Psychologist Richard Samuels began to answer questions from the jury deliberating the future of Jodi Arias, who killed her former lover in 2008.
Samuels was on the stand for a fifth day Thursday as the jury submitted more than 100 questions to Judge Sherri Stephens.
Many of the questions focused on Arias' lies, how Samuels could be sure she is telling the truth now, whether her memory loss could be fabricated and his opinions on premeditation.
"How can we be certain that your assessment of Ms. Arias is not based on her lies?" one juror question read.
"The diagnosis of PTSD is a function of an evaluation based upon my 35 years of experience in working with individuals with PTSD," Samuels replied, noting he can say with "all reasonable psychological probability" that she meets the criteria.
Jurors asked if Samuels could be certain that Arias wasn't still lying about the day of the killing.
"Not with 100 percent certainty," he said. "Psychology is the science of behavior so we're seldom 100 percent sure."
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Samuels testified previously that Arias was likely suffering from acute stress at the time of the killing, sending her body into a "fight or flight" mode to defend herself, which caused her brain to stop retaining memory.
The jury asked Thursday whether this scenario could occur even if this was a premeditated murder, as the prosecution contends.
"Is it possible? Yes. Is it probable? No," Samuels said.
"Can acute stress occur if someone plans to kill versus defending themselves from danger?" the panel prodded with another question.
"Homicide is of a different nature," Samuels said before being cut off by an objection from the prosecutor.
"Possible but not probable," he continued.
The jury later asked if it is possible for a defendant to trick a psychologist into thinking they have PTSD.
Samuels again said it was possible but unlikely, noting when a person is telling the truth their stories tend to change slightly, with increasing or decreasing detail, as they are questioned repeatedly. He noted how Arias' intruder story remained exactly the same during repeated questioning until she eventually told him it was self-defense.
"It is my feeling that once the story changed (from intruders) she was essentially telling actually what happened," he said.
Once juror questions conclude, attorneys on both sides will have the opportunity to question Samuels again based on his answers.
Arias faces the death penalty if convicted of first-degree murder in the June 2008 death of Travis Alexander in his suburban Phoenix home.
Samuels says he met with Arias a dozen times for more than 30 hours over three years.
Arias spent 18 days on the stand, finishing up last week, but the focus of the trial now is defense experts trying to back up or explain her contradictory testimony.
Authorities claim Arias planned the attack on Alexander in a jealous rage.
She initially told authorities she had nothing to do with his death, and then blamed it on masked intruders. She finally claimed self-defense nearly two years later.
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Copyright 2013 Associated Press. All rights reserved. CBS 5 (KPHO Broadcasting Corporation) contributed to this report.