The fatal shootings this week of the African-American men Alton Sterling and Philando Castile have stirred a public outcry and debate about the role race plays in the decision by police officers to use deadly force.
Such debate is nothing new. Scholarly discourse and research on the tumultuous connections between race and policing dates back decades.
And today, academics are drawing from foundational studies as well as new methods of measuring bias to continue unraveling that tangled relationship. Here's a look at some of their findings.
How much research has been done on police shootings and race?
A wealth of research and reports exists, not just from recent years but dating to the beginning of modern policing, in the early 20th century, said Tony Platt, a distinguished affiliated scholar at the Center for the Study of Law and Society at the University of California at Berkeley.
Mr. Platt, who has written about the history of policing in America, said racial strife between police departments and racial minorities is well documented.
He cited a congressional committee's report on the bloody East St. Louis riots of 1917, when roughly 3,000 white men marched downtown in the Illinois city and attacked African-Americans after black workers had been hired to replace striking white workers. The committee noted that the police at the riots "shared the lust of the mob for Negro blood."
Though research on police shootings did not begin appearing until the 1960s, previous works detailed the fraught, and violent, relationship between the police and black civilians.
Notably, in 1944, the economist Gunnar Myrdal published An American Dilemma: The Negro Problem and Modern Democracy, in which he portrayed the racial biases of Southern white policemen against the black neighborhoods they patrolled. The book was later cited in the Supreme Court's 1954 decision in Brown v. Board of Education.
In the 1970s, Paul Takagi, then a professor of criminology at Berkeley, examined police-caused deaths of male civilians over 10 years old. From 1950 to 1968, he found, the death rate for African-Americans was consistently nine times as high as that for whites, and he concluded that "the police have one trigger finger for whites and another for blacks." (In 2014, ProPublica found that young black males were 21 times as likely to be shot dead by the police as their white counterparts.)
James J. Fyfe, another major figure in criminology research, was the "grandfather of criminal justice," said Lois James, an assistant professor of nursing at Washington State University. Mr. Fyfe wrote extensively about police officers using deadly force, and in general he identified a link between an officer’s racial bias and his likelihood to shoot. But he emphasized there were numerous mitigating circumstances that muddied that relationship.
What methods do researchers typically use to analyze this question?
Typically, academics have relied on one of two methods, said Ms. James, and both can be problematic.
Some researchers simulate police-shooting scenarios on computers. Subjects sit in front of a screen and watch people of different races pop up either armed or unarmed. Then the subjects instinctively press a button to shoot or not shoot, and the length of their response time indicates their racial prejudice.
Lab simulations can provide precise data in a controlled environment. But that environment often bears little resemblance to real-world policing.
Debbie Ma, an associate professor of psychology at California State University at Northridge, helped conduct a simulated shooter study.
She and her fellow researchers found that police officers and civilians alike were quicker to shoot an armed black person than an armed white person. But she also found that when the virtual target was unarmed, racial bias showed up only in the trigger fingers of people who were not police officers.
Ms. Ma hesitated to extrapolate any larger conclusions from her study because the simulation is "fairly far removed from what police officers are asked to do in the real world," she said.
The other dominant method is to analyze existing police-incident reports of civilian shootings. But using real-world data can be limiting, mainly because no national database exists that compiles reports of police officers’ using deadly force on civilians.
Ms. Ma has another study in the works relying on that type of data. For that project, she and her colleagues had to find cases at the city level, and then map those onto census data as well as plug in disparate information from national sources like the Department of Justice. The process of just locating data took a year and a half.
Incident reports can also offer spotty information and erase some of the variables that could have influenced an officer’s decision to use deadly force more than race, said Ms. James.
Is there any consensus on how much of a role racial bias plays?
The disproportionate representation of African-Americans in fatal shootings by the police is not a fact people debate, Ms. James said.
But the reasoning behind why that occurs generally breaks down into two camps: the bias hypothesis or the threat hypothesis.
Proponents of the bias hypothesis say that police officers, like most people, have implicit racial bias ingrained in their thinking and decision making. Negative stereotypes about minority people influence when and how often officers choose to use their weapons.
Those who favor the threat hypothesis contend that, for a vast number of reasons — including poverty, lack of upward mobility and opportunity, and a history of oppression — African-Americans are more likely to be involved in scenarios where police officers use their weapons.
As for her own view, Ms. James said she is not convinced the debate matters much.
"Even if the threat hypothesis is true, there’s something very, very wrong and very, very damaged if young African-American males are more likely to be in those positions," Ms. James said.
Are academics pointing to any solutions?
Ms. Ma said researchers need comprehensive data at the national level to understand the larger, more complicated picture of the role race plays in modern policing.
"Unless those data start to get stored in a more systematic way, it’ll be really hard to actually address this question," she said.
President Obama and his Task Force on 21st Century Policing made numerous suggestions to improve policing in the United States, including advocating for the collection of national data.
Mr. Platt said any significant change would have to come from a major overhaul of policing culture, which has been militarized for a long time.
"It’s dangerous to have this myopia and this lack of history when we’re talking about solutions, and not understand how deeply rooted this is," Mr. Platt said.
But the social outrage over the recent deaths of young African-American men gives him some hope.
"I am encouraged that for the first time in a long time, there is this grass-roots activism around these issues and public debate," he said. "But I think we also need a deep historical perspective that shows us how structural these issues are."