When Dallas Police Officer Christopher Watson gave a false statement for why his partner shot a mentally ill man, it wasn’t because Watson was lying, according to the police union that represents him. No, the union said it was because Watson was questioned too soon.

“I believe it was the stress of the situation. It really messed him up,” Dallas Police Association vice-president Frederick Frazier said, noting Watson, who had nearly six years of experience, was questioned only hours after the shooting.

Watson’s account of what happened on Oct. 14, 2013 — that 52-year-old Bobby Bennett came at officers with a knife — was proved false days later by video from a neighbor’s surveillance camera.

The video showed Bennett standing still, his arms at his side, before Dallas police officer Cardan Spencer fired four times, hitting him once in the stomach.

But rather than firing Watson for the false account, police chief David Brown instituted a new policy that critics contend is self-serving: Officers involved in a shooting, whether they pulled the trigger or not, cannot be questioned by investigators for three days. It also allows an officer to review video of an altercation before telling investigators what took place.

The policy, introduced just before Thanksgiving that year, does not grant the same privilege to civilians.

Dallas is not the only police department with a waiting period. A Scripps News investigation found that 19 of the 50 largest police forces in the United States grant officers involved in shootings some kind of waiting period, either for an internal probe looking into whether an officer followed policy or a criminal investigation.

Dallas’ chief declined repeated requests from Scripps News for an interview, but told a Dallas Morning News reporter in 2013 that the policy would prevent an officer from providing a statement while still experiencing the effects of stress.

“It is my belief that this decision will improve the investigation of our most critical incidents,” chief David Brown said.

Bobby Bennett’s attorney saw it differently.

“You’re concerned about whether they’re going to get their stories straight,” said Don Tittle, who has frequently sued the Dallas Police Department. “There is no doubt in my mind that this rule is to prevent instances like what happened in this case, where an officer issues a statement that they find out later is directly contradicted by video evidence.”

Proponents of the waiting period insist there are no such ulterior motives.

“A police officer’s put in many, many stressful situations constantly,” said Frazier, whose union advocated for the Dallas waiting period policy. “When he comes to the one that’s almost overwhelming, we want to give him the chance to … give the best information he can of exactly what happened.”

Most wait periods are 48 to 72 hours. In Memphis and Kansas City, for example, interviews for criminal and administrative investigations do not occur until at least two days after the incident, unless the officer chooses to talk sooner.

In Albuquerque, officers aren’t questioned until two to three days after an officer-involved shooting. That’s what happened in March 2014, when officers there were called to the foothills of the Sandia Mountains, responding to a call that a man had been camping there for the past month.

The man, James Boyd, had been treated for mental illness. When approached by officers, he brandished a knife, prompting a five-hour standoff that involved 19 officers.

It ended with Boyd appearing to surrender, according to special prosecutor Randi McGinn, only to be shot and killed. The officers involved were not questioned until two days after the shooting, and have since been charged with second-degree murder.

In Baltimore, where Freddie Gray died in police custody on April 19 of last year, officers there and throughout the state of Maryland are afforded 10 days before having to answer questions, though a special task force recommended last month that the wait be cut to five days.

In addition to shootings, the waiting-period in Maryland applies to any other investigation related to an officer’s conduct.

Even if an officer chooses to waive the right to a waiting period, some police organizations caution them from speaking to investigators.

In its 2013 officer-involved shooting guidelines, the International Association of Chiefs of Police said officers with an immediate need to talk should do so “solely with an individual with whom they have privileged communication,” such as a priest or attorney.

The IACP does recommend that officers supply a “public safety statement” shortly after a critical incident, such as the type of force used, whether the suspect is still at large and the location of other witnesses. However, it cautions officers from explaining why force was used, saying that should be discussed later during a formal interview.

Critics of the policy see a double standard, saying officers aren’t the only ones who experience stress when a crime takes place.

“If you’re a civilian who’s experienced a trauma or a stressful incident, the same logic as a human being would apply to you,” said Matt Barge, the Independent Monitor of the Cleveland Police Department.

Barge is vice-president of the Police Assessment Resource Center, which consults with some of the country’s most troubled police forces.

Barge says that in addition to stoking public suspicions, waiting periods allow officers who might have crossed the line to alter their stories. ”Certainly, there can be poor motives,” he said.

But giving officers time to recover from the stress of a shooting — especially if they’ve taken a life — isn’t just humane, according to the International Association of Chiefs of Police, it allows for more accurate recall.

“It’s not that the officer is trying to lie to anybody or persuade somebody in a different direction,” said Wellesley, Mass., Police Chief Terry Cunningham, president of the IACP. “It’s just a physiological change that they go into as a result of that really stressful time.”

The rationale for delaying officers’ interviews is based on a highly-debated scientific principle on how trauma affects memory.

On one side is William Lewinski, professor emeritus of psychology at Minnesota State University and the founder of the Force Science Institute, which studies officer-involved shootings and holds training seminars nationwide.

Lewinski, arguably the waiting period’s biggest advocate, declined to be interviewed for this story, but said in a statement: “It is well documented that immediate interviewing, sleep deprivation and interrupted sleep can hinder accurate recall and the consolidation of memories particularly traumatic ones.”

Bjoern Rasch, a professor of psychology at the University of Fribourg in Switzerland agrees, saying that studies have shown that increased cortisol levels — a hormone released during stress — can impair memory retrieval.

But other memory experts contend that the opposite is true, that accurate recollection — especially regarding details — will get worse after a day or two of rest, even though the memory itself might seem easier to recall.

Charles Morgan, an associate professor psychology at Yale University who has studied soldiers’ memories, called Lewinski’s rationale for waiting periods “a bit of pseudoscience.”

“There’s a very real danger in letting people wait or sleep on something,” he said. “It only increases the likelihood that they’ll be exposed to other information that can affect their memory.”

Deryn Strange, an associate professor at the John Jay College of Criminal Justice, says sleep “can improve the logical sequence of an event,” but that can come at a cost, including lost details or contamination from other witnesses, family or co-workers.

“We absolutely know that the best thing as far as memory is concerned is to get those details out as soon as possible,” Strange said, who is puzzled by why the law enforcement agencies only apply the rule to police officers.

“I think if your argument is that the science supports this conclusion,” she said, “why wouldn’t you want that to be applied to all people?”

More than two years since Bobby Bennett was shot, his lawyer is asking the same question.

Bennett survived his gunshot wounds, sued the city of Dallas and was awarded a $1.6 million settlement last year. The officer who pulled the trigger is awaiting his trial on charges of aggravated assault.

Tittle is pleased he was able to get justice for his client, but is troubled by the policy that was born from his misfortune.

“There’s a cost to the system by allowing one particular type of witness, a police witness, a privilege that no other witness is allowed,” he said. "It serves the purpose of the police. It does not serve the purpose of justice.”

Contact Scripps National Correspondent Ross Jones at ross.jones@scripps.com.