BARNSTABLE, Mass. — A below-average IQ and the stress of a lengthy police interrogation could explain the contradictory stories Christopher McCowen told detectives last year about his interaction with murder victim Christa Worthington, a defense witness testified Friday.

Addressing the key issue of whether McCowen's statements to police were coerced, defense psychologist Eric Brown told jurors that McCowen did not fully appreciate what he was saying to police. He told them he had consensual sex with the fashion writer and that another man was responsible for the Jan. 5, 2002, stabbing death.

McCowen's lawyer contends that the stress of the interrogation caused him to be confused about the sequence of events. He says McCowen had consensual sex with Worthington, 46, a day or two before the killing.

"My opinion is that Mr. McCowen lacked the cognitive function necessary to intelligently participate in that kind of interview, and was unable to comprehend what was going on," Brown said. "He was unable to follow the sequence of questions, was unable to respond in a meaningful manner to that whole process."

Brown reached his conclusion after reading the case file, interviewing McCowen and administering an IQ test that measured his responses to simple questions. He judged McCowen to have an IQ of 78, below the national average of 100. Mental retardation is considered to be 69 and lower.

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On cross-examination, prosecutor Robert Welsh III implied that McCowen was hoping the $200-per-hour psychologist could help him beat the rap.

"Isn't it in his best interest to intentionally score low on the [IQ] test?" Welsh asked.

"Theoretically," Brown responded.

"He knows he's charged with murder ... He was aware that a key part of his defense was to contest the voluntariness of his statement to [troopers], wasn't he?" Welsh prodded.

"I believe he was aware of that," the witness agreed.

Welsh then asked the psychologist if he thought people with below-average intelligence were incapable of lying.

"What I am suggesting is that a person with a verbal IQ of 78 is prone, susceptible, to manipulation and would have difficulty ascertaining his thoughts, or his explanation, for something that happened," Brown said.

According to McCowen's last statement to police, he had consensual sex with Christa Worthington on the floor of her beach house. He claimed the details were fuzzy, but remembers Worthington getting into a dispute with McCowen's friend, Jeremy Frazier. McCowen claims Frazier stabbed Worthington to death.

The defense also called University of California social psychologist Richard Ofshe to the witness stand. Ofshe is a nationally recognized expert in false confessions and is expected to testify in detail Monday about the ways in which police can coerce confessions, and how that might be relevant in McCowen's case.

The trial is being streamed live on Court TV Extra.