CINCINNATI — Jurors bring their beliefs and life experiences into the courtroom.

Though many jurors try to remain neutral, their deeply held biases can affect the life-and-death decisions they are charged to make, say legal and brain science experts. The issue has come to the forefront in recent years when a white police officer has stood trial for killing an African-American.

In the avalanche of reaction last week to the announcement that former University of Cincinnati police officer Ray Tensing would not be tried a third time, his attorney provided the most convincing piece of evidence that implicit bias played a central role in the two mistrials.

Stew Mathews said it appeared both juries were racially divided. The five African-American jurors — two in the first trial and three in the second — all voted to convict Tensing on both murder and voluntary manslaughter, he said. Only one of the nine white jurors in the second trial voted for a murder conviction, and two wanted a manslaughter conviction.

Tensing, who is white, was charged for shooting and killing unarmed black motorist Sam DuBose during a July 2015 traffic stop.

In announcing his decision to drop charges against Tensing, Hamilton County Prosecutor Joe Deters said his polling of jurors convinced him "that we will never get a conviction. … So many things bled into the jury room related to race."

One of those things is implicit bias — the attitudes or stereotypes that scientists say affect our understanding, actions and decisions in an unconscious manner.

The presence of these hidden personal biases is the major reason that many police trials and other racially charged trials can end in acquittal or hung juries, say many psychologists and legal experts.

The inability to convict cops, even in cases bolstered, like Tensing's, with video evidence, is most often tied to racial and police biases.

"Do you trust the police? What kind of interactions have you had with police? Is there a racial difference in how your community views police?" are among the relevant attitudes and experiences that jurors have, said Calvin Lai, one of the world's leading experts on implicit bias.

Lai, an assistant professor of psychological and brain sciences at Washington University in St. Louis, said unconscious bias can cause jurors to trust a police officer or other witnesses more or less than others.

"The idea that we can judge something in a vacuum is not true," he said. "There is always some degree of interpretation."

Juries often do not represent the racial demographics of a community — a "jury of their peers" — because of how courts form their jury pools. There's an effort to change that by expanding the sources of potential jurors in a few states, including Ohio, where only logs of registered voters are now used.

Another challenge, some legal scholars say, is that the threshold to convict anyone of murder is high and requires agreement with all points in a case. Then jurors are subject to the instructions of judges, who control what evidence can be admissible in a trial.

'You can rationalize your biases' in jury room

Keenly aware of the potential for a case to hinge on the personal biases of jurors, attorneys use questionnaires in the jury selection process, otherwise known as voir dire, or "to say what is true."

Even then, no guarantees exist that a jury will be impartial. Questionnaires in the first Tensing trial asked for potential jurors' attitudes toward Black Lives Matter and police, if they were related to a police officer and whether African-Americans were more violent than other races.

"Biases can be explicit. It does not have to be implicit," said Lai, who has worked with the National Underground Railroad Freedom Center in the development of its Open Your Mind learning lab on implicit bias.

Even the time-consuming nature of deliberations does not protect against bias.

"Implicit biases are so tricky that when you think harder you might become more biased," Lai said. "You can rationalize your biases."

The first Tensing jury deliberated 25 hours, the second about 30 hours.

Implicit bias, though unrecognizable by definition, is real.

"You'd be hard-pressed to find someone who says it's not a major concern," said Thaddeus Hoffmeister, a professor at the University of Dayton School of Law whose course offerings include one on juries.

In the jury selection process, attorneys appear uncertain how to bring up the topic of implicit bias about race and other issues with potential jurors, he said. Explicit bias is much easier to raise.

Jury selection for the first Tensing trial lasted only four hours. It stretched over four days before the retrial.

"The prosecutor's office underestimated race in this case," Hoffmeister said. "They're usually on the other side and aren't prosecuting police officers. They don't have that experience, and it showed."

Explicit, implicit biases both concerns

Juror bias attracted the attention of the nation's highest court this spring.

In March, the U.S. Supreme Court ruled 5-3 against racial bias in jury rooms. The majority said that reports of racist comments by jurors during deliberations can be cause for a mistrial and reason to hold a new trial.

Justice Anthony Kennedy wrote that reports of overtly biased racial comments allow judges to ignore the long-held rule of "second-guessing" what has transpired during deliberations.

Since March, an 11-minute video about implicit bias has been shown to prospective jurors in two courthouses in the U.S. District Court for the Western District of Washington.

The presence of African-American or other minority jurors is no guarantee that a jury will vote completely along racial lines.

In May, a jury with three African-American members acquitted Tulsa, Oklahoma, officer Betty Shelby, who is white. She had been charged with manslaughter in the shooting death of unarmed black motorist Terence Crutcher, who had his hands in the air when shot.

In June, a jury with two African-American members acquitted Jeronimo Yanez, a former St. Anthony, Minnesota, police officer, in the shooting death of black motorist Philando Castile. Castile, who is African-American, had told Yanez, a Latino, during a traffic stop that he had a permit to carry a firearm.

The bias starts on the street with police, say civil rights activists. Black people are more than twice as likely to be killed by police as white people, according to data collected by The Washington Post since mid-2014.

Civil rights leaders say African-Americans are shot more because they are more likely to be pulled over.

All of this despite results of a 2011 U.S. Department of Justice report, revised in 2016, that shows black men are less likely to have drugs on them than white men when searched.

Racial bias is present throughout the legal system and was in play in the Tensing trials, say Cincinnati area civil rights leaders who had issued a statement demanding a third trial.

"I think the race of the jury is a big part of it," said Donyetta Bailey, an African-American attorney and president of the Black Lawyers Association of Cincinnati. "There appears to have been a strong bias toward cops, no matter what they do. 'You're a cop. I believe you.'"

Bailey said the judges in the trials, particularly the second one, Leslie Ghiz, hurt the prosecution with several decisions. Ghiz did not permit admission of the Confederate flag T-shirt that Tensing wore under his uniform the day he shot DuBose. She also did not allow into evidence Tensing's record at UC that showed he had by far the highest percentage of African-American traffic stops of any member of the department.

"She also allowed Stew Mathews to say over and over that Ray Tensing is not a racist," Bailey said.

Expectations 'very high' for criminal prosecution

The desire for justice – for the victim's family, for the community – is strong.

Yet the search for justice on the local level is often unrealistic, said another national expert, Michael Scott, a clinical professor in the School of Criminology and Criminal Justice at Arizona State University, Tempe.

The standard of proof in homicide cases is extremely high, Scott said. And the "central element to the case is the officer's intent to cause death or bodily harm. An officer rarely intends to kill a person other than protecting themselves or another person."

"They expect criminal prosecution to deliver a measure of justice that it simply may not be able to do," Scott said.

The Tensing mistrials left the DuBose family dumbfounded.

"It's not like you're in 2017," said Terina DuBose Allen, the victim's sister.

Prosecutors face increasing amounts of pressure to announce charges and prosecution within days of a police shooting, rather than taking their time to carefully review evidence. Deters announced charges against Tensing on July 29, 2015, just 10 days after Tensing killed DuBose.

Some legal experts and members of the general public alike have questioned Deters' bravado during his 2015 news conference and suggest he over-charged Tensing. Others have wondered aloud if a conviction could be won if the defendant police officer in a fatal shooting were nonwhite and the victim white.

The country will watch one of those rare cases unfold in the weeks and months ahead.

Late July 15 in Minneapolis, Minnesota, a white Australian woman, Justine Damond, was killed by a single shot to the abdomen. Mohammed Noor, 31, one of nine Somalis on the Minneapolis police force, fired the fatal shot.

Corrections and clarifications: An earlier version of this story featured an incorrect photo of Calvin Lai.