TYLER, Texas — Before she killed her children over Mother's Day weekend last year, Deanna Laney harbored a "grandiose delusion" that she and Andrea Yates — the Texas mom serving life for drowning her own children — might be "chosen" as God's only witnesses when the world came to an end.

The revelation and link to another sensational Texas murder case came Tuesday during the testimony of Dr. Park Dietz, a well-known forensic psychiatrist who spent two hours with Laney at the Smith County Jail in December.

Dietz concluded that Laney could not distinguish right from wrong when she stoned two of her sons to death and seriously injured a third. He also worked on the case of Andrea Yates, the Houston-area mother who drowned her five children one by one. Dietz concluded that Yates also suffered from severe mental illness.

"[Laney] thought they would be the ones to survive and preach," Dietz testified.

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Despite defense representations to the contrary, Dietz is not expected to say that Laney, 39, was legally insane when she killed her kids. He does maintain that she did not know right from wrong because she suffered from severe mental disease that, left untreated, caused her to experience delusions and hallucinations.

Asked by prosecutor Matt Bingham if he has an opinion about whether Laney was insane, the only issue at her murder trial, Dietz replied, "I think that's for the [jury]. That's not for the expert to say."

Dietz has been involved in numerous high-profile trials, including those of the Menendez brothers, Jeffrey Dahmer, the D.C. snipers, John Hinckley and others.

The expert testified that he concluded Laney was mentally ill after reviewing her life history and after she told him about several incidents that he concluded were actually hallucinations.

On some occasions, Laney detected a sulphur-like smell and believed her ability to detect it came from God.

She also had premonitions that members of her family were about to die in car accidents, or that someone would break into the Laney home and kill everyone.

Laney believed her son Aaron's "baby talk" was actually God telling her that he understood her when she spoke in tongues in church.

"There really is no questions the defendant was mentally ill, is there?" Bingham asked.

"There is absolutely no question the defendant was severely mentally ill," Dietz said.

Before court recessed for the evening, the defense complained outside the presence of the jury that prosecutors were trying unfairly to get Dietz to say that Laney never divulged an extramarital affair she had 15 years ago. Prosecutors said they were trying to counter the defense's portrayal of Laney as a devoted, faithful wife.

"She had a sexual encounter with a co-worker 15 years ago. Her husband was aware about it. They worked it out," defense lawyer F. R. "Buck" Files said. "It was a one-time event."

Judge Cynthia Kent ruled that the affair was not relevant to the core issue of the case and would instruct jurors to disregard any questions about it.