock was identified by police as the gunman in the deadliest mass shooting in modern U.S. history. Here's what you need to know about him. (Monica Akhtar/The Washington Post)

If the perpetrator is Muslim, are we quick to label an incident as “terrorism”?

One question often raised in these incidents is whether violence is more likely to be characterized as terrorism when the perpetrator is a Muslim. On Twitter, journalist Glenn Greenwald wrote that “in the early stages of mass shooting, ‘no signs of terrorism’ means: ‘shooter isn’t Muslim.’”

In new research, we find that the identity of the perpetrator matters for defining terrorism, but it’s not the only thing that matters. How we define terrorism also depends on other contextual factors that might — or might not — be highlighted in the media.

How do we decide what counts as terrorism?

To help explain this contentious question, we’ve been conducting studies to explore how members of the public decide which violent incidents count as terrorism, and which don’t. In a new article in the American Journal of Political Science, we turn to experimental methods to better understand how the public decides whether violent incidents are acts of terrorism.

If the responses of ordinary citizens constitute a central means through which terrorism operates, understanding what ordinary citizens think counts as terrorism is a crucial prerequisite to understanding how they react to it.

How we did our research

We fielded a survey experiment on 1,400 adult Americans. We showed each respondent summaries of a series of incidents, with randomly generated attributes: the type of tactics used (for example, a shooting versus bombing); if there were casualties; what the target was; where the attack took place; who the perpetrator was (including whether or not they were identified as Muslim); what their motivation was (for example, a personal dispute, policy change, hatred); and so on. We then asked participants to tell us whether they thought of each incident as being an act of terrorism or not. For more on the methodology, see here.

Our results showed that ordinary citizens classify terrorism based not only on relatively straightforward facts on the ground, such as the number of casualties — but also information about the perpetrator, which is often highly ambiguous in the immediate aftermath of incidents.

Considerations about the type and severity of violence matter: If the attacker in Las Vegas had used a bomb rather than a gun, for example, we would be much more likely to think of it as terrorism. But our respondents are also heavily influenced by relatively subjective descriptions about the perpetrator’s identity and motivations. These are the “who” and “why” questions — and the media has considerable latitude in how it chooses to answer them, especially in the hours and days following an attack.

Here’s why this matters — predictive models based on our experimental results show that subjective descriptions of the perpetrator make it more/less likely that Americans see the event to be terrorism.

For example, the Daily Mirror speculated about Paddock’s family history of mental illness. Other outlets emphasized other factors. Was he motivated by hatred, or was his motivation unclear? Did he have foreign ties, as the claims by the Islamic State in the immediate aftermath seemed to suggest? And what if he was Muslim?

Our models suggest that how the public understands an attack depends on which of these factors are mentioned or highlighted. And the media decides which factors make it into the descriptions of attacks that occur.

Here’s the test: building off our experimental results, we calculated the changes in the predicted probability that an event with the characteristics of the Las Vegas attack will be perceived as terrorism if we code the attack’s attributes in different ways. If the perpetrator is described as having a history of mental illness, we have no information about his identity and his motivation is described as unclear, our model suggests a 35 percent likelihood that the public will perceive this as terrorism.

If instead we hold all else equal but describe the perpetrator as Muslim, the probability of viewing it as terrorism increases to 46 percent. If we then omit the references to mental illness, the probability inches up to 53 percent. If we then attribute the incident to political goals, the probability jumps to 73 percent. And, if we also suggest the potential of foreign ties, the probability jumps further to 81 percent.

It isn’t just the attacker’s identity that matters

Tweeting after the attack, Piers Morgan said, “If the shooter was Muslim, we’d call this a terrorist attack.” Our results suggest it’s not that simple: The effect of social identity on terrorism classifications is real (especially among respondents who self-identify as politically conservative) but modest compared to many of the other factors we analyze.

The broader problem is that the media treats Muslim perpetrators differently — they are less likely to be referred to as suffering from histories of mental illness and their actions more likely to be attributed to political motivations.

These results suggest media coverage profoundly shapes how the public comes to understand violent events. Many of the considerations we rely on most heavily when categorizing events as terrorism are relatively subjective — and require information that’s not available for days or weeks after an incident occurs.

This means pundits and policymakers have considerable leeway in how they choose to frame acts of violence — and our results show that what they report may shape public opinion long before the full details are known.

Connor Huff is a PhD candidate at Harvard University.

Joshua Kertzer is an assistant professor of government at Harvard University.