Over two days in the North Highlands cabbie murder trial, jurors watched a two-hour video in which Jermaine John Campbell admitted to Sacramento sheriff's detectives he participated in the fatal robbery of James Walker.

On Thursday, Campbell's lawyer put on an expert witness who suggested there might be less to the audiovisual than what met the jurors' ears and eyes.

University of Nevada psychology professor Deborah Davis was restricted from offering her flat-out opinion of the damning statements Campbell made about himself in the video played in court beginning Jan. 24 and concluding Monday. But she emphatically testified that confessions such as the one Campbell made in the Oct. 18, 2010, robbery killing of the cab-driving Walker should be viewed with a healthy dose of caution.

"In terms of absolute numbers, there are quite a number of documented false confessions," Davis said. "We don't know what percentage of confessions are false. We just know that there are a very large number of … confessions that are proven to be false."

Campbell, 21, is on trial along with Jonathan Steven Hudson, 21, and Antoine Lee Taylor, 22, in the shooting death of Walker, 54. Prosecutors said the defendants called Yellow Cab Co. about 3 a.m., that they robbed the responding Walker and that one of them shot him dead.

In their videotaped statements, Campbell and Hudson both put themselves in and around the cab at the time Walker was killed, although neither took responsibility for the shooting.

Investigators said they recovered the gun used to shoot the cabbie when Campbell threw it as they were taking him into custody the day after the killing.

Davis testified Thursday before the two Sacramento Superior Court juries hearing the cases of Campbell and Hudson. Under questioning by Campbell's lawyer, Assistant Public Defender David Lynch, Davis told jurors that history is replete with anecdotal evidence of false confessions, such as those offered by the first defendants charged in the rape of the Central Park jogger in New York.

The psychology professor provided some statistical backup, testifying that 25 percent of the 301 people in the Innocence Project exonerated by DNA tests in serious crimes such as murder, rape, burglary and kidnapping had confessed to the offenses that they were later proven not to have committed.

Davis also cited a North Carolina Law Review study that documented 125 false confessions, including 100 that were offered in murder prosecutions.

"There's no limit to what people falsely confess to," Davis told the two juries in front of Judge Cheryl Chun Meegan. "There are just many, many documented cases."

One British study found that 12 percent of convicts surveyed reported that they had falsely confessed. Of those, some 40 percent "said 'I did it,' " Davis testified, "to be let go, which seems insane, but in looking at how police interrogations are conducted, that is the message that is given."

The false hope of release is one tactic that authorities hold out while interviewing criminal suspects, which leads them to believe that confessing to the crime "can be in their best interests," Davis said.

"The whole point of an interrogation is to convince them they will be better off," she testified. "It's a con job."

Davis said criminal suspects will "do anything" to get out of the stress, "the sense of hopelessness," of the interrogation. "They come to believe they will be better off in the eventual consequences if they confess rather than if they don't," even if it seems "crazy and nuts" to people who have never been in the police detectives' hot seat.

Clever investigators can elicit false confessions by ingratiating themselves with the suspects – the "likeability" factor, Davis called it. It's also a legal and time-honored tactic for detectives to lie during interrogations about what evidence they have linking the suspect to the crime.

Lynch claimed in his opening statement that what he called Campbell's "low-dull" 75 IQ made the client even more susceptible to confessing falsely.

Davis testified that people who are "smarter than Einstein" also have done themselves in – even though they were innocent – when detectives were able to penetrate their resistance.

Under questioning by Deputy District Attorney Kevin Greene, Davis conceded that "there are true confessions as well as false confessions" and said, "I don't think there is anything in the police repertoire" designed to elicit bogus admissions.

Davis agreed that guilty people give true confessions amid all of the same circumstances that she said produce false statements, such as wanting to relieve the stress of the interrogation; seeing the admission as being in their own long-term self-interest; wanting to please the interrogator; and succumbing to the circumstantial falsehoods laid out by investigators that are meant to trap the guilty.

She also testified that she was being paid $150 an hour by the defense for the estimated 12 to 14 hours she put into the case.

The trial will continue Feb. 13.

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